phd guiding basic use and troubleshooting

8/13/2019 PHD Guiding Basic Use and Troubleshooting

1/15

9/7/13 PHD Guiding Basic Use and Troubleshooting - Imaging - Tips, Tricks and Techniques - Stargazers Lounge

stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/

Sign In Create Account

Imaging Imaging - Tips, Tricks and Techniques SGL Code of Conduct View New Content

PHD Guiding Basic Use and TroubleshootingStarted by IanL , Jun 20 2013 04:12 PM

phd guiding, guding, long exposure imaging,

Like most sites, SGL uses cookies in order to deliver a personalized service , we remember and store information about how you use it. This is done using a simple text file casit on your device. these cookies are completely safe and secure and will never contain any sensitive information. They are only used by stargazerslounge.com.

By using SGL or closing this notice you accept our policy.

The SGL policy on cookies can be found >>HERE


2/15


stargazerslounge.com/topic/188777-phd-guiding-basic-use-and-troubleshooting/ 2

There are many cameras on the market that can be used for guiding. Many people start by experimenting with webcams. These can work if

your guide scope has a particularly fast focal ratio, but the lack of a long-exposure feature usually means you will be looking for a proper

guide camera fairly quickly.

If y ou are a planetary imager, you may already have a colour planetary camera; provided it can be set to take long exposures of at least a few

seconds in duration and has an appropriate driv er for PHD (or other guiding software), it will work as a guide camera,

Alternatively you can buy a dedicated mono guide camera for between 100 and 150; keep an eye on the second hand market as these come

up for sale regularly.

The final option is an auto guider, which tend to be more ex pensive than the cameras above. The adv antage of an auto guider is that you do

not need a laptop to guide; all the brains are built into the camera and guiding commands are sent via the ST4 port to the mount without a

computer involv ed**. I havent used an auto guider but reports of their success tend to be mixed; either they work or they dont, and if they

dont it can be hard to diagnose the issue. On the plus side, most auto guiders can also be used as a dumb guide camera in conjunction with

PHD or other software.

** Note that not all cameras with a built in ST4 port have auto-guiding capabilities, so check the specifications.

How to Choose the right Guiding Set-up

There are three basic methods of guiding with various pros and cons:

1. Using a separate guide-scope and guide camera.

A second, cheap telescope or finder and a cheap(ish) guide camera is mounted piggyback on top of the imaging scope, or side-by-side using adual bar.

Pros:Relatively cheap to purchase. Quick and easy to set up and use. Two telescopes on one mount looks really cool, like some kind of

awesome space cannon.

Cons:Flexure, mirror shift, flexure and flexure.

When to use:On lower end mounts like the HEQ5, NEQ6 and cousins, a separate guide scope works best with shorter-focal length scopes.

2. Using an off-axis guider (OAG).

An OAG sits between the focuser and the imaging camera. It uses a small prism to deflect part of the incoming light into the guide camera.

Pros:Relatively cheap to purchase. Much less weight on the mount as there is no second scope. Reduces problems with flexure and mirrorshift.

Cons:Some people swear by OAGs, others swear at them. They have a reputation for being fiddly to set up and use for beginners, as you

must get both the main camera and the guide camera parfocal and also locate a suitable guide star by moving and rotating the prism.

When to use:A better option for long focal length scopes, and a much better option for moving-mirror scopes like SCTs.

3. Using a self-guiding imaging camera.

A camera with a main imaging sensor, and a second (smaller) guiding sensor next to it.

Pros:Much less weight on the mount as there is no second scope. Reduces problems with flexure and mirror shift. Much easier to set up than

an OAG.

Cons:Only a few higher end (expensive!) CCD cameras available.

When to use:A better option for long focal length scopes, and a much better option for moving-mirror scopes like SCTs.

Exposures Deeper than your Pockets?

If y ou are just start ing out in deep-sky imaging, it is important to realise than long-focal length imaging is hard on low-end mounts. Very

hard. Dont be fooled by all those glossy adv erts in the magazines, anything in the sub 2,000 bracket is definitely low-end. Dont take that

remark the wrong way! Mass-market GOTO mounts have made deep-sky imaging accessible to us mere mortals, but understanding the

capabilities and limitations of your equipment is key to succ ess in this game.

The first tip is to learn your craft using a short focal-length, fast f-ratio scope. Guiding is much easier, and youll get better

results with shorter ex posures (a virtuous circle). Aim for a focal length of no more than 800mm and an f-ratio of no more than f/6 (and if


3/15



you have the budget to get to f/5 or f/4 with decent optics, so much the better).

If you have an existing longer-focal length scope you might use a focal reducer, or if you are looking for a new scope, invest in a mid-range

refractor (the ubiquitous Skywatcher 80ED with the matching 0.85x focal reducer/flattener is where many people start out, but there are

many similar models to choose from at a variety of price points from reasonable to eye-watering!)

I learned lesson this the hard way. I started out with lunar and planetary imaging through an ancient 8, f/10 SCT (a relic of the pre-GOTO

age). The 2,000mm of focal length was great for that sort of thing, but as I soon discovered when I purchased a shiny new NEQ6, it isn't so

great for learning how to do deep-sky imaging:

- 2,000mm is probably approaching (if not beyond) the limit of what an NEQ6 can track in the hands of an expert imager with years of

experience. In the hands of a beginner like me, results were frustratingly poor and I wasted many nights getting nowhere fast.

- f/10 is very slow for DSO imaging, necessitating long exposures. But long exposures were not possible (see above). A vic ious circle not a

virtuous one.

Minimise Weight

All mounts will quote a maximum weight that they can carry. Firstly, check whether the quoted weight is just that of the scope and other

equipment or whether it also includes the counterweights (which has been the case in some more dubious marketing materials).

The next tip is that once you know the maximum equipment carrying weight of your mount, divide it in half. That is the

target weight for all of your imaging equipment; the imaging scope, guide scope, cameras, dov etail bars, scope rings and anything else you

want to hang off the mount. Y ou will be much more likely to succeed with grossly over mounted equipment than you will with a mount that is

struggling at close to its maximum carrying capacity.

I know the feeling well, If I buy the HEQ5 instead of the NEQ6 I can spend the extra money on a bigger scope.. Bad decision. Buy the big

mount and the small scope; if youre serious about imaging there will always be another scope along later!

Managing weight is always a compromise. Finder-guiders are cheap and lightweight but may have too short a focal length for your imaging

scope (see below). Losmandy-style dovetail plates and solid tube-rings add weight, but lightweight alternatives may make it hard to avoid

flexure (again see below). You will find it helpful to weigh your components or check the specifications of any potential purchases and keep a

running tally of the total.

Choose the right Guide Scope and Camera

In order to have the best chance of guiding successfully , you need to match your guide scope/camera to yo ur imaging scope/camera. Now I

know you may have read some receiv ed wisdom that when using PHD Guiding you dont need to worry about matching the guider and imager;

that is true but only up to a point.

The next tip is to is to match the pixel scales of the guider and imager. By pixel scale I mean the angular resolving power of the

scope/camera combination in arcseconds**.

If y ou are using an OAG or a self-guiding camera, even though y ou are guiding and imaging through one sco pe, it is still worth understanding

whether your imaging camera, guide camera and scope work well together or not. (It is unlikely that yo u will have a problem unless you have

a ridiculous difference in pixel sizes between the two, but knowledge is power!)

PHD Guiding can track the centroid (centre point) of a guide star to sub pixel accuracy . Basically it is able to calculate the position of the

guide star down to a fraction of a camera pixel. This leads people to the erroneous conclusion that they dont need to worry about matching

pixel scales of guider and imager because PHD can track to a 10th of a pixel or even a 50th of a pixel (depending on who you ask).

In theory that is true, but in practice you are not likely to get PHD guiding reliably to more than one quarter of a camera pixel. So a good

rule of thumb is your imaging pixel scale should be no more than four times your guiding pixel scale. You mightbe able to

exceed 4x by some margin, but sticking to less than that will make your life much easier.

