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A Digital Astrophotography Primer

- The Sequel -

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Table of Contents

A Digital Astrophotography Primer...........................................................................................................................................................1 Table of Contents.......................................................................................................................................................................................2

Introduction............................................................................................................................................................................................3 AESTHETICS .......................................................................................................................................................................................4

General...............................................................................................................................................................................................4 Focus..................................................................................................................................................................................................4 Composition.......................................................................................................................................................................................4 Standards............................................................................................................................................................................................5 Look 4 a Hook ...................................................................................................................................................................................6 Location, location, location................................................................................................................................................................7

EQUIPMENT ........................................................................................................................................................................................9 Tracking mounts ................................................................................................................................................................................9 Telescopes..........................................................................................................................................................................................9 Cameras .............................................................................................................................................................................................9 Lenses ..............................................................................................................................................................................................10 Filters ...............................................................................................................................................................................................10 Intervalometer ..................................................................................................................................................................................10

TECHNIQUES ....................................................................................................................................................................................11 Polar Alignment ...............................................................................................................................................................................12 Guiding ............................................................................................................................................................................................12

LET’S GET STARTED.......................................................................................................................................................................13 KISS.................................................................................................................................................................................................13 What to expect from your first images.............................................................................................................................................14

Some New Terminology ......................................................................................................................................................................15 Signal ...............................................................................................................................................................................................15 Noise ................................................................................................................................................................................................15 Signal to Noise Ratio .......................................................................................................................................................................16 Integration time................................................................................................................................................................................17 Signal to Noise Ratio .......................................................................................................................................................................18 THE GOOD NEWS:........................................................................................................................................................................19 THE BAD NEWS:...........................................................................................................................................................................19

SHOOTING DARK FRAMES............................................................................................................................................................20 How to take a dark frame.................................................................................................................................................................20 What a dark frame does ...................................................................................................................................................................20

SHOOTING BIAS FRAMES ..............................................................................................................................................................21 How to take bias frames...................................................................................................................................................................21 What a bias frame does ....................................................................................................................................................................21

SHOOTING FLAT FIELDS................................................................................................................................................................22 How to take a flat field.....................................................................................................................................................................22 What a flat field does .......................................................................................................................................................................22

Combining/stacking image files...........................................................................................................................................................23 Processing image files..........................................................................................................................................................................23

Calibrate Your Monitor....................................................................................................................................................................23 Find some software ..........................................................................................................................................................................24

Summary..............................................................................................................................................................................................25 Sources & Suggested Reading .............................................................................................................................................................25

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Introduction

While I realize that sequels are never as good as the original, I was prompted to write a Part Two by necessity.

This paper is a logical step or progression in my learning curve. When I wrote Part One I thought I had reached

the end of my astro-photographic journey. It didn’t take me long to realize that the items I wrote about in Part

One were merely the tip of the iceberg, so to speak. I hadn’t written a definitive work, I had merely brushed on

the basics of astrophotography and digital slr’s.

One evening, not so very long ago, fellow astrophotographer, Dave G and I were surfing the web and looking at

some of the truly stunning Astrophotos that were posted on the www. I’m talking about posts like you see on

APOD, (Astronomy Photo Of the Day) or Nasa’s Space Image of the day. Stunning photos like you see in Sky

& Telescope & Astronomy magazine. You know the kind of images I mean. You look at these photos and

exclaim, WOW!

Dave appropriately called it the “WOW FACTOR”.

“What”, Dave asked, “do we have to do to get images like that? Not images that people will look at and say,

“Oh, that’s nice’. Images that cause that little rush of excitement in folks, and create the little “WOW” that

involuntarily escapes their lips. You know what I’m talking about. You’ve heard it a thousand times. That

WOW you hear when a child sees the moon through your telescope for the first time, or an adult see the rings of

Saturn through a telescope for the first time.

This paper is the result of my personal search for WOW! Some

of the topics we discuss will be technical in nature. Some topics

will be simply common sense, nonsensical guidelines, and some

topics will deal with intangible things like esthetics.

To achieve WOW everything has to click.

We have to master the technical aspects of our equipment. We

have to know our gear intimately. We have to know the range

and the limits of our tools.

We have to follow common sense guidelines and techniques to assure repeatability and achieve the ultimate

effects of our gear.

And, lastly, we need to have a true love of the beauty of astronomy. We need an artist’s eye. To capture WOW

we have to be able to see it in our viewfinders. We need to recognize it when we see it on our PC’s and we need

to be able to feel it in our guts. We need to remember back to the days of olde when we used to say WOW.

Then we need to shoot accordingly…..

This paper will not teach you to see or feel beauty. You already know how to do that, or you wouldn’t be here.

What it will strive to do is to bring together some of the skills and techniques you’ll need to create an image of

beauty. My goal is to teach myself (and hopefully, you, too) how to point our cameras and telescopes

heavenward and capture a little piece of that heaven. To save images to our screens that will prompt others to

look upon them and say WOW !!!

