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i am mounting it to a custom made mount for my telescope. i have no idea what i want to shoot...

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The object you wish to image will determine the lens choice. If you don't know what you want to try, I'd pick the 70-200 since it's shorter focal length will be less challenging.

Does the mount track or not? And is it equatorial or not? If it is an equatorial mount, set your lens to f/2.8, use the 5D, set your ISO to 800, and try thirty second exposures. I'd recommend trying out Rho Ophiuchus as a subject if you have a nice view of the Southern horizon--just center the field of view on Antares. It's a neat part of the sky and well suited to the focal lengths you are talking about.

If the 30s exposures give you stars that are nice and round you can try increasing the exposure length. Unless you have a really good quality mount that you have drift aligned, you will probably max out around 1 minute or so before stars start to become football shaped (due to tracking inaccuracies).

If your mount tracks but is "alt-az" in nature rather than equatorial, you can try the same thing but keep the exposures down around 20s or so. When you stack your different exposures, you will need to correct for field rotation with an alt-az mount.

If your mount doesn't track at all on its own, I would recommend you try out some "star trail" astrophotography. Point your mount somewhere near the north celestial pole and try a ten minute exposure. Generally, it works best if you include a little terrestrial scenery in your exposure for a frame of reference. This is a way to create some really nifty celestial images without the hassles of an equatorial mount and without having to stack separate subexposures. It's a great way to get started.

Best of luck!

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- Jared Willson

• Fluorostar FLT-110 w/ TEC optics
• Vixen VC200L
• Astro-Physics Mach1 GTO
• Stellarvue SV80S
• Takahashi Teegul SP Mount
• STL-11000

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