So how do you calculate your pixel scale?

pixel scale in arcseconds per pixel = (camera pixel size in m / scope focal length in mm ) x 206.3

So I image with a Canon EOS 500D with pixels that are 4.7m square and a Skywatcher Evostar 80ED and 0.85x Reducer with an effective

focal length of 510mm:

(4.7m / 510mm) x 206.3 = 1.9 arcseconds per pixel (http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?

BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-

m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22goto

DecD%22%3A%22-

2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%222.51%22%2C%22f
http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%222.51%22%2C%22fovHeight%22%3A%221.67%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%221.90%22%2C%22fovAppV%22%3A%221.90%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%2222.33%22%2C%22ccdHeight%22%3A%2214.89%22%2C%22ccdFocalLength%22%3A%22510%22%2C%22pixelWidth%22%3A%224.70%22%2C%22pixelHeight%22%3A%224.70%22%2C%22pixelHRes%22%3A%224752%22%2C%22pixelVRes%22%3A%223168%22%2C%22pixelFocalLength%22%3A%22510%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%220.85%22%2C%22equipRawFL%22%3A%22600%22%2C%22equipFocalLength%22%3A%22510%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%224.7%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%224.7%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%2222.33%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%2214.89%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%224752%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%223168%5C%22%7D%22%2C%22equipCameraText%22%3A%22Canon%20EOS-50D%20%2F%20500D%20%2F%20Rebel%20T1i%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22600%5C%22%7D%22%2C%22equipScopeText%22%3A%22Skywatcher%20Evostar%2080ED%20DS%20Pro%22%2C%22zoom%22%3A2.5762355244524544%2C%22showReticule%22%3Atrue%7Dhttp://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%222.51%22%2C%22fovHeight%22%3A%221.67%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%221.90%22%2C%22fovAppV%22%3A%221.90%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%2222.33%22%2C%22ccdHeight%22%3A%2214.89%22%2C%22ccdFocalLength%22%3A%22510%22%2C%22pixelWidth%22%3A%224.70%22%2C%22pixelHeight%22%3A%224.70%22%2C%22pixelHRes%22%3A%224752%22%2C%22pixelVRes%22%3A%223168%22%2C%22pixelFocalLength%22%3A%22510%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%220.85%22%2C%22equipRawFL%22%3A%22600%22%2C%22equipFocalLength%22%3A%22510%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%224.7%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%224.7%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%2222.33%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%2214.89%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%224752%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%223168%5C%22%7D%22%2C%22equipCameraText%22%3A%22Canon%20EOS-50D%20%2F%20500D%20%2F%20Rebel%20T1i%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22600%5C%22%7D%22%2C%22equipScopeText%22%3A%22Skywatcher%20Evostar%2080ED%20DS%20Pro%22%2C%22zoom%22%3A2.5762355244524544%2C%22showReticule%22%3Atrue%7D


4/15


stargazerslounge.com/topic/18 8777-phd-guiding-basic-use-and-troubleshooting/ 4

ovHeight%22%3A%221.67%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%

3A%220%22%2C%22fovAppH%22%3A%221.90%22%2C%22fovAppV%22%3A%221.90%22%2C%22fovScopeRes%22%3A%221.45%22

%2C%22ccdWidth%22%3A%2222.33%22%2C%22ccdHeight%22%3A%2214.89%22%2C%22ccdFocalLength%22%3A%22510%22%2C%

22pixelWidth%22%3A%224.70%22%2C%22pixelHeight%22%3A%224.70%22%2C%22pixelHRes%22%3A%224752%22%2C%22pixelV

Res%22%3A%223168%22%2C%22pixelFocalLength%22%3A%22510%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipRedu

cer%22%3A%220.85%22%2C%22equipRawFL%22%3A%22600%22%2C%22equipFocalLength%22%3A%22510%22%2C%22equipApert

ure%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%224.7%5C%22%2C%5C%22pi

xelHeight%5C%22%3A%5C%224.7%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%2222.33%5C%22%2C%5C%22ccdHeight%5C%22

%3A%5C%2214.89%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%224752%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%2231

68%5C%22%7D%22%2C%22equipCameraText%22%3A%22Canon%20EOS-

50D%20%2F%20500D%20%2F%20Rebel%20T1i%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%228

0%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22600%5C%22%7D%22%2C%22equipScopeTex t%22%3A%22Skywatcher%20Evostar%2080ED%20DS%20Pro%22%2C%22zoom%22%3A2.57623552445 24544 %2C%22showReticule%22%3Atrue%7D)

I guide with a QHY5 with pixels that are 5.2m square and an Orion ST80 with an effective focal length of 400mm:

(5.6m / 400mm) x 206.3 = 2.67 arcseconds per pixel (http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?



DecD%22%3A%22-

2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%220.95%22%2C%22

fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22

%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%

22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%

22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVR

es%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReduc

er%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipApertu

re%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pix

elHeight%5C%22%3A%5C%225.2%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%226.66%5C%22%2C%5C%22ccdHeight%5C%22%3

A%5C%225.32%5 C%22%2C%5C%22pixelHRes%5C%22%3A%5 C%221280%5C%22%2C%5 C%22pixelV Res%5C%22%3A%5C%221024 %

5C%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-

II%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3

A%5C%22400%5C%22%7 D%22%2C%22equipScopeT ext%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22%3A1

.170083278378049%2C%22showReticule%22%3Atrue%7D)

If you dont feel like doing the maths, use my Imaging Toolbox (http://www.blackwaterskies.co.uk/p/imagingtoolbox.html) to do the hard

work for yo u. The upshot is that I my imaging resolution is about one and a half times my guiding resolution (2.67 / 1.9 = 1.4 1). That is well

within the 4 x rule we established above.

Dont forget, you can use a Barlow lens or a focal reducer to adjust the relative pixel scales of imager and guider. For example, the quality of

the image through the guider is not critical so a cheap Barlow can be used cut the number of arcseconds per pix el if needed.

Now there is a lot more that could be said about the optimal pixel size of imaging cameras vs different scopes, but that is another topic . The

best advice I have read is that your first priority is to have a scope and camera combination that can fit your target in the frame, and to worry

about everything else later! Again using the Imaging Toolbo x you can try different combinations of scope and camera to see what works.

** One arcsecond is 1/3600th of a degree, which is a pretty small angle.

Eliminating Flexure

Differential Flexure occurs when the imaging rig and the guiding rig end up pointing in different directions ov er the course of one

exposure. There are two main causes of flexure:

- The guiding rig and the imaging rig are not joined together in a mechanically sound way. As the mount moves to track the sky, gravity or

cables pull on one part more than the other and they end up pointing in different direc tions. Usually this is simply a matter of loosemechanical connections between the two scopes allowing them to bend or twist out of alignment.

- Parts of one of the rigs move, causing the image to shift. The two most common causes of this are the mirror shifting or flopping in SCTs

(and other moving mirror scopes), or a drooping focuser tube which bends under gravity. Tec hnically these are not Differential Flexure but

the net effects are the same, as are the solutions.

The fundamental problem is that PHD will be successfully keeping the guide star in one spot on your guide image, but because the guide scope

image is moving relative to the main scope image, the main image ends up trailing.

If you are using an SCT or similar scope, an OAG or self-guiding camera is the surest way to address this problem. As the mirror flops under

gravity , the guider sees the image shifting in exactly the same way as the imager does and compensates for it.

