Telescope Reviews: FAQ: prime focus film astrophotography primer

OK, you've just dug out your old film SLR, and are wondering, "what do I need to try taking photographs through my telescope with this thing?"

You need:
1) your camera
2) a way to attach it to your 'scope
3) a way to focus it
4) film
5) a way to guide your telescope (for longer exposures)
6) a way to polar align your telescope

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1) The camera

If you've got an SLR, you can take astrophotos with it. Period.

If you've got a point-and-shoot, or a rangefinder, it will be much more difficult to get going. Go get a used SLR!

It MUST have a "B" setting (I think all do). That's it. Mirror lock-up is nice, mechanical "B" (otherwise you'll be going through batteries like M&Ms) is nice, replaceable focus screen is nice, all that stuff is nice, but all you really need is a camera to hold the film and a long term shutter. So, if you have ANY SLR, you're all set. If you don't, the following is a great discussion of which ones to look for from Jerry Lodriguss' website:

http://www.astropix.com/HTML/I_ASTROP/I08/I0805/I0805.HTM

2) Attaching it to your scope

You need 2 things, a "T-ring" specific to your camera, and a nosepiece that screws into the "T-ring" and allows you to attach the camera to your eyepiece drawtube.

Get T-rings here:

http://www.telescope.com/shopping/product/detailmain.jsp?itemID=56&itemType=PRODUCT&iMainCat=6&iSubCat=29&iProductID=56

Drawtubes come in 1.25" and 2" flavors - if you have a 2", get a 2" nosepiece to reduce vignetting, otherwise, well, you have to use a 1.25" nosepiece. Alpine Astro has very nice nosepieces from Baader in Germany (they call them "T2" system components, but they're just regular "T" threads):

http://www.alpineastro.com/eyepiece_adapters/eyepiece_adapters.htm#Nosepieces

If you have a refractor or SCT/MCT, then you'll be probably able to bring your scope to focus right away. If you have a Newt, you might not have enough in-focus to get to focus. You have four choices, easy to hard:
- Use a 2x barlow to get the focal plane "out" closer to the end of the drawtube - the downside is that you've increased magnification through your telescope
- Replace the focuser with a "low profile" focuser intended to allow you to get your camera closer to the focal point
- Move the main mirror forward (usually an inch will do it) in the OTA get the focal point "out"
- Buy a new scope!!!

3) Focusing your camera

You might think that you can just look through the screen and focus your camera, but there are a couple of things working against you.
- The typical "microprism" focusing area goes black at the slow focal ratios typical of telescopes (after all, your telescope is acting like a REALLY long telephoto lens)
- The rest of the focusing screen will probably be very dim
- Your eye "accomodates" meaning it's pretty flexible at making something that's slightly out of focus look IN focus

For initial shots of the moon, etc., you can just eyeball it and you'll probably get close, but if you get serious and want REPEATABLE focus, you need an auxilliary method that bypasses your screen and eliminates the effects of eye accomodation.

I use a Ronchi focuser for my Nikon cameras:

http://www.stellar-international.com/

Again, Jerry has a great write-up about focusing:

http://www.astropix.com/HTML/I_ASTROP/I08/I0812/I0812.HTM

4) Film

Kodak Elite Chrome 200 slide film or Fuji Provia 400F slide film. Your lab can't screw up astro slides (other than cutting in the wrong place -- make sure you take a couple of regular daytime shots at the beginning and end of the roll) and since the image is positive, you can visually evaluate your slides without scanners, etc.

You might think faster (ISO800 and up) is better but a lot of faster films have poor reciprocity failure characteristics, so that the Kodak and Fuji end up being effectively as fast because they don’t lose sensitivity over long exposures as much as other films.

5) A way to guide your scope

With the previous 4 steps you’re all set to take pictures of the moon. You don’t have to polar align, or even be equatorially mounted, since the exposures will be short (fractions of a second). You WILL have to deal with “mirror slap” vibrating your telescope. If you have mirror lock-up, use it. Use a cable shutter release. If you still have vibration problems, try the “hat trick”:

http://www.company7.com/library/questar/quephoto.html

For longer (10s of seconds and more) exposures, you need a way to guide your scope.

Since you’re just starting out, an auxilliary guide scope with reticle eyepiece is the easiest way to go. Any scope that can be rigidly mounted next to your imaging scope, of at least half the focal length of your main scope, can be used. Orion sells a relatively inexpensive guide scope setup:

http://www.telescope.com/shopping/product/detailmain.jsp?itemID=5222&itemType=PRODUCT&iMainCat=6&iSubCat=29&iProductID=5222

Astro-Physics also makes a more expensive guidescope:

http://www.astro-physics.com/products/accessories/photo_acc/photo_acc.htm#12

I like the following reticle eyepiece:

http://www.company7.com/rigel/products/pulsguide.html

There's an excellent discussion of HOW to manually guide (and the tolerances required) in Robert Reeves excellent book "Wide-Field Astrophotography"

http://www.willbell.com/ccd/widefieldast.htm

6) Polar drift alignment

If you want to take long exposures, you need to be accurately polar aligned to avoid field rotation. Drift alignment is the most accurate way to align to the pole. Here’s my favorite webpage on how to do it:

http://www.aa6g.org/Astronomy/Articles/drift_align.html

If you have an STV (then you aren't a beginner!), you can use it to polar align:

http://home.att.net/~lee.suk/root/diy/STVPolarAligning/STVPolarAlignment.htm

Good luck!
Suk

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http://www.siliconvalleyskies.com