Thought I'd share an update....Read a post on making sure the primary and secondary are concentric by looking down the tube with the secondary removed. Thought I could see it being out a bit but it was hard to tell so I came up with a little trick. Drew concentric lines on a clear sheet of plastic and taped it over the end, then looked down and made sure all circles from the plastic, primary, and secondary holder were concentric. I then adjusted the set screws a little with the outer ring holding screws loosened. Went back and fourth a few times to get it, but I did find that I did have to adjust it...a couple millimeters would be my guess. I read somewhere that the holder can have some slop in the corrector plate, so while I wasn't able to remove it, it could have been fastened down out of center by one of the previous owners. Still being indoors looking out through cloudy light with an admittedly flawed collimation, I'm not going to call it until I can actually get out there having done a proper collimation, but there does appear to be an improvement in the sharpness of the image...if only just a little.
Advice is all over the map here. The artificial star idea is certainly valid. You can make one or buy one. The cheapest available are around $20 from Hubble Optics. Depending on the size of the star you use, you do not have to be 300 feet or 300 miles from the artificial star to achieve accurate collimation. Now, you do have to be at around 300 feet or so (there is a formula for it) to star test your optics for various aberrations. Closer than a certain distance, and spherical aberration presents itself. That will not be an issue for collimation. To collimate your optics, you only need to be far enough from the artificial star so that you cannot resolve it. For the 50 micron artificial star I have, this distance for me was about 100 feet (~33 meters). It could have been shorter, but probably not by much. This allowed me to see an airy disk of the light point created by the artificial star. For a C8 and C9.25, this was all I have ever needed. The C11 might present another challenge...
With my specific C11 unit, collimation on an artificial star does not lead to perfect collimation when looking at a real star. The C8 and C9.25, at least the units I have, there is no discernible change in collimation when looking at an artificial star in a horizontal position and looking at a real star at an apparent elevation of 40 degrees or more. Collimation holds regardless of the angle of the OTA (horizontal/flat or nearly vertical and looking near zenith). My C11 presents mirror flop when angled at the sky. What happens is, the mirror tilts ever so slightly once the OTA is no longer pointed at a target that is on or near the ground, like an artificial star. Once the OTA is "lifted" at a angle, looking at objects in the sky, the angle of the mirror changes (the forward tilt)/relaxes. This changes collimation. This has to do with the weight of the mirror in the C11. Some people have experienced this with smaller OTAs such as the C8, but that has not been the case for me.
Anyway, your telescope can be perfectly collimated when looking at a star, but be a significant amount out of collimation when looking at the horizon - or vice versa. Because of this, the only thing an artificial star allows me to do with the C11 is practice collimating it during the daytime or a cloudy night. Practice should not be dismissed. This will allow you to know which collimation screws you should adjust in order to collimate properly on a real star. Of course, you can skip this step and go straight to a real star. I needed practice, but not everyone is the same in this regard. I'm not the bravest or most confident soul when it comes to mechanical things.
I've found this graphic on this page to be quite useful for knowing which collimation screw to adjust:
Of course, you can figure this out on your own at the eyepiece within a few minutes.
As a word of warning, your first collimation experience might take some time. Well, it did for me (see: aforementioned confidence in mechanical skills). Still, once you get the hang of it, either with an artificial star or a real one, you'll be able to do it in five minutes or less for a telescope that is already near acceptable collimation. Tighten down the screws properly and collimation generally holds really well. I've never had to collimate my C9.25 in the 4 years I've owned it. My C11 and C8 have not required collimation in the last 9 months, and unless I drop one of them, I doubt they'll require any adjustments in the near future (year or more). Still, I do check quickly each session. One other thing to note, when I write "tighten" the collimation, they should not be so tight that you have trouble turning them back in the opposite direction if you need to. One description I've read is that you shouldn't have to grip the screwdriver with enough force to break an egg when turning the screw. I don't know how helpful that is as I don't purposely eat eggs or crack them, but it might be useful for you. Eventually, you get a feel for "too tight." The collimation screws on my C8 and C11 are roughly of even tightness. I don't have to struggle to turn them, but they certainly wouldn't turn of their own accord with vibrations typical of careful transport (i.e. not dropping the OTA down a flight of steps).
Edited by gfstallin, 23 May 2019 - 11:51 PM.