I hope someone can help me with my question here: I am attempting to capture the moon using variable projection photography.  My camera is a Canon Digital Rebel XTi and my scope is a Celestron 8" SCT.  My scope is polar aligned and tracking with PEC enabled.  I am using a 26mm Plossl eyepiece inside the variable tele-extender. Now here's the problem - looking through the camera eyepiece I can focus very clearly.  But when the picture is taken the picture is slightly blurred on both the camera dispaly screen and equally so on computer screen when downloaded.  I have messed with exposure times/ISO/mirror-lockup/etc., etc.  There does not appear to be any telescope shake when the shutter fires.  But no matter what the final image is slightly blurred.  I have even tried adjusting camera focus either side of crisp but it only worsens.  Any ideas from anyone?  Also, is anyone aware of different focusing screens available to put in this camera more suited to astrophotography purposes?  Any help is greatly appreciated!

I don't think that Rebel model has "live" image magnification on the back panel, but if it does then use that to help check focus.

Your problem is most likely image shake. Remember you're shooting a greatly magnified image (that SCT is something near 2000mm focal length, right? So any movement at all (slight breeze, a footstep near the tripod, shutter button motion, etc.) will cause it to blur.

In fact, if your shutter speed is not faster than about 1/30th second you will be prey to those motions.

If you're not using a remote cable release or the wireless remote to fire the shutter, use the self-timer mode. That will help.

One thing to remember is that the viewfinder is low-resolution compared to the downloaded frame, so it will always appear to be sharper than it really is.

A tip I can offer for finer focusing is to use a large wooden spring-clamp clothespin around the focusing knob on the SCT. Focus normally and then clamp the clothespin around the shaft of the knob. You can then focus in finer increments using the clothespin as an indicator of how much the shaft has turned.

I don't know about focusing screen replacements for that camera, but I do know there is a downloadable program you can find using a Google search. It works with any Canon camera which has the Digix chip set and provides a highy magnified view of the center of the field of view on the LCD after you take a photo. It's very handy. It's freeware, but I don't know who wrote it.

You can also get the program DSLR Focus, which works with that camera. It's not freeware, but is excellent if you're using a computer in the field.