I expect some parallax with any sight tube, but the variation with the LightPipe is enough to warrant another tool for assessing and correcting secondary mirror tilt (either a good thin beam laser or an autocollimator). Either the LightPipe (or the TeleCat) should be fine for assessing and correcting secondary mirror placement (three circles).
Did you also check the focuser travel using the laser beam on the far side of the tube?
Generally speaking, I suggest starting with the pupil near the apex relative to the native focal length (where the apparent primary mirror diameter is a bit smaller than the apparent secondary mirror diameter. From the apex, you want to make these three circles concentric:
the bottom edge of the sight tube,
the actual edge of the secondary mirror, and
the reflected edge of the primary mirror.
When the three circles are concentric, you'll need to assess and correct the focuser axial alignment by adjusting the secondary mirror tilt to align the sight tube cross hairs (or the outgoing thin beam laser, or P-3 in an autocollimator) to the primary mirror center marker.
Assuming the three circles are still more-or-less concentric and the focuser axial alignment is correct, the final alignment is the primary mirror axis. This is best assessed and corrected with a Cheshire derivative (ideally with a calibrated Cheshire).
Once the telescope is correctly collimated relative to the native focal length, you'll need to assess the vignetting profile in your imaging train. Usually, assuming the secondary mirror is large enough to fully illuminate the coma corrector field lens (or nearly so), the vignetting profile is then attributed solely to the coma corrector.
Thanks for all your help Vic!
I did a more thorough measurement of the focuser use a sheet of paper wrapped around the tube, locating the center of the focuser and then rotating the paper 180 degrees and drilling a small hole in the tube. The sight tube cross hairs and laser land just on the edge of the hole, so there may be a tiny bit of tilt, but it's just as likely that my measurements were a fraction of a mm off...
I didn't check the focuser travel, since the focuser is always at the same position for all my imaging, never moving up or down by more than a couple of millimeters. I noticed early on that there is a lot of play in the focuser when at its limits. So I keep it in the middle where there is least movement after a meridian flip and have a one inch spacer ring around the outside of the camera adapter to keep the camera at a precise distance from the focuser.
After using the Light Pipe to get the circles concentric I use to the Catseye XLK and Catseye Cheshire to align the center marker reflections.
I'm not sure I'm going to get any closer than this, but thankfully even the corners of my images look much better than before, with nearly perfectly round stars, and vignetting has been greatly reduced.
Edited by Catchlight, 19 April 2025 - 09:30 AM.