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AESTHETICS

To get a WOW your image must be aesthetically pleasing. There are several aspects involved in producing a

pleasing image. Of course, the obvious rules apply.

General

Obviously, we need to capture photons to make our image. In astrophography this usually involves an exposure

that is built up over time. Which means, most likely, there will be tracking issues. Trailing stars and bouncing

stars and skipping stars are all OK for run of the mill images. When we’re talking WOW Factor, though, we’re

talking ROUND stars. People know stars are round and appreciate it when you show them images of round

stars. Later we’ll learn how to assure that we have round stars in our images.

Focus

The image should be in focus. Yea, so you

captured the comet that only comes around once

every 86 years. For you it may be a lifetime

achievement. But, nobody cares, if it’s out of

focus. Nobody ever says, WOW, really nice out of

focus image, buddy. Good job!

There are some really neat programs available that

do a remarkable job of focusing using Full Width

Half Medium routines. These programs measure

the diameter and brightness of a selected star. As

we focus and the star becomes brighter and smaller

in diameter the program records these changes and

aids us in determining when the best possible

focus exists

Composition

Next is composition. If your shooting an image of the moon, compose the image so the

moon is centered. If half the moon is missing from the image, or the moon is off to one

side of the image people will be confused. They’ll be too busy questioning why the

moon wasn’t centered to say WOW. Similarly, notice the orientation of the moon. It’s

easy to rotate a telescope around and have your camera positioned at an odd angle.

People will wonder why Tycho is in the top of your lunar image instead of where they

are used to seeing it.

Just like you compose an image when shooting terrestrial scenes, you need to take some

time and learn to “see,” compose and frame your astrophotos.

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Standards

Stick with the standards. Everybody knows that the Orion Nebula is red and the Pleiades are blue. Just because

we CAN manipulate images in Photoshop, doesn’t mean we should. Modifying your image of the Orion Nebula

so the red hues match the red in the neon Budweiser sign might make you and your drinking buddies happy, but

probably won’t get a WOW from other folks.

Likewise, people know that the sky is dark. Our camera chips sometimes produce red, brown and even pink

skies, depending on stray light and pollution levels. Now we need Photoshop to modify our images to achieve

standards or results that people expect.

Some folks like to see a jet black sky, while others prefer a deeper blue cast. I use both to good effect depending

on the circumstances. The general rule of thumb I use is that stellar objects that have no worldly association get

a black background. These otherwordly objects exist ONLY in the blackness of space, so I prefer to give them a

black sky background. Worldly objects, sometimes seen in twilight, like comets, planets, ISS, satellites, etc; can

look good with a blue sky or a black sky depending on the circumstances of the image. Use your judgement in

these cases.

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Look 4 a Hook

Every image needs a hook. There needs to be something in an image to invoke a WOW. There needs to be

something that makes the image unique and/or wondrous. Lunar images are a dime a dozen. You’ll rarely get a

WOW from a straight Full Moon photo. But, take a shot of the full moon eclipsing half of Saturn’s rings, and

the WOW’s are numerous. Or, do a closeup of the moon. You know the kind, one of those where it feels like

you’re hovering above the surface looking down. Or, better yet, the ones where it feels like you’re hanging in

space and the lunar surface is gliding overhead. These images always illicit at least a slight WOW.

And sometimes the hook can be subtle. Recently, while focusing on Comet Holmes I noticed an M object off to

the side of the frame. “Wow”, I thought, “that’s neat.”

Then it occurred to me. Duh, compose the image to include the M object. I got a lot of comments from that

photo. It escalated the image from plain to WOW! Evidently, what I thought was neat pleased other folks, too.

Go figure!

Another example is the photo to

the left. I was awaiting Comet

Holmes to rise out of the treeline

in my yard. I was focusing on

the comet while it was still in

the trees, so I would be ready to

shoot as soon as it climbed

above the treetops.

While focusing I noticed how

the trees framed the comet and

M34. The addition of the

“earthly” trees gave the image

an eerie, other worldy effect,

while grounding the image at the

same time.

“WOW,” I said, “that’s neat!”

CLICK…

Some hooks are impromptu, like when a jet flies in front of the eclipsed sun or moon, or a meteor streaks

through a timed exposure, but some hooks can be planned. Look for transient events like iridium flares. While

iridium flares are nice, after a while, they even become blasé. Watch for an iridium flare that will cross the

surface of the moon, or zip by Saturn. Recently, Comet Tuttle passed by M33 and Comet Holmes passed by the

California Nebula. What stunning images. WOW!

And sometimes, it pays to think outside the box. A friend once handed me an odd photo of the full moon. I

immediately recognized that it was framed upside down, so I flipped it over to view it in the proper

configuration. I noticed a strange ripple in the center of the moon, almost as if he had bent the negative while

printing the image. Then I noticed an upside down caption on the mask, so I flipped the image back around. It

read, “A reflection of the Moon in Grampa’s Pond.” Now I understood why the moon was upside down and

what had caused the ripple in the image. WOW! Nice image. Nice hook.

