Gert Which holder part are you using? Is that a regular SCT small finder holder?
Yes, Gert. This is on a C6A.
The left (fixed) SCT ring has slightly larger diameter than 1.25". I lined the camera body with a strip of 1mm thick cork sheet -- not visible under the black ring. Holds it quite snugly.
The three plastic thumb screws on the right side of the SCT holder clamps on to the Fujinon's body.
I start by using a large sensor with the SCT. Plate solve and center a bright star. I then watch the small camera and center the same star using the thumb screws. This then allows me to center to a planet by plate solving with this auxiliary camera. Basically, using the small camera as a "finder" scope.
Comically, the 75mm focal length and an ASI290MM will actually autoguide an RST-135 mount to about 1.5 arc second RMS. But that is not what it is used for. It is just a finder when I use the ASIAIR, since the ASIAIR cannot plate solve when a small-FOV planetary camera is used with the C6A.
Chen
Edit: I just checked my old Amazon orders, and found the cork sheet that I used:
https://www.amazon.com/gp/product/B07RZ5W3NV
I have used it in multiple places, like shimming Bahtinov masks to lining OTA clamshells.