Hi Chris,
You could do the following during daytime....
1. Place the telescope on tripod or mount. I actually prefer a table, but aiming is more difficult.
2. Remove any eyepiece if fitted.
3. Point the telescope at a distant terrestrial object, like a tree or building which nicely fills the frame.
4. Take a flat sheet of white paper /cardboard and whilst holding it behind the telescope near where an eyepiece would be, move the paper backwards and forwards until the light coming through the telescope forms a clear image on the paper of the distant object you chose. The image will likely be upside down.
I sometimes like to do this on a tabletop as you can mount the paper to a small box or block and slide it back and forth for better precision. I've also used velum / tracing paper held in a frame and slid back/forth on the table top. The telescope can be restrained form moving using some towels or boxes on either side of the tube.
When the image is in crisp focus (you can use a magnifier to help judge) simply measure the distance from the paper (The Focal plane) to whatever reference surface on the telescope you need. Hey presto done! You simply then need to ensure that using spacers, focusers and taking account of the sensor being recessed in to the camera, that the sensor ends up in the same position as the focal plane position where you obtained your crisp terrestrial image.
NOTE .... Make sure so chose a distant object which can be considered as optically at infinity. For your ~800mm focal length, I would say an object atleast 200m away would be good
Best
JA
Last edited by JA; 06-12-2024 at 02:11 PM.
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