Focusing a Ritchey Chrétien Telescope
A very important note here, when
using the OGS RC or any RC for that matter, it is important to know the design
focus of the telescope. Seeing that my 10" is designed at 9.75" of back focus,
the best optical performance will be achieved at or very near this distance.
What this means is that if I were to figure out the distance from my CCD chip to
the rear cell of the optical tube taking into account everything that exists
from the camera's CCD surface to the rear cell, it should measure 9.75" or very
close for best imaging performance. I find it extremely helpful to keep a
notebook or in my case, a Marble Composition Book, in the observatory to keep
important notes that I refer to very often. The information below and a fair bit
more are all at my finger tips. The table below list these values using
available information from SBIG.
|ST8XE CCD Camera Distance from
chip surface to exit
|CFW8 Direct Connection
|AO7 Direct Connection
|AP 2" Nosepiece
|SBIG CFW8 Plate
|AP 2.7" 2" adapter
|AP 2.7" Focuser fully in
|AP Spacer Ring
|AP Spacer Ring
|Rear Cell Adapter
Using the information you can determine the proper
distance for your imaging train to the rear cell. As an example, I often use the
ST8/CFW8/CFW8 Plate/AO7/AP 2" Nosepiece/AP 2" Adapter/AP 2.7" focuser
arrangement. This tells me that the total distance with the focuser in the full
in position gives me a total distance of 9.03" . Now if I move the focuser out
.75" and then focus using only the secondary mirror, I'll be at design focus or
If I add the rotator into the above imaging train I'll
have a total of 10.7" with the focuser in the fully in position. Thus exceeding
my ideal focus distance of 9.75". To resolve this I'll eliminate the AP focuser
bringing me down to 6.7" and install the rear cell adapter and both the 1.25"
and .75" AP spacers. Total distance now is extremely close at 9.7", .05" off. It
may be necessary at times to use the below listed distances due to spacing
Acceptable focus ranges are as follows
for this telescope:
|9 3/16" - 9 5/8"
|8 7/8" - 9 1/8"
|9 1/6" - 10 3/8"