afocal astrophotography

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  • Afocal Deep Sky Astrophotography

    I use a 8.5 inch F6 Newtonian on and HEQ5 mount plus Olympus 2020Z camera. I wrote this stuffbecause I got fed up with reading on various forums how it was a waste of time imaging anything

    except the moon and planets with a cheap digital camera. I guess if you have much a smaller

    telescope aperture you might struggle to do much with the limited exposure times available to non-

    DLSR digital camera. But I have never tried, so I don't really know!

    Update for 2008 onwards: I now have a 4 inch F6.5 Celestron Nexstar on a GOTO alt-az mount - it

    works just as well for afocal imaging as the 8.5 inch - better on large objects in fact.

    Update for 2010 onwards: well, I finally went down the DSLR route and started prime focus imaging.

    I did seriously consider a new P&S, but it is now very difficult (almost impossible in fact) to find one

    with a threaded lens.

    Anyway, for what its worth, here is how I used to do it ...

    Pros and Cons of Point&Shoot digital cameras cf DSLRs

    Pros

    Cheap Light

    Easy to vary image scale

    Focus position similar to visual

    Fixed lens, so no problems with

    dirt falling on the CCD

    Cons

    Cameras allegedly noisier than DLSRs (not

    sure this is true!)*

    Light loss & optical distortions due to

    camera lens/eyepiece

    Limited exposure times (can be overcome

    by stacking)

    Small effective camera apertures don't full

    cover eyepiece exit pupil**

    *You will often see it said that DSLRs are better because their CCDs have larger pixels which can hold many more photonsand hence reduce the random noise. For everyday photography this is true - but the reason this works is that DSLRs havemuch bigger lens apertures and so collect much more light for a given exposure and F-number. In astronomy, the amount of

    light is fixed by the telescope not the camera lens, so this advantage vanishes (except see below!). **Most compact digital cameras have very small apertures on their wide field setting, e.g. my 2020Z has a 6mm focal lengthat f2 => 3mm aperture. A low power eyepiece for optical viewing may have, say, a 6mm exit pupil, so I am effectively limitingmy telescope to one of half the aperture. Zooming in, either with the camera or the eyepiece, helps (gives a larger cameraaperture or smaller exit pupil) but, of course, reduces the field of view on the sky.

    How-To Image DSOs

    The basic idea is to take lots of short exposures and digitally add them together in a computer. Roughly

    speaking, this is just as good as taking one long exposure. You will still need a mount which can track though

    (for the length of exposures we are talking here, either a cheap motor driven alt-az or an equatorial mount will

    be fine). The camera needs fastening securely and square on to the eyepiece. Ideally, the lens aperture should

    be about the same size as the exit pupil of the eyepiece and be positioned at a distance equal to the eye relief

    of the eyepiece. There are various devices around to assist with this, but there is no real substitute for a

    threaded lens screwed on to a threaded eyepiece! Unfortunately there are not many compact digital cameras

  • with threaded lenses. A camera with manual control is almost essential and you will need some way of

    triggering the shutter remotely. Focus both telescope and camera on infinity. Take various test exposures to

    refine the focus (this means examining the shots on computer - I have never found looking through the camera

    LCD successful). Choose a target - exposure for as long as you can (or as long as your mount will allow).

    Repeat. At regular intervals take dark exposures in order to remove hot pixels from the images.

    Camera

    There are several desirable features of the Olympus 2020Z which lend themselves to astrophotography.

    Whether one can still find them in more modern cameras is debatable:

    1. Infra-red remote control of the shutter

    2. Threaded lens

    3. Full manual control4. 16sec maximum exposure time

    5. F2 lens6. 4 micron pixels

    Mounting

    With a threaded lens the 2020Z can be attached securely to an eyepiece by means of a T2 adapter whichfastens to the top of the eyepiece with three metal screws and an Allen key. This assumes the eyepiece has a

    suitable groove in which to site the adapter. For eyepieces without I have used a Scopetronics adapter whichsimply holds the eyepiece with nylon screws. However this proved very difficult to align correctly, especially

    in the cold and dark! If you do not have a threaded camera lens (and most modern compacts - but not all -do not) then you will need to look at one of the proprietary devices available (there are some specifically for

    astronomy, but also try looking up 'digiscoping' on the WWW - people who photograph birds have beendoing this sort of thing for ages). Whatever you choose, make sure to make a light tight seal between the

    camera and the eyepiece.