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I remembered that image and the hook, and many years later shot an image of Comet Hale Bopp, reflected in

silvery waters of Lake Erie.

Location, location, location

One of the most frequently overlooked yet most

important considerations in astrophotography is

location. Probably the most significant

contribution you can make to your images is to

improve your signal to noise ratio. By signal I

mean starlight and by noise I mean light pollution,

stray light, etc. Basically, noise is unwanted

signal. (More on this later.)

We, as astrophotographers recognize this truism,

but we tend to disregard it for the sake of

convenience. We build our observatories in less

than optimal conditions because it’s convenient.

But convenience doesn’t always lend itself to

WOW images. More often than not, it detracts

from them.

Our professional counterparts, of course, realize this necessity and build their observatories on remote

mountaintops, high above weather patterns and city lights.

Not to despair, we can take steps to minimize

the effects of our locations. Many of us build

observatories. While they may not be in the

best location, they are used as much to store our

equipment and provide us a place to take

photos as well as providing shields from

obtrusive stray lighting.

We can use special filters to block unwanted

wavelengths or pass desirable wavelengths. We

can modify the way we use our equipment in

light polluted areas to achieve optimal results.

We can learn to shoot small, narrow sections of the sky

from our city locations, and save those wide panoramic

vistas for our visits to the countryside. We can learn to

shoot lunar and planetary images from our light polluted

downtown locations and save those dim emission

nebulas for our trips to dark skies.

There are numerous ways to combat the negatives of

shooting in a less than optimal environment, and we will

address most of them in upcoming chapters, but

recognizing the fact that we need to increase the signal

to noise ratio is the first step in taking care of the

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problem. After all, we can’t all move to the mountaintops. If we did, there would be light pollution there…

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EQUIPMENT

Tracking mounts

This is my portable mount, a Celestron CG5-GT. I

selected it because of it’s portability, light weight

and strength.

As you can see in the photo I have a 120mm short

tube refractor AND an 80mm short tube refractor

mounted atop this mount. Even with all this weight I

can repeatedly take up to 3 minute exposures.

This type of mount is also capable of being auto

guided by a computer.

Owning a small mount like this is an excellent way

to access dark sky sites for those really important

shots that you just can’t get from suburbia.

Telescopes

The best telescopes for astrophotography are fast telescopes. Today

there are some really nice systems on the market, but most of the fast

telescopes are VERY expensive. You can find astrographs and

Schmidt cameras out there in the many thousands of dollars range.

For a few thousand dollars you can purchase Newtonian reflectors

optimized for astrophotography, and an impressive line of

Apochromatic refractors. For about one thousand dollars you can buy

some small apo’s and some decent camera lenses that serve the same

purpose as a telescope. If your budget is under a thousand dollars you

can still purchase some nice equipment like medium fast reflectors,

semi apo and ED refractors.

Cameras

Digital SLR cameras are abundant and offer many features. I don’t feel qualified to discuss particular cameras

that I haven’t owned, so will only speak in generic terms. My experiences are primarily with Canon digital slr’s.

I’ve owned a digital Rebel and currently own a 30D. I’ve learned that it is extremely hard to focus dslr’s. Before

you purchase a camera, look closely at how they focus and what the viewfinder looks like to you. The ability to

actually see stars in the viewfinder was the selling point that convinced me to trade in my Rebel for the 30D.

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Lenses

Like telescopes you want fast lenses.

Most cameras come with 2 kit lenses and these are usually

the lowest quality lenses made by the manufacturer of your

camera. It’s basically a hook to make you think you’re

getting more than you really are. For really tack sharp

images you will probably wish to purchase fast lenses and

prime lenses. Prime lenses and usually higher quality lenses,

they have a lot less glass inside than a zoom and are

therefore inherently faster, lighter and all around better made

lenses.

Filters

Filters is sort of a no brainer. If you want WOW and a filter is needed, get it. If you have an achromatic

refractor and get purple halos around bright stars, by all means, buy a minus violet filter. If you live downtown

and the Merc-vapor lights turn the sky, in even your shortest images, into brown murky goo, buy a light

pollution filter.

Filters are designed to enhance positive attributes in our images, or to block unwanted

attributes. Either way, the bottom line remains the same. If you want WOW and a

filter will help you, buy it. I know, filters are expensive.

You’re not rich and neither am I. I’ve learned to prioritize my filter needs, set goals,

place them in my budget and then acquire the filters as the priorities & budget

dictates. Remember that the journey of a thousand miles begins with the first step….

Intervalometer

I’ve never been much of an observer. In my mind

observing is not the goal, it’s a task performed in

preparation of capturing an image. I have observed

and I know how to, but I never really enjoyed

observing. That is, until a simple little electronic

device called an intervalometer came along and

changed all that.

