Getting galaxies at a decent scale is no small feat. Longer focal lengths require a rock of a mount- you won't get sharp images with a SCT on a wedged fork mount no matter how good the seeing. Remember Tyler's advice concerning mounts. The OAG is a great tool, I think you nailed it, Don. I have yet to try my old Lumicon OAG with my DSLR but it should work. I may have to come up with something else for my Starshoot camera. The current trend toward "mini" guide scopes minimizes the flexure concerns but at these plate scales I still worry about guiding accuracy. On Tue, Jan 28, 2014 at 2:16 PM, Don J. Colton <djcolton@piol.com> wrote:
I have been experimenting with imaging at longer focal lengths using the SBIG STT-8300 with FW8G-STT, filter wheel with built-in-guider. I have been using the Mewlon 250 with the Astro-Physics .67 focal reducer. This gives a focal length of 2010 mm or 79.13 inches. At this focal length a guide scope is difficult to use without getting flexure. Using off-axis guiding is the most effective way. The new SBIG filter wheel intercepts some of the light with a small off-axis pick off mirror and passes it through a focal reducer before going to the separate built in guide camera bypassing the filters so you always guide with unfiltered light.
This setup allows long guided exposures at relatively high magnification. On Saturday I took 9 - 15 minute luminances and 8 each 5 minute RGB images and combined them for the image of M 51 (see below link). Not one image had any star trailing, which I find quite amazing. The seeing was not very good which I think affected fine detail.
http://www.slas.us/gallery2/main.php?g2_itemId=5755&g2_imageViewsIndex=1