lmclouth What can be causing this and what can be done to correct it?
Lamar,
The aberration that you showed does not appear to be related to back focus.
However, the diffraction pattern could come from the OAG prism sticking into the light cone of the star.
If you replace the OAG with a simple spacer with the same optical path, and the problem goes away, then for sure it is caused by the OAG.
You can also check if rotating the camera relative to the OAG will move the problematic stars to a different quadrant of the image frame. Use thin M42 washers that are 0.1 mm to 0.25 mm thick in between the OAG and the camera, if the OAG does not have means of rotating the camera angle.
Check if the Meade OAG has a way to slide the prism farther away from the optical axis. Should be in the manual -- usually a set screw to allow the prism stem to be adjusted.
The normal practice for mounting an OAG is also to make sure that the relative angle of the OAG and camera is such that the prism is inserted towards the longer axis of your sensor, this way it can reach in as much as possible and still not obstruct the light cone of any star that reaches the sensor.
It could also also be something else in the OAG that is obstructing the light cone, but in all likelihood, it is a prism that sticks in too far.
Typically, a star at the center of the sensor will not be in the shadow of the prism, unless the prism sticks in really far. So if you do see this aberration with a star right smack at the center of the frame, none of the above is true. Also, if stars in all 4 quadrants of a frame show the problem, the issue is not the prism either.
A prism's shadow not only causes star dimming, but optical diffraction causes other abberations too.
Chen