The new computer controlled intervalometers

allow us to practically automate our imaging

sessions. I can set the unit up to take, let’s say, 30

three minute exposures with a 5 second gap in

between each image. Then I depress the start

button and find myself with 90 minutes on my

hands. During these 90 minutes I can’t touch the

camera or telescope as it busily captures images

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for me. I can’t turn on the lights. So, what is there to do, standing outside ‘neath the starry skies?

Recently I’ve been setting up my portable scope on the deck outside my observatory. While the main instrument

is working away, I’ve started observing. I mean real observing. Like finding a dim fuzzy, or a blinking

planetary, or a nice globular and just observing them. No hurry to get the camera out. It’s already busy. Just take

my time and look, and look, and look some more. And stare at objects. Stare for a long, long time, like they

used to do back in the olden days. I’ve even started getting out my sketch pad and drawing images like I used to

do back in the sixties and early seventies before I owned a camera.

I’m impressed with how much detail I can see if I actually sit down, get comfortable, stare for a while, relax,

stare some more, and then some more. I think over the years I’ve become acclimatized to the “star party”

method of observing. That’s where someone finds an object, you run to his scope, take a quick peek and then

move on to the next telescope/object.

When I’m doing this, I always want to look just a little bit longer. But, hey, you know the feeling. You’re

standing at the eyepiece and the guy behind you is staring a hole in the back of your neck and fidgeting around,

wondering how much gawdawfully longer you’re going to stand there, semmingly frozen to the eyepiece. So

you look quickly and then reluctantly pull your eye away from that wondrous little jewel in the eyepiece. You

don’t want to get a rep as an eyepiece hog, now, do you?

My intervalometer has changed all that. Now I can stand and look at M33 for a full twenty minutes or so, if I

dare. I can remain at the eyepiece until that rare little window of seeing opens up and offers me a visual view of

dust lanes and motes and knots of dark & light matter just too faint to be seen except under those fleeting

seconds of transparency we never seem to see during those brief glimpses in the eyepiece at a star party.

So, get your self an intervalometer. Let it do the work for you while you enjoy your hobby of choice.

TECHNIQUES

There are two important processes you will need to familiarize your self with before doing any serious

astrophotography. They are polar alignment and guiding. (Not to be confused with tracking!) Of course you are

familiar with these terms, but I can’t stress how very important they are to successful astrophotography. You

can have the very best equipment money can buy and have the darkest skies on the planet, but if you don’t

properly polar align and guide your equipment you seriously limit what you can do with your equipment.

Take some time to learn an alignment technique that works for you. Find a program or technique for guiding

that works best for you. Don’t scrimp, and don’t worry about how long it takes. It’s a pain, but the rewards for

doing it right will be the “WOW” you hear when you show folks your images.

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Polar Alignment

As we discussed earlier, imaging astronomical objects using

exposures greater than a few seconds makes it necessary to

carefully align the telescope mount so that the primary axis of

rotation ( the polar axis ) is oriented so as to be "exactly"

parallel to the Earth's axis of rotation. This can be a long

process for those who are not experienced at it, but the

following guidelines will make the task a bit shorter and a lot

easier.

A POLAR SCOPE can be useful for approximate polar

alignment IF it is well designed and well aligned with the polar

axis of the telescope. However, due to the mechanical

imperfections inherent in most telescope mountings AND the

effects of refraction, a POLAR SCOPE can only provide an

approximate alignment setting for the telescope. The main

function of using the polar scope is to provide for a first order

approximation to polar alignment which then translates directly

into a savings of time when performing the accurate polar

alignment using the DRIFT PROCESS.

The DRIFT PROCESS is the most accurate method available for portable mountings. If you use the drift

method properly you will be able to do long exposures with virtually any lens or telescope and NEVER see the

image ruining effects of FIELD ROTATION ( Photos taken near the horizon are still subject to the effects of

differential refraction which is altogether a separate and different effect!! )

Drift alignment can be accomplished by eye, using your camera or using some very accurate and worthwhile

astronomy programs available on the www.

Guiding

Many people, myself included, get confused on this one. Guiding is not the same as tracking. Tracking is what

your RA motor does when you turn the telescope on. Your tracking motor slowly slews the telescope westward

at the same speed as the earth rotates eastward. Guiding, wether done by hand, or with computer aided devices

is a totally different beast. Guiding involves setting up on a stellar object and guiding the telescope to follow

that object and keep it motionless while shooting your image.

There are many guiding programs and different methods of guiding out there today. Meade, with their DSI and

Orion with their Starshoot are two of the most popular commercial units. High end users with deep pockets will

frequently go with units like SBIG. If you want to take the inexpensive route, there’s PHD guiding and the

GPUSB adaptors.

