In your images above, note how the view through the collimation cap allows your eye to be closer to the diagonal so the diagonal mirror appears larger than the primary reflection further away at the end of the telescope tube. But it provides a wide white background making it difficult to assess your diagonal position as Don mentioned earlier.
The view through the site tube, however, provides a thinner white background making it easier to assess your diagonal position. Unfortunately, you cannot see the entire primary reflection. That is because the Cheshire combo pupil high above the focuser places your eye further from the diagonal mirror, so it appears smaller than the primary reflection.
You should be able center your diagonal and align the focuser axis. If the cross hair is on the primary center, the primary reflection will be centered under the focuser. If the diagonal is centered under the focuser, it will be concentric with the focuser and the primary reflection. Often, though, the act of tilting the diagonal to align the cross hair can push the diagonal away from focuser center. So, focus on the axial alignments and seeing the entire primary reflection in the diagonal.
Astigmatism is common with a poorly mounted diagonal. It's a worthwhile modification if it can be done so the diagonal can still be centered under the focuser. I've done it a few times. One time, removing a shroud with a lot of batting forced me to redrill the spider vane mounting holes to move the entire spider toward the primary mirror. Check your spider installation to see if this can be done easily. Edit: If you can move it slightly, it may not be binding and causing astigmatism.
Edited by Asbytec, 20 April 2024 - 09:14 PM.
