Hello. I recently picked up a used Celestron C8, and been trying to get collimation on it. I can get collimation on the center start pretty well, but stars outside the center are a bit all over it seems. I am not sure if this is user error, equipment error, or something else.
Gear list is below, but I'm using the Celestron f6.3 corrector/reducer and and ASI385. <snip>
Celestron C8 (used)
Celestron 6.3 reducer/corrector
106mm back focus (cannot get spot on 105mm with current setup)
Take off everything:: and use the telescope straight through (not even a diagonal, and for sure not a reducer.)
Use a high power EP, something like 3.5mm (7mm in a Barlow/PowerMate.) Which for most applications would be considered stupidly high magnification. Even better if the EP has a reticule so you can properly center the star near the reticule. You ARE, after all, looking at diffraction patterns not fields of view.
Use a star high in the sky to minimize atmospheric effects. Do not attempt to collimate if the sky has rivers of air running through them (or use an artificial star from a long way away in daylight.)
First order of business: As you get close to focus, there will be a ring of light around a dark circle (as seen in your posted image). You want this ring of light to be evenly illuminated around the whole ring--AND--you want this ring of light to collapse into focus, and then spring back into a ring of light just beyond focus.
Based on your photo it does not look terrible as we see stars with a nice disk (with a hole in it). I can see that the collimation is off because the disks to the left shows significantly more coma than the disks to the right. But I can also see the center bright star is WAY TOO BRIGHT. you should be using a 4M star bright enough to see the disks and patterns, not so bright as to over illuminate the whole view.
ETA:: after you get it collimated for visual, add back in the diagonal. If collimation changes, collimate the diagonal with paper shims if necessary so that the collimation does not change as you rotate the diagonal around the holder.
After the diagonal is collimated, then you can try the reducer. If it alters collimation--I don't know what to advise.
Edited by MitchAlsup, 21 February 2021 - 07:09 PM.