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LET’S GET STARTED

KISS

Have you ever heard of the KISS (KEEP IT SIMPLE, SILLY) principle? Have you come to the conclusion that

this astrophotography stuff is just too much work, way too expensive and requires too much equipment? If so I

must apologize for leading you down the wrong path. In fact, let me show you how very easy astrophotography

can be. The only equipment you need is a dslr, the kit lenses that came with it, a tripod and your computer. Most

cameras these days come with 2 lenses, a wide to medium and a medium to tele lens. The former is somewhere

in the range of 15 to 70mm and the latter somewhere between 75 to 300mm or so. We’ll use these lens

configurations for this experiment.

Take your camera out under the night sky, place it on the tripod and point it heavenward. Find a bright star and

focus. Slowly rock the lens in and out of focus. Notice the point where the stars seem to “POP” into sharpest

focus. Do this several times until you are sure you’ve reached the finest point.

For this exercise set your ISO level to 800. Further experimentation will illustrate what iso level your particular

sky can support.

Since you don’t have a fancy intervalometer yet, you need to shoot your image without introducing vibrations

into the equation. Go to your cameras’ settings and set it to take a delayed image. (Some cameras use a clock

icon to mark this feature.) A 2 second delay or longer is fine.

Use the exposure chart below to set your cameras’ exposure time without getting star trails.

Lens Aperture Exposure Length

in millimeters in seconds

14 30

28 15

50 08

75 06

100 04

150 03

200 02

OK, that’s all done. Compose your image and start having some fun. Take some pictures. Take a couple dozen,

or even more if you’d like, at 14 millimeters for 30 seconds. Then go to your computer, upload the image files

and stack them using Deepsky Stacker. You can download it for free on the web.

If you took 20 exposures at 30 seconds, you shot the equivalent of a 600 second, or 10 minute (20x30=600

seconds) exposure. If your final image is noisy, reduce the iso level and try again. If there’s no noise, consider

yourself quite lucky and increase your iso level.

As your profiency level grows, move down the chart. Do your own experimentation. Have fun. You’ll know if

and when it’s time to move up to the next level. Additionally, commit this chart to memory. If you ever find

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yourself out with a camera and no other gear, you can use the guidelines above to get that once in a lifetime

shot.

What to expect from your first images

If you’re anything like me, when you view your first astrophoto, you will be a little disappointed. Digital

cameras are, after all, setup to take daylight photos. That’s where the majority of the buyers want the cameras to

be set up to, so that’s how it is. As soon as you release the shutter button, the camera starts running your image

through different algorithms to make it friendly to the mass buyer. This program does things to an image like

adjust color parameters, adjusts brightness and contrast and a whole slew of other things it’s probably best we

don’t concern ourselves with.

But, you say, I’m doing astrophotography! I don’t want the camera doing all

this “stuff” to my image. And that’s where the raw image comes into the

picture. (Excuse the pun.) When you record a raw image, the camera collects

light while the shutter is open, saves the pixel counts and displays the image

(well almost) EXACTLY the way it was shot.

Also, raw images are beginning to become accepted as the “negative” of the

digital world. The Digital Negative Group has done a lot of work developing

standards and methods of foolproofing the raw image information.

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Some New Terminology

Signal

The goal of astrophotography is to capture those faint little photons that have travelled all those hundreds and

thousands and millions of light years through the murky depths of interstellar space. We want to capture them in

our meager little telescopes and focus them onto our camera chips. For the purposes of this article we will call

this starlight SIGNAL.

Noise

However, trying to capture that faint signal streaming in from the far reaches of the universe is no easy task.

When we point our instruments heavenward, we are catching more than just starlight. We’re also capturing

unwanted light, or NOISE. Ice crystals in the upper atmosphere sometimes light up the sky with what we call

skyglow. For our intents, skyglow is noise. Light pollution is also noise. Stray houselights, nearby streetlights,

traffic, etc can cause flare and reflections in our optics. This is another form of noise. Yet another form of noise

comes from the electronics in the cameras we use to capture our signals from space.

Skyfog Light Pollution

Chip Noise (Upper Right) Vignetting, dust on lens, noisy chip

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Signal to Noise Ratio

Basically, then, our ultimate goal is to capture as much signal as possible while limiting or eliminating noise.

The higher the signal to noise ratio, the better our photos.

In this example exposing past 20 seconds will result in

the sky pollution being as bright as the starlight.

Exposing for more than 22 seconds will result in an

image with light pollution, camera read noise and skyfog.

Here, in a heavily light polluted area we see that exposing

for more than 12 seconds will result in the sky pollution

being as bright as the starlight. Exposing for under 12

seconds will yield us nice images.

By keeping our images below the noise thresholds and

through the usage of flat frames, dark frames and bias

frames we can greatly reduce and/or eliminate the effects

of skyfog, light pollution, poor optics and electronic

noise from our cameras.

There are two sure fire methods of increasing signal to noise ratio.

