Another clear night – that’s 3 in a row, possibly the only time I’ve had such a chance in a decade.
I spent it investigating my guider collimation problem.
Imaging with an SV80S (480mm f/6 triplet refractor) I’m getting very elongated stars in my guide camera, seeing through the off-axis guider port on the QSI583WSG camera. So elongated that I doubt guiding will be accurate. And considerably worse than I remember seeing a couple of years ago. What’s going on?
My assumption was that the pick-off prism or the guider camera mount was somehow out of collimation – that it wasn’t orthogonal to the incoming light path. That would seem consistent – the distortion in the guider stars is exactly vertical, as though the prism was out of alignment in the vertical direction, but fine horizontally.
However, before that it occurred to me that the entire camera/guider system might be out of alignment. Better check that first.
Using CCDInspector, I discovered a minor out-of-plane error in the main camera, which I was able to correct by snugging it better into the focuser drawtube.
Next, I did a field analysis of an image from the guide camera, which confirms what I could see with my own eyes – there is major distortion, almost exactly vertical, toward the bottom of the image field.
Now we come to the problem. I think I need to tweak the alignment of my off-axis guider port’s “pick-off” prism. However, it doesn’t seem to have any such collimation adjustments. It’s solidly machined, as is the threaded guide camera mounting port. It’s not supposed to be out of alignment, and there doesn’t seem to be any way to adjust it if it is.
By now I have run out of evening time and patience, and I have to get up for work tomorrow, so I’m stopping, stumped, for now.
On the weekend I will use daylight to take the camera off and apart and look more closely for some way to adjust or shim the pick-off prism or guide camera mount.
My fall-back plan if I can’t adjust that will be not to worry about it. I’m pretty sure the problem is greatly exaggerated by the rather tight field curvature in this scope – evident in the curvature map shown above. The pick-off prism is very close to the edge of the field, where curvature is strongest. Maybe that’s why I don’t remember having this problem before – because I was using a different scope, with a famously flat field.
With this short focal-length scope I should be able to take 5- to 10- minute images unguided and, if guiding is needed, at this short focal length a separate guide scope should work well.
So, I’m going to have a look at the mechanics on the weekend. If I can find a way to adjust the prism alignment, I will try that. Otherwise I’m going to declare “don’t care” and use this short scope without the off-axis guiding. Its season is nearly over anyway – I’ll be returning to a longer focal-length, and flatter-field, scope for the summer. If I have a guide-star problem with that scope I’ll pick up this problem again, but I’m guessing I won’t.
So, assuming I don’t find an adjustment, next clear night will be spent refining my PEC training now that I have perfect focus, and then I’ll do some tests of unguided exposure length to see what the Paramount can do at a short focal length. I expect to be impressed.