If you are using a piggy-back or side-by-side guide scope, you may find that you get good results when the scope is pointing in one direction,
http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%222.51%22%2C%22fovHeight%22%3A%221.67%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%221.90%22%2C%22fovAppV%22%3A%221.90%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%2222.33%22%2C%22ccdHeight%22%3A%2214.89%22%2C%22ccdFocalLength%22%3A%22510%22%2C%22pixelWidth%22%3A%224.70%22%2C%22pixelHeight%22%3A%224.70%22%2C%22pixelHRes%22%3A%224752%22%2C%22pixelVRes%22%3A%223168%22%2C%22pixelFocalLength%22%3A%22510%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%220.85%22%2C%22equipRawFL%22%3A%22600%22%2C%22equipFocalLength%22%3A%22510%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%224.7%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%224.7%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%2222.33%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%2214.89%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%224752%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%223168%5C%22%7D%22%2C%22equipCameraText%22%3A%22Canon%20EOS-50D%20%2F%20500D%20%2F%20Rebel%20T1i%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22600%5C%22%7D%22%2C%22equipScopeText%22%3A%22Skywatcher%20Evostar%2080ED%20DS%20Pro%22%2C%22zoom%22%3A2.5762355244524544%2C%22showReticule%22%3Atrue%7Dhttp://www.blackwaterskies.co.uk/p/imagingtoolbox.htmlhttp://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%220.95%22%2C%22fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVRes%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%225.2%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%226.66%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%225.32%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%221280%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%221024%5C%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-II%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22400%5C%22%7D%22%2C%22equipScopeText%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22%3A1.170083278378049%2C%22showReticule%22%3Atrue%7Dhttp://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%222.51%22%2C%22fovHeight%22%3A%221.67%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%221.90%22%2C%22fovAppV%22%3A%221.90%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%2222.33%22%2C%22ccdHeight%22%3A%2214.89%22%2C%22ccdFocalLength%22%3A%22510%22%2C%22pixelWidth%22%3A%224.70%22%2C%22pixelHeight%22%3A%224.70%22%2C%22pixelHRes%22%3A%224752%22%2C%22pixelVRes%22%3A%223168%22%2C%22pixelFocalLength%22%3A%22510%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%220.85%22%2C%22equipRawFL%22%3A%22600%22%2C%22equipFocalLength%22%3A%22510%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%224.7%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%224.7%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%2222.33%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%2214.89%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%224752%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%223168%5C%22%7D%22%2C%22equipCameraText%22%3A%22Canon%20EOS-50D%20%2F%20500D%20%2F%20Rebel%20T1i%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22600%5C%22%7D%22%2C%22equipScopeText%22%3A%22Skywatcher%20Evostar%2080ED%20DS%20Pro%22%2C%22zoom%22%3A2.5762355244524544%2C%22showReticule%22%3Atrue%7D


5/15



but not in another. This is often a sign of mechanical flexure of some sort, as the effect of gravity causes things to bend more in some

orientations than others.

The next tip for guide scope users is to sort out your mechanical connections:

- The biggest culprit is adjustable three-point guide-scope rings (like a set of giant finder-rings). The o nes with plastic screws are an absolute

disaster, and even the plastic-tipped metal screw variety can cause problems. Avoid at all costs and use a set of solid scope rings for your

guide scope: We've all got a set of three-pointers in the bottom of the junk box having learned the hard way!

(You do not have to have the guide scope and image scope perfec tly aligned for short focal length exposures of 1 0 or 20 minutes, and with a

decent guide camera you wo nt need to hunt around for a guide star.)

- If you are using a finder-guider, try to use a pair of solid rings. Avo id three-point finder rings if possible (especially plastic screws), and

never use a the rubber o-ring type of finder holder that you get on many cheap scopes!

- Check for cable snags or pulls. It is tempting to leave a nice long loop of cable fro m the cameras to the computer to avoid it snagging on the

mount, but the weight of the cables can be enough to cause flex ure. The best solution is to run the cables from the cameras up the scope tube

to the midpoint of the scope without leaving a big loop, fix them there with Velcro or a tie and then run back to the computer from that point.

- After that you will need to look at the connections between mechanical components for signs of flexure. If you can hold the imaging scope in

one hand, the guide scope in the other and make them move visibly by applying gentle to moderate force, you may well have an issue! Even if

things look solid, it only takes a few arcseconds of flexure to ruin an exposure, and the amount of movement between the scopes needed to

cause that is imperceptible to the human eye. Tighten up bolts (but beware of over-tightening and stripping threads), file any contact

surfaces flat, add shims, washers, extra bolts, screws and brackets as needed to keep everything rock solid.

- Check your fo cusers for droop. The focus tube will be pulled downwards under gravity and as the scope rotates it can cause the guide image

or the main image to shift. Make sure your focuser tensioning screws are properly adjusted; you should be able to point the scope straight up

and not have the weight of the camera cause it to slip. Also apply the lock screw as part of the focusing process if you have one. Using it can

cause the focus to shift in cheaper models, but it will reduce the tendency to droop or slip, so learn where to set the focus point to allow for

this final shift. Ultimately yo u may need to fit an upgraded focuser if you have a heavy camera.

Other Causes of Trailing that are not Guiding-related

Camera non-orthogonality causes elongated stars. This means that the camera sensor is not exactly perpendicular to the telescopes

focal plane, so the stars get stretched in the direction of greatest tilt. You can diagnose this problem as follows:

- Aim the telescope at a group of bright stars about 40 or 50 degrees above the horizon and take a short exposure of a few seconds, long

enough to capture the stars but short enough that you would not get trailing due to poor tracking. Look at the image and see if all the stars are

elongated in the same direction. If so, it is likely that you have non-orthogonality .

- Now rotate the camera through 90 degrees with respect to the telescope and take a second short image. Figure out which direction is down

towards the ground in each of your pair of images. If the elongation in both images is in the up/down direction, then yo u most likely have a

problem with the focuser or other element drooping under gravity.

- If the direction of elongation changes as you rotate the camera, then you probably have a mechanical problem in the imaging train. Very

occasionally the problem can be with the camera sensor not being parallel to the mounting ring on the camera body, but it is more likely that

something is cross-threaded or not fully screwed together. I recently had a problem with a retaining ring in a light pollution filter that was not

fully screwed in and this caused the camera to about 0.5mm off true.

My next tip is to obtain a set of digital callipers.These are invaluable in checking orthogonality of the various elements in the

imaging train, as it can be pretty hard to spot small misalignments by ey e. Use them to measure the gaps between the same points all the way

around the focus tube or other element to ensure that it is the same. As a bonus, you can use them to measure and subsequently set the foc us

position of your scope at the start of the night.

Optical aberrationssuch as coma can cause elongated stars. This is easy to diagnose as aberrations cause elongation in different

directions acro ss the image (typically pointing outwards from the centre towards the corners), whereas guiding issues will cause trailing or

elongation in the same direction for all stars in the image.

Vibration is a common problem that is easily mistaken for poor guiding. There are two main causes of vibration:

- Environmental causes, typically y ou, dear reader. You need to do your best to isolate your mount from external sources of vibration. A

solid tripod or pier, anti-vibration pads, etc. The list of cures for vibration is endless and Ill leave you to do your research on that one. If

your imaging software allows it, program in a pause before the first image is taken. Use that time to walk away from your scope since your

size tens are more likely than not to be a source of problems even on a solid concrete pad.

- Camera-induced vibration is typically a problem for DSLR imagers. When starting an exposure, the mirror makes a very satisfying clunk

as it flips up out of the way befo re the shutter opens. That tends to induce plenty of v ibration in the imaging rig. Most astro-DLSR


6/15



applications have the ability to pro gram a pause between the mirror lock-up and opening the shutter. I recommend setting this pause to at

least five or ten seconds to allow vibration to die down.

Windis also a big issue. If you find you are getting erratic guiding or stars that look like they have been drawn with a spirograph (wiggly lines

all over the place), it may well be that the wind is causing your scope to wav e around like a windsock. Bigger the scopes like Newtonians have

more surface area and can suffer from this problem more. Look for a sheltered spot or wait for better weather.

Poor Seeingis also a problem. There are techniques that can help with this (see below) but you will find that the quality of y our guiding

definitely v aries according to the seeing conditions. If you get it right, youll end up with bigger (but still round) stars on nights of poor

seeing, but if you get it wrong you can end up with trailing or wiggly lines as above.

ST4 Cable or ASCOM Pulse Guiding?