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Integration time

Integration time means exactly what it implies. The more time we expose our chips to starlight the more signal

we acquire. However, by increasing integration time we are also increasing the possibility of acquiring noise.

The good news is that we can take steps to subtract noise from our images.

A common mistake of novices and old time film photographers alike is to expose their chip to the sky until it is

totally fogged over with noise, and then to try to process the signal out of the noise. I’ve done this myself. When

I take an image and look at the back of the camera and see a purely black image, it’s plain to see that the

exposure time is too short. When I shoot longer images, I can see stars, nebulae, etc. Yea, so the sky is pink. I

can fix that with processing. WRONG! What I really need to do is find the exposure time where the sky is still

black, (NO skyfog) and the stars are white (Maximum signal to noise ratio) and then take enough of those

images to stack (integration time) and achieve the desired image.

Properly exposed image Overexposed image Image after processing

Properly exposed image Over exposed image Under exposed image

Actually, an image that has been adequately exposed should in fact NOT have a black background. The SNR

(Signal to Noise Ratio) in your final, stacked and processed images has a direct relationship to how much

skyfog you have.

Total integration times used in stacks of frames need to be in direct proportion to how much skyfog you have at

your site. Six times more skyfog (eg semi-rural suburbia), six times longer integration time. So to reproduce

those beautiful DSO images that took 3-hours integration at a dark site will require 18-hours integration at the

light polluted site; for the same SNR. Basically, extend integration times to the limit of your stamina, at any

site.

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Signal to Noise Ratio

With proper processing we can remove most sources of camera noise, but not all. With the new offerings of

low-noise DSLRs and the use of darks, flats and bias frames we can actually remove most noise very

effectively. But not the Read-Noise of the camera and noise in the skyfog. Longer and ever longer integration

times will reduce the skyfog statistical noise by a factor proportional to the square root of the integration time.

But when we stack frames, each with a short

exposure, we are also stacking the camera's

Read Noise in each frame! We need to

separate this Read Noise from the skyfog

statistics. Fortunately there is a way to do this.

Expose each frame so that the skyfog peak is

well removed from the origin of the histogram

display on the LCD screen on the back of the

camera. Expose so that the skyfog peak is at a

quarter to a third of the way from the left side

of horizontal axis of the back-of-camera-

histogram.

In the illustration to the left, You can see an

image of Mars on the viewfinder LCD screen.

To the right of the image is the Histogram

window. You can toggle the histogram chart

on/off with the info button

My own rule-of-thumb at a dark site is to use the highest ISO (usually 1600) your camera offers, but not one

where you have to invoke some special function, like the H setting on most of the newer cameras. ISO 1600

actually seems to have lower Read Noise than lower ISO’s. Beware, though, high ISO in heavily polluted areas

increases skyfog, thus reducing integration time. (More on this later.)

Expose each frame long enough that you can see a distinct gap

between the TRAILING edge of that skyfog mountain and the

origin of the histogram on the back of the camera. Tracking is

usually the main limitation to how long each exposure can be.

(More on tracking in another chapter.)

In the illustration to the left you can see the histogram skyfog

mountain is about 1/3 of the way in from the left side of the

chart. This is right where we want it to be. This gives us

latitude to stretch the histogram to keep the sky dark and

enhance details in the lighter elements of the image.

You will find that from a dark site, the use of fast optics, eg f4, and ISO 1600 allows you to to shoot individual

exposures no longer than a couple of minutes to do excellent DSO imaging. Of course, in less than pristine

conditions, shorter individual exposures means you’ll have a larger stack of images to achieve the same

integration time. Most DSOs need 2 to 3 hours integration time at a dark site to come out nicely, so shooting for

a whole night on just one DSO is not uncommon. When morning comes and you begin processing, you’ll

probably wish you had shot more frames.

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THE GOOD NEWS:

Modern DSLRs have low enough thermal noise at reasonable ambient temperatures (25deg C and cooler) that

with proper procedures (darks, flats and bias frames used in calibration) they are capable of beautiful astro

images of very dim DSOs, and even narrowband imaging using modified DSLRs. They also have low Read-

Noise compared to many astroCCDs. This enables their use in the desirable, so-called skyfog-limited mode with

quite short exposures using relatively fast optics; a couple of minutes per frame with f4 optics, even at very dark

sites. Much shorter exposures may be used at light polluted sites and still remain in the skyfog-limited regime.

Many people with excellent tracking prefer to shoot longer exposures per frame, perhaps even at a lower ISO.

What really matters is that the skyfog statistics in each frame are not interfered with by your camera's Read-

Noise and that you have ample integration time.

