I did what you suggested and it worked, but there is still one aspect that I am unsatisfied with.
My starting point is a USB camera with an OV9712 sensor (1280 x 800 pixels, 3x3 microns each). This has a 5-50mm focal length zoom lens (manual focus), which I set to focus at infinity. I opened the aperture and put it close to the eyepiece. Here are four resulting images.
At the shortest focal length:
My issue here is the following. The green circle does not extend out to the boundaries of the recticule. That boundary would be a circle with a diameter about 1.5 - 2x the diameter of the visible green circle. This is like the image I get if I look through the eyepiece, but with my eye too far away from the eyepiece.
To say it another way: what I *should* be seeing in the previous photo is a green background that covers almost the entire rectangular area, with the corners black, and a sharp circular dividing boundary between the black corners and green circular area.
If I zoom in by increasing the focal length I get this:
The issue is the same. If I look through the eyepiece, this entire rectangular region should be fully illuminated. Meaning: I should see a rectangular green background not a circular green background. There should be no black in the background.
If I increase the focal length more:
Now you can see that the green area is gradually filling the frame.
Finally, if I make the focal length close to maximum:
This is extremely nice to use. It's very easy to repeat 0.1 arcsec at this level of zoom. It's also nice that the entire field of view is a uniform green.
Do you have any idea how I could fix the issue that I am describing in the first photos?
Could it be that the autocollimator is actually forming two images? If I make the green light source bright, put a sheet of paper over the eyepiece, and move the paper up and down, I can see that the eyepiece brings the green light to a sharp focus at a point about 19mm above the top surface of the upper lens. If I put a cmos array at that focal point, which has about a 1mm diameter, I can see that this makes the AC act like a telescope, and gives a sharp view of the object at the location of the mirror (but not of the target or reticule).
If I have understood correctly, the eyepiece should be collimating the rays coming from the reticule/target, i.e. making them parallel as if those objects were at infinity. So the eyepiece is focusing one set of rays to a point, but making a different set of rays parallel. It is the latter ones that I want to image, not the former.
Could you or someone else here suggest a modification of my camera setup which would enlarge the green circle to full size, so that what I see is comparable to what I see with my eye? Is the angular field of view too small?