The Shmidt Cassegrain Telescope means that the optical tube has a combination of lenses and mirrors. It incorporates elements of both refractors and reflectors. One advantage this produces is the ability to have a very short optical tube. My 8"(primary mirror diameter) SCT has a focal length of 2,000 mm (78 inches) but the optical tube is only 16 inches long. The disadvantages of this design show up most if used for Astrophotography. The main problem encountered by Astrophotographers using an SCT is mirror movement during an exposure. As the telescope tracks across the sky, the center of gravity of the mirror changes often causing the mirror to shift. The stars move across the night sky at about 15deg/hour. So during one of my 2 hour exposures the telescope moves quite a bit. This can cause very high guiding errors and/or blurred photographs. It is recommended that astrophotographers using SCT's should NOT use a separate guide scope even though this guiding option seems very appealing due to it's ease of setup with a CCD autoguider.
The recommended procedure then is to use an Off Axis Guider instead. This cancels out the effects of mirror shift. OAG setup is much more difficult and frustrating than using a separate guide scope. Star images viewed through the Off Axis tube often appear fuzzy and resemble something more like sea gulls than stars due to the small pick off prism. This can cause problems with a CCD autoguider. I have found that this problem can be overcome by careful focusing of the guide star on the CCD chip. A little extra time spent here is well worth the effort even in 24 degree weather when your fingers are numb! I have now successfully shot 1 1/2 and 2 hour exposures with this setup without noticing the effects of mirror shift. Good Luck!