THE BAD NEWS:

Integration time! There is no obvious way around having to achieve proportionately longer integration times at

light polluted sites compared to a dark site. Shorter exposures per frame simply implies having to take even

more frames. Let’s work out some numbers: Assume that your mount can deliver satisfactory tracking for 6-

minute exposures at our desired focal length. At a very dark site we can often achieve a satisfactory (not great,

but satisfactory) image of a faint fuzzy with about an hour integration time. A "great" image will typically

require 3 to 5x as much integration time, even at a dark site. On the other hand, star clusters such as globulars

require much less integration time. Depending on the focal ratio of our OTA we may find that at ISO 1600 there

is a nice gap between our skyfog mountain and the origin, on the back-of-camera histogram using 3-minute

exposures. So we decide to use 20x3minute exposures to deliver the one-hour integration time. Back at our

home base in semi-rural suburbia we find that the skyfog is 6x brighter than at the dark site.

This is not atypical and we need to shoot 6-hours integration time to achieve the same final SNR. We will also

likely find that at ISO 1600 there is already a nice gap between the origin and the skyfog mountain with

exposures as short as one-minute. So we have a choice, shoot 360x1min frames at ISO 1600 or use longer

exposures at a lower ISO? We wish to achieve 6-hours integration time in all cases. Perhaps we decide to settle

on 60x6min frames at ISO 400. Either route is valid and the resultant SNR after stacking should be similar,

20x3min at ISO 1600 at the dark site compared to either 360x1min at ISO 1600 or 60x6min at ISO 400 at the

light polluted home base. There is no getting around having to increase the integration time enormously.

To keep the total number of frames reasonable it is often wise to use a lower ISO at a light polluted site. In all

cases you also wish to have a clear gap between the trailing edge of the skyfog mountain and the origin, so that

Read-Noise does not mess up your skyfog photon statistics. The greater the number of frames in your planned

stack, the clearer the gap has to be and the more dependent you become to the absolute linearity in the response

of your camera. I often try to use exposure length and ISO that keeps the total number of frames in a stack

under a hundred.

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SHOOTING DARK FRAMES

I normally take my dark frames at the end of each session. Some camera operating manuals say you can re-use

dark frames and re-calibrate them before subtracting them from your image. I have found that although it might

work in principle, in practice it does not work, or if it does, it does not work as well as taking one prior to,

during or immediately after each session. The key consideration here seems to involve temperature. Here in

northern Ohio we can have some very wide temperature swings throughout the year and matching darks to

lights taken at different temperatures is critical to achieving good calibration.

On a night where the temperature will only fluctuate several degrees I will wait until the end of the session to

take darks. However on a night where we will have very large temperature fluctuations I will shoot darks

throughout the night. I try to keep my eye on the thermometer and take darks every time the temperature moves

more than a few degrees from the prior reading.

How to take a dark frame

The dark frame is taken by covering the CCD chip, usually by covering the end of the telescope so no light can

reach the chip. The thing to remember is to take the dark frame at the same temperature and for the same time

period and at the same ISO setting as the astronomical images you shoot during the session. I use a very

lightweight strip of black cloth that I swing overtop of the dewshield. Then I shoot just like I’m taking lights.

An intervalometer helps with this needed but mundane task and allows me the freedom to do other chores while

the darks are being taken. I frequently take my darks while closing down for the night. I throw the scarf over the

front of the tube, and start taking darks. While the darks are being shot, I’m closing up my observatory, packing

away my eyepieces, lenses, filters, adapters, etc.

What a dark frame does

At a given temperature and for a given exposure time a CCD chip will accumulate a certain amount of noise

that is not related to an image. This noise is not random. That is if you repeat the same exposure under the same

circumstances (temperature & time) the noise can be repeated. This means that if we take a dark frame and then

take an image at the same temperature and for the same time as the dark frame the unwanted noise that is

accumulated and recorded during the exposure on the dark frame will be on the image as well. So if we subtract

the dark frame from the image we can remove all the unwanted noise.

IMAGE (including unwanted noise) - DARK FRAME (the image of the unwanted noise) = IMAGE (without the

unwanted noise).

Note that although I said the noise was repeatable it is not quite. There is a very small amount of variation

between exposures, most likely caused by very slight temperature variations. This can best be accounted for by

taking several dark frames (I usually let the amount of temperature change dictate how many darks I take.) and

average them together before subtracting them from the image. Note that DSS does this for us.

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SHOOTING BIAS FRAMES

I take the bias frames just before the dark. As with the dark frames, some camera manuals say the bias frames

can be taken and then saved to be recallibrated later to match the temperature of the dark frames and image

frames. I take fresh bias frames during each session as I think I get better results.

How to take bias frames

Cover the telescope just like you would for the dark frame to stop any light from getting on the CCD chip. Then

simply take an exposure for as short a time period as your camera will allow and at the same temperature as the

image exposure will be taken at. I usually take a minimum of six exposures and then average them all together

before applying them to the dark frames and the final image. It is important to take the bias frames at the same

temperature as the dark frames and the final image frames are taken.