As you may be aware, there are two typical methods of sending guide correct ions to the mount. Some cameras may be connected directly to

the mount by means of an ST4 cable, so guiding corrections are sent by means of electronic signals to the mount controller

board. Alternatively, guide corrections may be sent through software via the mount driver (typically the ASCOM platform and drivers on a

Windows PC).

Which is better? The reality is that neither method is intrinsically better than the other. There is no performance advantage in using an ST4

cable over using ASCOM pulse guiding, as the time taken for corrections to reach the mount is minuscule compared to the duration of even a

1 second guiding exposure.

If y ou are using an auto guider which calculates its own guiding corrections on the camera, then using ST4 may make things simpler as you

can do away with one set of drivers on the laptop (or not use a laptop at all if capturing images directly on a DSLR memory card). We are

talking about PHD guiding here though, so Ill assume that isn't the case.

- If yo u wish to use an ST4 c able with PHD guiding, you should select On Camera from the PHD Mount menu to tell it to send guide

commands via the cameras ST4 port.

- If yo u wish to use ASCOM pulse guiding you would select ASCOM from the mount menu, and choose the appropriate mount from the

dialog when you press the connect to mount icon on the PHD tool bar.

- For non-ASCOM mounts supported by PHD, youd choose the appro priate mount from the menu instead.

To PEC or not to PEC?

Periodic Error Correct ion (PEC) is a feature of some mounts and/or their drivers. All mounts will suffer from Periodic Errors in tracking in

the RA axis, which relate to the rotation of the RA gears (gear periods, hence the name). As the gears rotate, machining imperfections in the

gears cause the tracking to drift East and West of the correc t position in a repeating cycle. The PE of a high end mount might be a few

arcseconds, but on a low end mount (sub 2,000) a PE of 10 arc seconds is considered good, and figures much higher than that are notuncommon.

PEC records those errors by monitoring guiding corrections for at least one full cycle of RA worm gear. Subsequently the mount (or driver)

plays back those error s to correct the tracking before it drifts off target. When used properly , PEC should be superior to guiding alone since

it prevents most of the tracking errors before they happen, not after they are detected.

Whether you should use both PEC and guiding together is an ecumenical question! If used properly , then PEC should give superior

performance to guiding alone, but I am definitely in the camp that thinks it is rather a lot for a beginner to take in conjunction with learning

the ropes of guiding.

If y ou are using an HEQ5, NEQ6 or related mount with the EQMOD drivers on your laptop and you also want to use EQMODs PEC features,

you should definitely use ASCOM pulse guiding. The driv ers have been written to properly combine the PEC correc tions with pulse guide

commands from PHD (or any other sof tware that produces pulse guide commands). This avoids conflicts between PEC and PHD which could

otherwise over or under-correct guide errors, whereas using an ST4 cable with EQMOD PEC may result in such problems.

(Thanks to Chris Shillito, EQMOD guru and all-round good guy, for his many patient posts on this subject which I am paraphrasing here).

Proper Preparation m akes for Pixel Perfect Performance

My next tip is to do as much testing as possible during the daytime. It is much easier to solve problems when you are warm,

indoors and have plenty of time. Trying to solve pro blems in the dark, cold and when you feel like y ou are wasting precious imaging time is a

recipe for silly mistakes.

- Do a dry run of y our entire set-up and imaging routine. Check you know where all the cables go and that they are not loose or prone to

disconnection. Check that all the software can see the cameras, mount, etc. Ensure that all drivers are installed and properly

configured. Last of all do a complete systems check to ensure that everything works together. I have had many issues where individual

components work fine, but fail when asked to work together, e.g. EQMOD works on its own, but not with PHD connected, or APT works but


7/15



not with EQMOD, etc .

- Check your guiding rig can achieve focus. You may find that you dont have sufficient back-focus on your guide scope or finder and you

need to add an extension tube. Focus on a distant object in daylight to verify this.

- Note the focus position of your guide scope and your imaging scope. You can get close during the day but you will need to refine this by

focusing on a star. Again I have wasted many hours struggling to get cameras focused; unlike eyepieces it can be incredibly hard to tell where

the focus point is until you are almost there, and how do you even know if you have a star in frame if you are way out of focus?

- Once you do have good foc us on both scopes, measure the length of focus tube that is out of the focuser body . Yo u could mark it with a pen

(not good for resale value), or make a focus measure with a strip of metal or stiff plastic that just fits between the camera and the focuser body

when you are in focus. You can sit it on the focuser tube and instantly get close to the focus point with no effort.

- Review the software settings described in the sections below until you are familiar with them and have a reasonable idea of what they do. It

can be very confusing at the outset with so many knobs and dials to fiddle with, so try to learn as much as you can during the day or when it is

cloudy.

(http://stargazerslounge.com/index.php?

app=core&module=attach&section=attach&attach_rel_module=post&attach_id=95663)

- Test that PHD can connect to your c amera and mount, and that it can send guide commands which are acted upon:

Camera: Firstly connect to the camera by pressing the camera icon on the left of the PHD tool bar. A dialog box will pop up and give you a

choice of camera types to choose from. Yo ull have to read the manual (or search the forums) for advice on which option is the right one for

your camera as they do vary, and debugging camera drivers can take a fair bit of time when you first start out!

It is worth knowing that some cameras allow you to adjust the c amera gain (brightness) from within PHD using the Cam Dialog button at the

right of the PHD tool bar, but for other c ameras you can only adjust the settings at the point where yo u first connect to it. If so, y ou will need

to use the camera button to disconnect/reconnect the camera each time you want to make an adjustment.

You will not see anything on screen when you connect to the camera. The next thing you need to do is press the Loop button, the blue

circular arrow third along on the tool bar. This will start taking repeated exposures (by default 1 second exposures). Yo u can test whether

the camera is working by c over ing and uncovering the scope (the PHD display should go black when cov ered and brighten up when

uncovered). If yo u see nothing, then it is time to check your drivers, adjust dialog settings to increase/dec rease gain, etc.

Mount: Next y ou should test your mount connection. Make sure you select the appropriate mount from the PHD Mount menu and then

click the Telescope icon second from left on the tool bar. You may be presented with a dialog to choose the mount driver depending on the

model; again check the forum for advice on your specific mount.

Test Guiding:The final check (for now) is to ensure that your guide commands are making it through to the mount. Once you have

successfully c onnected to the mount, y ou should set it to sidereal trac king rate (using EQMOD, the mount handset or whatever method is

appropriate to your mount).

Listen carefully to the mount and you will probably be able to hear the mount motors driving the mount as it tracks (it may be a very quiet

singing/whistling sound for example). Yo u wont see the mount visibly moving though, as it will take 24 hours to rotate 360 degrees in RA.

Next go to the PHD Tools menu and select Manual Guide. You will see four buttons to send short guide commands in North, South, East

and West Directions. Start by repeatedly clicking the West button and listening carefully to the tracking mount. You should hear the motor

sounds change slightly as you click , which indicates that the guide command was received and acted upon. Next try the East button. In this

case you may hear the motor sounds change or more likely stop momentarily. Finally try the North and South buttons and you should hear

the Dec motor starting up and stopping.

Bear in mind these sound are quite subtle and you may not be able to detect them. If y ou can point the main scope at a distant object and use

a high-power eyepiece, you can visually check whether the mount is moving when you click the manual guide buttons. If not, y ou will have to

wait until you can get it out under the stars, but it is better for your sanity if you can confirm that PHD is in control of y our mount during the

hours of daylight.
http://stargazerslounge.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=95663


8/15



Aligning the Mount for Imaging

Now when I said Polar Alignmentwasn't that important, I didn't mean you can forget about it entirely . You should certainly perform a

reasonable polar alignment using the polar scope and the appropriate procedure for your mount for two main reasons:

1. Whilst some new mounts and/or software can get yo ur scopes GOTO aligned however bad y our polar alignment, most mounts still rely on

a reasonable polar alignment to get the process started. If you cant get your scopes alignment routine going, you aren't going to be imaging

very much.

