Gary, those are terrific for beginner work. The first shot shows a focus problem, possible tracking error, but obvious shift of the focal plane during the exposure. The second shot is better, but still displays the same traits. Let's take these one at a time. Focus problem: Try focusing on a first-magnitude star, Venus, or a mountain-top tower light at least 20 miles away. You also need to know that Schmidt-Cassegrain telescopes have an inherent design flaw in that they focus by moving the primary mirror back-and-forth. Because the primary mirror isn't held firmly in place, it can and does shift when the telescope is moved from one part of the sky to another. Many people using these telescopes for astro-imaging lock-down the primary mirror and focus using an aftermarket focuser attached between the camera and telescope. There are also specialized tools that can help you achieve a precise focus such as a knife-edge focuser matched to your camera. You can also bracket the focus of your shots if in doubt- digital imaging means never running out of film! Tracking error: You do need to be polar-aligned, and not being able to see the NCP complicates the task. But there is a way to achieve a more precise alignment using the "star-drift" method. It does take time, though, and can be confusing for a beginner the first few times, unless someone walks you through it. Focal plane shift: This can be related to the "mirror flop", or shift, mentioned above, but it can also be due to the ground the telescope is set up on. Vibration can be transmitted through the ground and up the tripod, ruining an exposure. Even a concrete pad or porch transmits vibration. A wooden porch would be even worse. Wind can jiggle the telescope, or even movement by the operator. These are just some general areas to look into and there may be others. I'm not right now in a position to direct you to Web resources or books discussing them in detail, but I'm sure some of the active astro-imagers on the list can help you in a more specific manner. Kurt has been doing a lot of work on precisely polar-aligning a portable mount, and Patrick is a treasure-trove of practical imaging advice. Joe has been doing a lot of imaging since retirement, as well, and Tyler sets a high benchmark for everyone who is into imaging. Lots of people who can offer you specific advice on this list. Again, very good work, don't give up. Persistence is key to success! On Sun, Dec 21, 2008 at 8:29 PM, Gary Bulk <garybulk@the-logans.net> wrote:
It's cloudy tonight, so I guess I'll send an email instead of looking at the stars.
The Salt Lake southern skies cleared a little last night, so I took my friend's Celestron C8+ (8" CST with a motor drive - the one I'm dusting for him) and my Nikon D80 with a prime focus adapter out to my front porch and took some shots of the Orion Nebula. After about a 100 shots, I'm getting closer to a clear picture, but the problems are getting harder to figure out..
My Orion Nebula emphasizing the fuzzy stars: http://glogan.smugmug.com/gallery/6872266_UTskH/1/#440661533_PeEFc-A-LB
The above link is a crop that shows fuzzy stars in the pictures. That was a 30sec. exposure at iso640. The stars definitely have movement, which I'm guessing is my poor polar alignment. My porch faces South and I can't see the North star from it, so I kept moving the tripod until the star trails got short (at the time I thought they went away). I am also wondering how long of an exposure I should expect to be able to get as far as accurate tracking using this Celestron C8+?
But the stars should be smaller also, more pin-pointish. I can't seem to be certain of my focus through my camera viewfinder, as the stars are too dim. Are there techniques or equipment that others use to help focus when mounting a camera? Also, (whine) the Celestron seems to have a pretty coarse focus knob, even with the regular eyepieces.
My best Orion: http://glogan.smugmug.com/gallery/6872266_UTskH/1/#440655195_68tXH-A-LB