What a bias frame does

A CCD chip has thousands of separate collectors called pixels on it that measure the amount of light that falls

on them. When you download an image you are downloading the value of each individual pixel, then you put

them all together on the monitor screen in the right order to make an image. If you zoom in on an image you can

see the individual pixels. So a pixel with a low number is darker than a pixel with a large number. The problem

with this is that the pixels do not all start with a value of zero. This means that if two pixels close together

receive the same amount of light they will have different values. The value of the light received plus the the

value they started with.

Pixel 1. Start-up value (50) + light received value (1000) = total value (1050)



To remove these unwanted values we simply subtract the bias frames from the dark frames and the image

frames. This means that every pixel starts with the same value.

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SHOOTING FLAT FIELDS

Dark frames & bias frames are both designed for correcting unwanted noise in the CCD chip. Flat fields are

designed to correct problems in the optics i.e. dust, dirt, internal reflections & even vignetting. Because of this

every time you change the optical system you should take another flat field. This means even if you just refocus

or change a filter.

How to take a flat field

The flat field must be taken at the same temperature as the dark, bias & image frames. The time should be as

short as possible while at the same time getting the correct saturation level. For the correct saturation level the

flat field’s average value should be approximately one third of the maximum saturation level of your camera.

To take a flat field you need an evenly illuminated surface. There are several ways of doing this. The simplest

method is to get a piece of white card & position it parallel to the telescope. Then shine a light onto the card.

Take several images with different exposure times to get the right average saturation level. The light can be a

porch light or any low powered light. The card can be exchanged for a blanket over the washing line or the

inside of your observing dome or a wall of a roll-on roll-off observatory. Paint a section (Larger that the

diameter of your telescope) of the wall white, and surround it with small, low wattage light bulbs. Place a baffle

in front of these bulbs to stop the light from shining directly into the telescope. The light from the bulbs is

reflected off the baffles and the observatory walls, providing the white illuminated surface needed for flats.

What a flat field does

When you take an exposure any dust or dirt in the optical system will show to some extent on the image. A

speck of dust on the mirror will be out of focus and appear on the image as a doughnut shaped smudge.

Generally the nearer the dirt is to the camera the darker the smudge will be. Vignetting (a gradual darkening

towards the outside of the image) may also show up if the optics have this problem. (Many zoom and kit lenses

and fast optical systems exhibit vignetting.)

Because the image you are taking is even (that means the image reflects the same amount of light across the

whole of the image.) any imperfections in the optical system will be recorded on the flat field image. These

imperfections will also be on the final astronomical image. To remove these imperfections we divide the final

astronomical image by the flat field. This is different to dark & bias frame processing because we are trying to

flatten the background not remove hot & cold pixels.

Take 2 pixels that receive the same value of light.

Pixel 1.

Image (1000) + extra value caused by light reflecting inside the telescope (200) = 1200

1200 \ flat field value (400) = 300

Pixel 2.

Image (1000) - light reduction caused by dirt on mirror (400) = 600

600 \ flat field value (200) = 300

The internal workings of the camera can also create localized hot spots, which will alter the sensitivity of the

pixels in that area.

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Combining/stacking image files

Once you’ve finished the session and have all the lights, darks, flats and bias

frames, you need a program to do all the dividing and adding and subtracting

and multiplying that needs to be done before during and while stacking your

images. Doing all this work yourself would take countless hours. Using the

new software available today only takes minutes of your time. Two programs

I recommend are Deep Sky Stacker (Free!) and Images Plus. Both are quick,

easy, clean and efficient. There are many other stacking programs and you

would be best suited to try some for yourself and see which you prefer.

Processing image files

Once you’ve finished stacking your image, you are almost ready to process it.

Calibrate Your Monitor

Before processing, however, you need to perform one step. You will only need to do this once. Go to your

monitor settings and calibrate your monitor. You’ll see a series of blocks, aligned in steps, black on the left,

white on the right, with shades of gray in between. Make sure you can see every shade of block. If you can’t

follow the on screen instructions until you can.

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Find some software

There’s a lot of really good software out there today. Probably the premium suite is Adobe’s Photoshop CS2.

It is a HUGE program with a steep learning curve and is quite expensive. It’s also worth every penny.

In the middle ground of expense there are quite a few programs. Images Plus, Corel Paint Shop Pro X and Iris,

to name a few.

To learn more about software or any of the topics discussed in

this paper, check out Yahoo Groups. There are groups and

forums for each topic we discussed here and all are manned

with intelligent and caring people who are ready to help

novices at a moment’s notice.

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Summary

This paper is not intended in any way to be an inclusive treatise on digital cameras and/or astrophotography.

Each item I’ve touched on in this article could probably have a book written on it. (And probably has!) This

article is intended more as a beginner’s guide to provide a starting point for your personal exploration of digital

astrophotography.

Information for this article came from my personal experiences and numerous sources, some of which I’ll list

below.

Sources & Suggested Reading

Your Camera’s Users Manual

a digital astrophotography primer - o'neal web site · the basics of astrophotography and...

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