If we get too theoretical, we can stray from the practical of what we can do and get caught up in the soupy mire of what we might do if only we could...
With that in mind, here are some items to consider:
1. We can talk about axes and planes all we like, but at the end of the day, we're working with 6 adjustment screws and an additional central screw. I like to tell my customers, "There are 10 ways to do anything and 9 of them are probably wrong". There are infinite wrong endpoints for these 7 screws and far fewer endpoints that produce good results.
2. Terms like "centered" and "HOM" are visual terms. They have a basis in geometry, but all we're really doing is making something appear a particular way to our eye and hope that the view comports with the geometry behind what we're doing. We don't "do geometry", we turn wrenches and pray we don't drop one down the OTA (Helpful hint: Attach your hex wrench to a line and clip it to a secondary vane so that if you do drop the wrench into the OTA, it's can't reach the primary mirror and ruin your day. Also, trash bags make good rain-wear if you don't mind looking like a bum). Odds are poor we'll ever center a dot in the mechanical sense, but we can center it in a visual sense, and we can 2nd guess our visual centering ad infinitum if we allow ourselves to get caught up in a vicious cycle of obsessive/compulsive perfectionism. Two different observers can vary in their visual acuity (and the ability of the brain to interpret the visual) such that one observer may be able to judge center better than another. The two observers will probably argue over who has a better eye for the center (because why not?) and harsh words will be exchanged. At this point, it's helpful if an impartial 3rd observer happens along to inform them that they're both gormless twits and would you guys please knock off the arguing, it's upsetting the dog.
3. When discussing the Cheshire centering inside the secondary donut, it seems a precise centering won't likely result in optimal on-axis performance. Rather, we're starting to see that this instead gives us a good reference point for further on-axis optimization. For this, I've suggested the tri-bahtinov mask. The TBM is relatively cheap, needs no batteries, and has more diagnostic power than a round chunk of plastic has any right to have. You can even leave it out in the rain and the TBM will be no worse off for it. I've got in on my schedule to develop a recipe for using the TBM to bring the 3 equally spaced TBM axes into simultaneous focus via secondary screw adjustments with the TBM installed. Again.. what we can do, not what we might do if only we could...
4. I don't pay much attention to the whole focuser/primary mirror alignment issue since my focuser/primary happen to be very well aligned. I choose to use a laser to check this alignment, but others may not. Not everyone has an aligned laser and anyway they're over-priced and under-useful in many but not all cases. There are other ways to check/adjust this alignment as TS discusses. To each his own.
5. Once we have completed the iterations of visually centering the Hall of Mirrors using whatever visual cues we prefer (2nd bright reflection of the open end, bla bla bla) and we have achieved best on-axis performance (again, Cheshire probably not centered, but using tri-bahtinov mask at twilight), we have exhausted our ability to further improve the visuals. The photographic results we get are the results we get. We might observe, "Left corners look good, upper right looks good, lower right looks blobby", but fixing the lower right without probably screwing up another aspect of the frame will require a whole different procedure and a level of optical understanding that most of us don't possess. Perhaps someone will come up with a recipe for achieving this, but I suspect we're into such tiny adjustments here that we're in the realm of "improve one aspect, degrade another aspect". We're working with M5/0.8 metric screws. One turn will give 800 microns of translation. The entirely notional idea of "one twentieth of a turn" represents 18 degrees of rotation and 40 microns of translation. You don't make a twentieth of turn, you bump the hex wrench and pray it makes things better. I suspect that the optical quality of the mirror system can be made perceptibly better or worse by translations well below 40 microns, which is already a distance that we can only control precisely if we have vivid imaginations. In short, the "bump" of the wrench that resulted in a "perfect" image was pure luck and we won't reproduce that perfect result with any force other than another bout of pure luck.
See: Diminishing returns.
See: Leave well enough alone.
See: Don't mess with success.
See: Knock it off and image something
We didn't buy these things for the "joy" of adjusting them. That's just a necessary slog to complete. Stack some images and process something nice!
Edited by dg401, 10 August 2021 - 02:32 AM.