Hello, According to the back focus calculator spreadsheet, the STC-7 uses 25.6mm of optical path (including filters). Something about the camera has been bugging me since I got it though. The M42 threaded socket on the camera face is quite shallow at only 3.75mm. None of my M42 adapters and in fact the M42-> 2" nosepiece that the camera came with thread flush with the face of the camera (See attached image of the included nosepiece & digital calliper measuring the depth of the socket). What I'm wondering is this: does that figure of 25.6mm back focus account for the gap that results between a M42 adapter and the camera face? or do I need to measure the gap between the camera and the adapter flange and add it to the stated back focus in the spreadsheet to get a true measure of back focus of camera+whatever adapter I attach to it? I have a vast collection of m42 threaded adapters, but none of them will sit flush in the camera socket.
Yes, you will have to add that gap into your backfocus calculations. Probably if you're extremely tight on backfocus you could shave the nosepiece down a little, if you have access to a milling machine.
Thanks, that's good to know. I have plenty of back focus room, but my scope's design (Celestron EdgeHD) recommends having the sensor within 1-3mm of their specified distance.
