When using cameras on telescopes, there are times when the camera must be rotated to place a guide star on the guiding chip or to provide better framing of an object. If you only intend to only gather one night's worth of sub frames, it is not too important to keep track of the camera's relative rotation. However, if you intend to take sub frames over a number of nights or perhaps over a number of years, the rotation of the camera becomes very important as you typically will want to use the same framing or guide star to keep the sub frames consistent.
On the Sky90, I used a very simple system. I placed a reference mark on the focus tube coupler and used a piece of tape corresponding with the reference mark on the CAA with the object's name/number for future use. Any time I wanted to image the object I simply rotated the CAA until the object's tape label lined up with the reference mark. However, after a number of years, I ended up with a CAA totally covered with smudged pieces of tape that I could no longer read. I needed a better system...
For my first attempt at a camera indexing system for the FSQ-106ED, I tried another simple idea. I attached a paper tape marked with degrees of a circle around the metal collar immediately in front of the "captain's wheel". Next I fashioned a piece of heavy wire as a pointer and attached it to the Robo-Focus™ bracket I made. This system proved to be very successful. I could rotate the camera and record the relative degree of rotation from the pointer into a log I kept for each object I imaged. The next time I returned to image the same object, I could look up the rotation value in the log, and then rotate the camera into the same relative rotation I had used previously.
After changing components in the image train of the FSQ-106ED to incorporate a CAA, I found myself in need of a new system. This time I made a graduated collar from aluminum to fit over the Astrodon 92-98mm adapter I used in the image train immediately in front of the CAA. The collar has 72 index marks corresponding to 5 degreees of a circle for each mark. Red marks indicate 90 degrees while yellow marks indicate 45 degrees.
The picture shows the new image train and the installed collar. I plan to use a self adhesive label to mark a reference line on the CAA. Then when I rotate the CAA/camera assembly, I can easily record the degrees of rotation from the "0" (screw hole) reference on the collar.