2. Both equatorial mounts and fork mounts on wedges rely on polar alignment to track the sky pro perly. If you have a poor polar alignment,

your ex posures may well suffer fro m field rotation. Basically the stars will not stay as pinpoints but will form short curved arcs instead. This

is more likely the longer the ex posure length and/or the longer the focal length of your imaging scope. With a short focal length scope for

(say) ten minute exposures, a polar alignment done with just the polar scope will be fine. If y oure imaging with a long focal length scope and

going for hour-long exposures, then you may well need to drift align to avoid field rotation.

It is worth noting that using a guide scope that is not aligned with the imaging scope can also cause field rotation, but the same caveats

apply, i.e. for a short focal length/short exposure dont worry about it too much.

Before we go any further, there are some important points to understand about how the mounts mechanics work:

- All mounts track by means of motors which drive a system of gears; one motor and set of gears for the RA axis to track in the East-West

direction, and one motor and set of gears for the Dec axis to track in the North-South directio n. In a mass-produced mount, those gears are

manufactured to a price; they are not hand-crafted things of beauty and they will have various imperfections that cause tracking errors.

- In even the most perfectly made set of gears, there is an amount of backlash. Put simply, there has to be a gap between the gears; if they

fitted together perf ectly then the gears would be unable to turn at all. That gap causes problems when the gears reverse direction, since the

gear has to turn a certain amount to stop pressing on one side of the teeth and start pressing on the other, so there is a delay before the gears

start moving the opposite way . A high-end set of gears will have a small amount of backlash, but guess what? Were not dealing with a high-

end set of gears and there can be a lot of backlash in a typical low-end mount, especially if it wasn't well adjusted at the factory.

- Backlash should not be a problem for tracking in RA. When tracking, the mount should always be driv ing the scope from East to West and

will not reverse. There may be a small amount of backlash at the start of tracking if the mounts last GOTO was from West to East, but this is

quickly taken up. PHD guiding will speed up the RA motor if it needs to move West to correct a guide error. It will stop the RA motor if it

needs to correc t to the East, and wait for the sky to catch up. In other words once tracking has started, the RA gears will never rev erse and

enter into backlash.

- The same cannot be said of the Dec axis. In this case, if PHD needs to guide North, it drives the Dec motor North, and if it needs to guide

South it drives the motor the opposite way. If your set-up constantly requires PHD to keep guiding in opposite directions in Dec, the gears

will spend a lot of time in backlash, which delays the guiding correct ions from taking effec t and may cause PHD to over or under-correc t the

error.

The ideal situation is that all Dec guiding corrections be made in one direction only (North or So uth, it matters not) so that the gears stay out

of backlash. PHD will do its best to make that happen (unless you tell it not to), but there are things you can do to assist it.

The next tip is not to sweat too much over drift alignment. If y ou have been imaging using an unguided mount, you may well have

put a lot of effort in to drift aligning the mount to maximise exposure time. Contrary to your ex pectations, when guiding a small amount of

polar misalignment is not a major problem when guiding a short focal-length imaging scope..

If y our target is drifting slightly in declination (either North or South) that will ensure that ensure that guide corrections are made in one

direction only, thus keeping the Dec drive out of backlash. All things in moderation though:

- Yo u dont want such a misalignment as to cause visible field rotation during your exposures.

- Nor do you want so much misalignment that you cause PHDs calibration routine to fail (see below). If you have severe drift in Dec, PHDmay interpret the drift as r esulting from its calibration commands and will end up under-correcting when guiding in Dec.

As stated previously, I find that a polar scope alignment works really well for my short-focal length imaging, but the longer your focal

length, the more critical a good Polar Alignment becomes, and thus part of the reason why imaging at long focal lengths is hard.

If you do have perfect polar alignment (i.e. you are drifting so little it produces no visible effect on the image during one exposure), you can

switch off Dec guiding altogether. Click on the Brain icon on the PHD tool bar and set Dec Guide Mode to Off. This might be the case if

you have a permanent observato ry with a pier and a rock-solid mount adjustment. For the rest of us, it is probably quicker to leave Dec

Guide Mode on Auto. and let PHD work out what to do for the best.

Balancing the Mount

The next tip, if using an equatorial mount, is to think about Balancing it, or should I say not balancing it! The mount


9/15



manual will probably explain that it is important to check the balance o f the mount, so that the RA and Dec ax es will stay in a horizontal

position when the clutches are unlocked. It is certainly true that you do not want a massive imbalance in either axis, especially if using heavy

equipment, as this will put a lot of strain on the gears and motors and may lead to premature wear.

That said, obtain perfect balance is actually bad for tracking and guiding performance on low-end mounts:

- The usual advice is that the mount should be East-side heavy, so that it is working to lift whatever is on the east side of the mount. So if the

scope is on the East side of the mount (pointing West of the meridian) then you should move the counterweights up the counterweight shaft

slightly higher than the perfect balance point. Conversely , when the counterweights are on the East side of the mount (scope pointing East of

the meridian), move them down the shaft below the perfec t balance point.

You are aiming to have the RA worm (one of the gears) drive the main RA worm gear. By keeping the RA axis imbalanced to the East, you

force the gears to stay in contact with each other (driven imbalance). This tends to smooth out any sudden movements more than the

opposite (resistive) imbalance. In the resistive set-up, the worm gear may slip or jolt suddenly due to uneven machining. This can also

happen with driven imbalance, but most people report better results with driven imbalance than resistive imbalance.

My personal experience with my NEQ6 is that so long as I have some imbalance in RA, it doesn't particularly matter which side is heavy . In

fact when I am within 30 minutes of hav ing to do a meridian flip, I will often pre-emptively do a forc ed flip between exposures (so I can get

back to a frosty beverage of my choice and watching the TV on the nice warm sofa).

The scope ends up on the wrong side of the mount (East Side pointing East) with the counterweights higher than the scope until it track s

through the meridian. Id expect at least one bad exposure as the weights drop through horizontal and the gears switch from one face to the

other, but it never happens. Again I suspect that using a short focal length imaging scope gives me a lot of room for manoeuvre!

- The same sorts of consideration apply to the Dec worm and worm gear (unless you are not guiding in Dec of course). Yo u need to imbalance

the Dec axis by moving the scope fo rwards/backwards (or adding weights) depending on which direction you are pointing relative to the

zenith so that you achieve driven resistance.

If y ou have southward drift:

- If the scope is pointing South of the zenith, imbalance the scope so that the North (lower) end is heavy .

- If pointing North of the Zenith, again imbalance so that the North (higher) end is heavy .

If y our polar alignment is causing a gradual northward drift instead, then you rever se the imbalance:

- Pointing South of zenith, South/higher end heavy.

- Pointing North of zenith, South/lower end heavy.

PHD Basic Settings

We looked at a few of the basic PHD settings during testing above, so hopefully y ou already know how to get your camera and mount

connected to PHD ready to start guiding. Next we will look at a few of the more important settings.

Exposure Length

In the middle of the PHD tool bar is the Exposure drop-down, which allows you to choose the length of the guiding exposures in

seconds. Setting the exposure length is a trade-off between several factors:

- You need a long enough exposure to produce a measurable guide star. If your exposure length is too short (and/or yo ur camera gain too

low), you will either get no stars or stars that are too faint to use reliably. If you find you are able to lock on to a star and calibrate, but then

get frequent Star Lost errors flashing up, it may be that your chosen star is too faint. Increase the ex posure length (and/or gain).

(Bear in mind that cloud can cause the same problem as it drifts across and dims the guide star from time to time, so chec k the skies,

especially for high, thin cloud which can be hard to spot. Also check your guide-scope for dewing which has much the same effect. )

- You need a short enough exposure not to saturate the guide star. If your exposure length is too long (and/or your camera gain is too high)

you may get a Star Saturated error from PHD. This means that one or more of the stars pixels has reached maximum brightness and PHD

cannot calculate the stars centroid accurately. Reduce the exposure time and/or reduce the gain.