    Mounting kit disassembled - eyepiece;T2 adaptor; Olympus CLA-1 adapterplus 43mm-T2 step down (up?) ring.

    Eyepiece with T2 adaptor attached.Fully assembled, ready to screw onto

    the camera.

    Vignetting, or not?

    You will read on the web that afocal imaging is usually a waste of time due to vignetting. What is meant hereis the fact that the camera images the end of the eyepiece, and so you get a circular field of view (There is

    also optical vignetting, where the intensity of the sky background falls off towards the edge of an image. Thisis a separate issue and affects all telescopes/cameras to some extent). Personally I do not consider this a

    problem. What really matters is pixel scale and total field of view. After all, telescopes give circular images,and cameras take rectangular pictures, so one can never match the two perfectly. My preference is to see the

    whole field of view of the telescope, which means that I consider a perfectly matched combination of

  • eyepiece and camera to be one where the narrow width of chip is just filled by the image from the eyepiece.

    The full 2020Z image of the twilight sky (M44 actually),using a 32mm eyepiece. The view here is almost identical towhat you would see by eye through the eyepiece. I considerthis a good match between camera and telescope.

    The combination I use of F6 Newtonian, 32mm eyepiece and 2020Z camera, gives about 3'' per pixel (the

    pixels are 4 microns), and field about 1 deg across, which I reckon is just about right for most of the brightergalaxies, clusters and nebulae. There may be an issue with the camera aperture being too small for the exit

    pupil - I have still to investigate this. It is possible to zoom in with the camera, or change to a higher poweredeyepiece, to get a smaller number of arcseconds per pixel, although this puts more demands on the tracking.

    Exposure times

    Another comment you often see relates to the short maximum exposure times offered by non-SLR digitalcameras. It is rare even now to see much more than 16sec offered. However, again, I do not see this as a

    problem. In my light-polluted skies, exposures much longer than this will simply be bright orange! It alsomeans my mount doesn't have to track particularly accurately.

    Curiosity: Two times eight does not equal sixteen!

    Left: the sum of two 8sec exposures on the Orion nebula. Right: One 16sec exposure take at the same time with the sameISO. The colours (and saturation) are clearly not the same!

    I prefer the 8sec shots, but with the 2020Z there is a constant readout time penalty with each exposure of a

    few seconds, so observing with 8sec or shorter times is very inefficient.

    ISO

    When I started I just set the camera to maximum ISO (400 for the 2020Z) and got on with it. However,

    digital cameras are not like film cameras, and ISO is rather a fake concept. In 16secs you get 16secs worthof photons on the CCD, whether ISO100/200 or 400. Changing ISO just multiplies up the voltage before it

    goes through the A/D converter, so although you get brighter images (i.e. larger numbers) at ISO400 it is not

  • clear that the signal-to-noise ratio is any better. In fact, if the read noise increases it might be worse. What hasbecome clear to me is that the colour balance is much more `natural' at ISO200 than 400 (which tends to

    give too much red/orange - and even if you adjust the colour balance you still end up with a predominance ofred noise in the image.), so at the moment I take all my frames at ISO200 [I changed my mind on this one

    and moved back to ISO400, but having read more about unity gain (see below) I think more tests are

    needed. Another issue I hadn't considered before is that of colour resolution at low signal - with onlyone or two counts from an object, unless it is very strong coloured, it is going to come out grey, so in

    this situation higher ISO is potentially better.]. This, of course, could just be a feature of the 2020Z.

    Unity gain: in fact, theoretically, the best ISO to use is that where the gain is set so that one electron == onecount. A higher ISO clips the bright portions of the images, whereas a lower ISO drops the faintest areas (i.e.

    doesn't fully sample the noise). Assuming 12 bits in the A/D converter, then for compact cameras this

    seems to be ~ISO100 whereas for DSLRs it is > ISO1000* (see e.g. the ClarkVision site). However, if likeme you use jpegs, which are limited to 8 bits, it is not at all clear where unit gain occurs (it is not a figure that

    manufacturers seem to want to give you!) - it might be at 16x these ISOs! In principle, this could make youthink that DSLRs have much low

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