- You need an exposure length that is short enough that PHD can detect and correct guiding errors before they become visible on your main

image. In an ideal world wed take guide exposures as frequently as possible, but in practice there is a minimum length of exposure dic tated

by the need to pick up a guide star (see above). Furthermore it can be advantageous to have a longer exposure to reduce the effec t of

seeing. Really short ex posures can end up with PHD chasing the seeing and poor guiding, whereas a slightly longer exposure av erages out

the seeing and gives a better estimate of the guide stars centroid.

- When you have selected a guide star, the stars Signal to Noise Ratio is displayed at the bottom left of the PHD window on the status

bar. There is no hard and fast rule as to what this number should be. Generally speaking a higher number is always better, prov ided you are


10/15



not getting Star Saturated errors from PHD. Use the SN number to tweak your ex posure length, camera gain and focus until you get a

reliable lock with no Star Lost or Star Saturated errors.

If you are really struggling to get an acceptable SN number with your camera, you can go in to the Brain icon on the tool bar and change the

Noise Reduction setting from none to 2x2 or 3x3. This increases SN by binning pixels in software at the cost of reduced guiding

accuracy. If trying the 2x2 setting, multiply the camera pixel size by 2 (or by 3 if using 3x3 setting) and calculate the guider pixel scale using

the equation we discussed above. If yo u are still within the 4x pix el scale rule of thumb, you will probably be OK.

- The best advice I can give you for exposure length is to start with a two or three second exposure and work from there by reference to the

guiding graph, which well look at below. Over a few sessions youll get the hang of the right exposure length for your equipment.

- Bear in mind my earlier c omments that if you are pushing the envelope and imaging at a long focal length, you may need a more sensitive

guide camera, a longer focal length and/or faster f-ratio guide scope (hard to do both), to use PEC in addition to guiding or simply a higher

quality mount.

The next tip is that it might help defocus the guide scope slightly. If yo u are having problems with saturated stars, or you are

exceeding the 4x pixel scale rule, this may help. Defocusing will spread the light of the star over more camera pixels which can prevent

saturation, and also makes it easier for PHD to detect the centroid of the guide star more accurately. Don't go mad with defocusing or PHD

won't be able to lock onto the star and you may start getting Star Lost errors if your SN figure drops too much due to de-focusing.

In all cases your guide star still needs to have a brighter central peak and not turn into a hollow doughnut, but a bit of defocus on the guide

scope may help where you have SN to spare. Yo u can use the Star Profile option on the Tools menu to check the profile of your guide star

once selected. It should have a peak in the middle like a mountain. If it is flat on top you have a saturated star, and if there is a dip you need

to focus more.

Brightness Slider

Next to the exposure drop-down is a brightness slider. It is essential to realise that this slider only increases or decreases the brightness of the

guide image display. It has absolutely no effect on the camera settings or the actual brightness of the guide image. Yo u can use the slider to

make it easier to see stars on screen, but if the stars are too faint (or bright) for guiding you have to adjust the camera gain or exposure length,

as this slider has no effect on guiding performance,

Take Dark

Towards the right hand side of the tool bar is the Take Dark button. You should definitely use this. Click the button when ready to start

guiding. PHD will instruct you to cover the guide scope and then take a series of dark frames. Finally it will remind you to uncov er the guide

scope again (been there, bought the t-shirt!)

Failing to take a dark can cause two problems:

- Firstly you may end up selecting a hot pixel as your guide star. Normally PHD will spot this and complain, but you may waste a lot of time

trying to calibrate o n it if it doesn't (or even guide on it if retaining an existing calibration). The guiding graph looks great since the hot pixel

stays dead centre of the lock box, but the final images do not!

- Secondly as you guide, a hot pixel may be close to the guide star, and PHD may intermittently include the hot pixel as part of the star image

causing the centroid position to jump around erroneously.

Bear in mind that whenever you change exposure length or camera settings, you should re-take your dark using this button.

Getting ready to Guide

So now you are ready to go. You have found your target for the night, you have framed it nicely and focused yo ur imaging camera and set the

basic PHD guiding parameters above. The next steps are as follows:

- Click the Loop button to start taking guide exposures. Hopefully you will see a choice of guide stars in the PHD screen. If you dont go

back a few steps and adjust the exposure/ camera gain and try again (re-taking your dark if needed).

- Click on the brightest star but avoid using a star that is close to the edge of the image, as it may drift out of view over the course of the

imaging session (especially if y ou are dithering your images). Check that PHD is not complaining about the star being saturated, and ensure

that PHD has found the star (it should draw a green box around it if it is happy).

- Avoid picking a star that is close to another star, and also avoid using any known double stars. In both cases you may end up with

erroneous/f luctuating centroid calculations and spurious guide commands.

Calibration

- Next clic k on the PHD button and PHD will start the calibration proc ess. It will attempt to move the star some distance East and West in


11/15



RA, and then North and South in Dec in order to figure out which guide commands move in which orientations and also how long guide pulses

need to be..

A pair of lines will be drawn in a cross indicating the initial position of the guide star, and you will see the status bar indicating which guiding

commands are being sent. All being well you will see the star and the green box gradually drift along one of the lines, stop and drift back

again, and then do the same along the other line.

This process can take several minutes to complete successfully so be patient. A good calibration will usually require somewhere between

seven and forty steps in each direction. If the calibration completes in less than seven steps in any directio n, you are unlikely to get good

guiding performance and you should change the calibration step size (see below). If the calibrat ion takes more than forty steps in any given

direction it will still work, but again you may want to change the calibration step size to save time. My experience is that a good calibration

takes about 15-20 steps and between five and ten minutes to complete.

If the calibratio n fails with PHD complaining that the star didn't move enough, then you have one of two problems. Either the calibration step

size is too small (see below), or your guide commands on one or both axes are not being acted upon. We talked about using the Manual

Guiding option on the Tools menu to verify that PHD can control the mount, so if you havent done so already, put a high-powered eyepiece

in your imaging scope, centre a star and then use those controls to make sure the scope mov es in all four directions when commanded.

The f inal thing to be aware of is that you should recalibrate PHD whenever y ou slew to a new imaging target (unless it is within a few degrees

of your original target). You do this by click ing the Brain button on the tool bar and ticking the force calibration button. Yo u will not be

able to successfully guide if you try to keep the same calibration in different parts of the sky.

Similarly if you perform a meridian flip on an equatorial mount, you should probably re-calibrate. There is an option on the Brain dialog to

reverse the existing calibration after a flip, but my experience is that it is better to spend five or ten minutes recalibrating rather than

spending ten minutes exposing only to discover you have to recalibrate anyway!

Calibration Step Size and Guiding Rate

If y ou are have a good guide star (good SN figure, not saturated), and PHD is definitely able to command the mount but calibration fails, then

it is a fair bet that the Calibration Step Size number needs to be adjusted. Click on the Brain icon on the tool bar to bring up the advanced

PHD settings.

The Calibration Step Size determine the length of the calibration guide pulses. T he default setting is 500ms (half a second). This may or

may not be the right amount:

- The bigger the pixel scale of y our guiding rig, the longer the step size should be. I use 2,000 to 2,500ms for my 4 00mm/5.6m guide scope

at 2.67 arc-second per pixel. If yo ur pixel scale is a bigger number, increase the step size, if smaller decrease it.

- The closer yo u are to the celestial pole, the longer the step size should be. Ty pically I use 2,000ms near the celestial equator and increase

that to 2,500ms as I approach the pole. The appropriate range will vary acc ording to the pixel scale of your guiding rig.

- Your mount, or mount driver may allow you to set a custom guiding rate. The best starting point is to set your guiding rate to 1x, which

means that when guiding East the mount will stop completely, and when guiding West it will move at 2x sidereal rate. Many mounts of fer 0.5 x

guiding rates (and others), which are helpful when hand guiding, but probably not that much use when using PHD. keep it simple and use the

1x guide rate.

The next tip is that you should experiment with the calibration step size and keep notes of what works for each

declination. Prov ided you are calibrating in more than seven steps you should be OK, but if you are exceeding 40 steps you can almost

certainly save time by increasing step size. I aim for 10-20 steps at most.

Guiding (at Long Last!)

Once PHD completes calibration, it will immediately start guiding. The pair of lines will turn green and guide commands will be sent. Time to

start imaging at last.

I know it has taken a long time to read to this point, but in reality and with a bit of practice, the time from finishing

centring and focussing my imaging target in the main scope to having PHD guiding merrily away takes me seven

minutes on average, and for about 6 minutes and 30 seconds of that time, PHD is calibrating without any input from

me!

As far as guiding goes, the next thing you need to do is monitor the PHD status bar for a minute or two to make sure you are not getting Star

Lost or Star Saturated errors. The oc casional Star Lost error is nothing to fret about usually, but if they are flashing up repeatedly you

should do some troubleshooting (check exposure and gain as described above, look out for high cloud and dewing on the guide-scope).

Interpreting the PHD Graph

The most useful tool for check ing your guiding performance in the field is the guiding graph. Go to the Tools menu and select Enable


12/15



Graph.

(http://stargazerslounge.com/index.php?

app=core&module=attach&section=attach&attach_rel_module=post&attach_id=95664)

What you will see is a graph showing your guiding performance in RA (the blue line) and in Dec (the Red line). The name of the game is to try

to keep the two lines as close as possible to the centre line running from left to right across the chart.

The horizontal scale button (100 abov e) allows you to zoom in and out by reference to guiding cycles. By default 100 guiding cycles are

shown across the chart, this isn't a fixed time scale as such and the longer your individual guide exposures, the longer those 100 cycles will

take. Yo u can click the scale button to zoom out and see more cy cles, but to be honest the default 100 cyc les is most useful for diagnosing

guider problems.

The vertical scale button (RA/Dec) allows you to switch between RA/Dec corrections and x/y pixel correct ions. Again the first mode is most

useful since you want to see what is going on with your RA guiding and Dec guiding separately, whereas the x /y mode isn't (necessarily ) much

help in that regard.

Again the vertical scale is not too helpful at first sight. Each dotted line corresponds to one pixel of error on the guide camera. Since you

have calculated the pixel scale of your guide camera already, you can multiply it by the number of divisions to find your maximum guide

error. In the graph above I was using my ST 80 and QHY5, and the blue RA line averages out at about 0.2 pixels above/below the line,

making for an error of:

2.67 arcseconds per pixel x 0.2 divisions = 0.53 arcseconds.

This is what we call the Root Mean Square or RMS error, which basically means we square the all the errors (positive stay positive, negative

x negative beco mes positive), add them together, average the total and take the square root. If we didn't calculate the RMS then the negative

errors (below the line) would cancel out the positive errors (above the line) and our average error would be close to zero, which clearly it is

not!

Helpfully PHD calculates the RMS error for whatever you can see on the current graph scale and puts it on the bottom left of the graph below

the buttons. It only does this for the RA error however (the blue line) as it is assume you will not be making major Dec correct ions at all (not

always true though!)

The other f igure just below RMS is the OSC Index, i.e. the oscillation index. Again this is only calculated for the RA axis. The OSC index is

the probability that guiding will change direction at the nex t step (i.e. the blue line will cross the centre of the graph)

A mount with no Periodic Error would have a figure of 0.5, i.e. equally likely to have to correct in either direction, but a figure between 0.25

and 0.4 for a real mount is generally considered okay but there isn't a hard and fast rule about the OSC index:

- If you have a small RMS figure and a large OSC index you should be okay. The mount is constantly flipping between guide directions but

only by a small amount.

- If yo u have a large RMS figure and a large OSC index, that would tend to suggest the poor guiding is due to lack of stability in the set-up, and

that could be down to mechanical/balance problems, wrong exposure/gain on the camera creating a noisy guide star, incorrect guiding

parameters or just really bad seeing conditions.

Unhelpfully in my ex ample, the OSC index is greater than 0.5, which suggests the mount is correcting more in RA in one direction than the

other ov er time. I'm man enough to admit I dont know what that means! What I do know is that I got really good imaging results from that

session so clearly it wasn't a problem to worry about.

Common Issues from the Graph

- First of all, please bear in mind that you can have a pretty ugly looking guiding graph that produces little or no visible effect on the actual

image. The opposite is also true of course! You need to c onvert the pixel lines on the vertical scale and/or the RMS error to arcseconds of

error by perf orming the guider pixel scale multiplication described above. Now compare that to the pixel scale of your imaging rig. Y ou may

find that what looks like a major guiding issue is actually less than a pixel or two of error on your final image.

For example, looking at the RA error in the graph above:

0.2 RMS pixels error x 2.67 arcseconds per pixel = 0.53 arcseconds of error on the guider image

0.53 arcseconds of error / 1.9 arcseconds per pixel imager scale = 0.28 pixel RMS error on final image
http://stargazerslounge.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=95664


13/15



Looking at my graph, I can see that the RA error is roughly evenly distributed above and below the line so I should double the RMS error,

meaning my stars are perhaps half a pixel wider than they should be in RA. The Dec error looks pretty much the same. Sounds pretty good to

me, and indeed it looks good on the final result.

(If the guide error was predominantly above the line, or predominantly below the line, we could use the smaller 0.28 pixel RMS value

instead).

- If one or other of the RA/Dec lines is shooting off the graph it may mean that corrections in that axis are not being sent to the mount or are

not being responded to. Again perform the manual guiding checks to ensure every thing is working as you would expect. Also check that you

havent turned off Dec guiding under the Brain button. If ev erything checks out, ensure your c lutches are tight on the mount and look for

other signs of flexure or equipment flapping about.

(if you are using dithering, it is normal to have the RA and/or Dec lines shoot away from the centre line whilst dithering is in progress, and

then drop back to the centre line as dithering finishes and guiding settles. It does tend to make me jump off the sofa when I see it happening

on my remote monitoring screen though!)

Tweaking Guiding Parameters

The next tip is that your results may well vary according to the Alt and Az of the target. I consistently get better results on

higher targets nearer to the meridian. Targets lower in the east produce rougher guide graphs. Whether that is due to poor seeing due to

more atmosphere in the way of the guide star, something mechanical or perhaps needing to tweak the guiding parameters I know not.

If y our guiding graph is zig-zagging all over the place and you are getting many arcseconds of err or, and assuming you hav e checked all the

basics above, then you may need to tweak some of the guiding parameters. Helpfully the main parameters that you may need to change can

be adjusted using the controls directly below the guiding graph:

-RA Aggressiveness: How much of the calculated RA correction to apply. Start with the default figure of 100%, as hopefully PHD knows

what it is doing. I f you find that guiding in RA (blue line) is constantly overshoo ting the mark and then rapidly zig-zagging back the other way,

try reducing this to a lower figure, perhaps 70-80%. If y ou think your mount has sloppy gears, maybe try going to 11 0 or 1 20% to see if y ou

can compensate.

-RA Hysteresis: How much of the prev ious RA guiding trend to take into account. Good guiding is smooth and doesn't respond to rapid

changes in guide direction (usually caused by seeing or poor SN from the guide star). If you think that guiding is too jerky with sudden rapid

corrections, increase this number, or decrease it to make guiding more responsive. Aggressiveness and Hysteresis go hand in hand and you

should normally reduce or increase them together in small increments until you get best results.

-Min Motion:The smallest guide error that should result in a correction being made. This is in pixels and applies to both the RA and Dec

axes. So if I set min motion to 0.1 pixels, then PHD would not try to correct any err ors until they were more than that value, and for my

guider scale of 2.67 arcseconds per pixel, that means PHD would not try to correct any errors less than about a quarter of an arcsecond in

size. Again this is useful for smoothing out jerky guiding if your mount is poor or y ou dont have a good guider set-up.

-Max RA Duration: This sets the maximum single guiding correction that PHD will send in a single cycle (in this case 1,0 00ms = 1 second

guide pulse). Generally you shouldnt make this value too large (maybe less than half a second), since PHD will issue another correction if it

needs to, but if the first correction is too large and overshoots youre already in trouble. (In this case, the rest of the guiding is working well

and PHD never gets anywhere near the perhaps excessive 1 second maximum I have set, but your mileage will vary!)

- Max Dec Duration: This is the same as above. The default setting is smaller than the default for RA out of the bo x, and I've never needed to

change it.

To be honest I think that is the final tip: If it aint broke, dont fix it! It can be very tempting to dive in and start fiddling with

the advanced parameters in PHD in the hope of finding a magic formula for pix el perfect guiding. In reality 90% of the guiding problems are

external to PHD guiding. Only when you have investigated and fully understood all those externalities will you be able to make an informed

decision as to what to tweak in PHD itself.

There are many other resources on-line covering advanced PHD fettling, and plenty of people on the forum willing to

offer you customised advice and assistance, so please dont hesitate to ask for help. Youll probably find their advice

more useful if you have done a bit of homework first.

Tim Posted 20 June 201 3 - 05:01 PM

Thank you Ian for such a comprehensive post about PHD :-)

This post will be stickied for immortality!

Uranium235 Posted 20 June 201 3 - 05:02 PM


14/15



An excellent guide, should be made a sticky!

I have a little extr a for the pot in regard to PHD and finderguiders, the following link contains posts from Craig Stark (the author of PHD) as

to the settings that should be used for 5 0mm finderguiders:

http://www.cloudynig...psed/sb/5/o/all

(http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/2009880/page/11/view/collapsed/sb/5/o/all)

Also, how to work out whether you're getting backlash. It manifests itself in the form of an slow drift in dec, followed by an overcorrec tion. If

left for long enough you will notice a sawtooth pattern forming in the graph history .

Whoops! Tim beat me to it in the sticky request!

Edited by Uranium235, 20 June 2013 - 05:05 PM.

Jessun Posted 20 Ju ne 201 3 - 1 0:56 PM

Great write up Ian, many thanks for that!

Quote

Unhelpfully in my ex ample, the OSC index is greater than 0.5, which suggests the mount is correcting more in RA in one direction than

the other ov er time. I'm man enough to admit I dont know what that means!

Could that not be simply refraction towards the lower east or west?

/Jesper

Roberto Coleschi Posted 21 June 201 3 - 05:34 AM

Great guide, and Imaging To olbox is a wonderful aid, thanks..

Gasman Posted 23 June 201 3 - 05:12 PM

Superb guide Ian thanks. Answered a few querys for me

Steve

Gedan Posted 24 Ju ne 2013 - 01:26 AM

Fantastic, I'm just about to start with PHD.

thanks

Russe Posted 03 July 2013 - 08:14 AM

Thanks! Great guide!!!

IanL Posted 04 September 2013 - 02:49 PM

Just spotted a small error:

I guide with a QHY5 with pixels that are 5.2m square and an Orion ST80 with an effec tive focal length of 400mm:

(5.6m / 400mm) x 206.3 = 2.67 arcseconds per pixel(http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?



DecD%22%3A%22-


fovHeight%22%3A%220.76%22%2C%22f ovMax%22%3Atrue%2C%22fovColo ur%22%3A%22%2300FF00%22%2C%22fovRotation%22

%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%2

2%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%2

2pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVRes

%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer

%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipAperture
http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%220.95%22%2C%22fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVRes%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%225.2%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%226.66%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%225.32%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%221280%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%221024%5C%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-II%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22400%5C%22%7D%22%2C%22equipScopeText%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22%3A1.170083278378049%2C%22showReticule%22%3Atrue%7Dhttp://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%220.95%22%2C%22fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVRes%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%225.2%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%226.66%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%225.32%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%221280%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%221024%5C%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-II%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22400%5C%22%7D%22%2C%22equipScopeText%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22%3A1.170083278378049%2C%22showReticule%22%3Atrue%7Dhttp://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/2009880/page/11/view/collapsed/sb/5/o/allhttp://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%220.95%22%2C%22fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVRes%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipAperture%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pixelHeight%5C%22%3A%5C%225.2%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%226.66%5C%22%2C%5C%22ccdHeight%5C%22%3A%5C%225.32%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%221280%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%221024%5C%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-II%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3A%5C%22400%5C%22%7D%22%2C%22equipScopeText%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22%3A1.170083278378049%2C%22showReticule%22%3Atrue%7D


15/15


Back to Imaging - Tips, Tricks and Techniqu

Stargazers Lounge Imaging Imaging - Tips, Tricks and Techniques

%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pixel

Height%5C%22%3A%5C%225.2%5 C%22%2C%5C%22ccdWidth%5 C%22%3A%5C%226.66%5C%22%2C%5C%22cc dHeight%5C%22%3A

%5C%225.32%5C%22%2C%5C%22pixelHRes%5C%22%3A%5C%221280%5C%22%2C%5C%22pixelVRes%5C%22%3A%5C%221024%5C

%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-

II%22%2C%22equipScopeVal%22%3A%22%7B%5 C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3

A%5C%22400%5 C%22%7D%22%2C%22equipScopeText%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22% 3A1.

170083278378049%2C%22showReticule%22%3Atrue%7D)

That should actually read "(5.2m / 400mm) x 206.3 = 2.67 arcseconds per pixel

(http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?


m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-


fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22

%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%

22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%

22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVR

es%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReduc

er%22%3A%221.00%22%2C%22equipRawFL%22%3A%22400%22%2C%22equipFocalLength%22%3A%22400%22%2C%22equipApertu

re%22%3A%2280%22%2C%22equipCameraVal%22%3A%22%7B%5C%22pixelWidth%5C%22%3A%5C%225.2%5C%22%2C%5C%22pix

elHeight%5C%22%3A%5C%225.2%5C%22%2C%5C%22ccdWidth%5C%22%3A%5C%226.66%5C%22%2C%5C%22ccdHeight%5C%22%3

A%5C%225.32%5 C%22%2C%5C%22pixelHRes%5C%22%3A%5 C%221280%5C%22%2C%5 C%22pixelV Res%5C%22%3A%5C%221024 %

5C%22%7D%22%2C%22equipCameraText%22%3A%22QHY5%20%2F%20QHY5-

II%22%2C%22equipScopeVal%22%3A%22%7B%5C%22aperture%5C%22%3A%5C%2280%5C%22%2C%5C%22focalLength%5C%22%3

A%5C%22400%5C%22%7 D%22%2C%22equipScopeT ext%22%3A%22Orion%20(US)%20ShortTube%2080%22%2C%22zoom%22%3A1.170083278378049%2C%22showReticule%22%3Atrue%7D) "
http://www.blackwaterskies.co.uk/p/imagingtoolbox.html?BWSFOV=%7B%22version%22%3A1%2C%22searchSurveySelection%22%3A%22P%2FDSS2%2Fcolor-m%22%2C%22gotoRaH%22%3A%225%22%2C%22gotoRaM%22%3A%2240%22%2C%22gotoRaS%22%3A%2259.00%22%2C%22gotoDecD%22%3A%22-2%22%2C%22gotoDecM%22%3A%2227%22%2C%22gotoDecS%22%3A%2230.00%22%2C%22fovWidth%22%3A%220.95%22%2C%22fovHeight%22%3A%220.76%22%2C%22fovMax%22%3Atrue%2C%22fovColour%22%3A%22%2300FF00%22%2C%22fovRotation%22%3A%220%22%2C%22fovAppH%22%3A%222.67%22%2C%22fovAppV%22%3A%222.67%22%2C%22fovScopeRes%22%3A%221.45%22%2C%22ccdWidth%22%3A%226.66%22%2C%22ccdHeight%22%3A%225.32%22%2C%22ccdFocalLength%22%3A%22400%22%2C%22pixelWidth%22%3A%225.20%22%2C%22pixelHeight%22%3A%225.20%22%2C%22pixelHRes%22%3A%221280%22%2C%22pixelVRes%22%3A%221024%22%2C%22pixelFocalLength%22%3A%22400%22%2C%22pixelAperture%22%3A%2280%22%2C%22equipReducer%22%3A%221.00%22%2C%22equip

phd guiding basic use and troubleshooting

Documents