I’ve had my suspicions for quite a while now, but for some reason after just over 5 years it has finally jelled in my last two remaining neurons what the problem has been.
From way back when I was imaging with the original Sky 90 (bought from True Technology) piggy-backed on the C11 – I had this bunch of trailing stars in the corner of the field of view. It looked like polar rotation (or coma) but it wasn’t dependent on the sub-exposure time (even for very short subs) so polar rotation was written off. I got a reasonable collimation (overall) according to CCDInspector, but there was always this bunch of annoying stars in the corner. Then I saw Steve Cannistra had a similar problem which was due to flexure in the camera rotator – I did his fix and the problem stars were still there.
Recently I bought another Sky 90 from an Irish astronomer and this went on to the mini-WASP array as one of the two main imaging scopes. I was able to collimate the M26C to this scope very quickly, and unlike the other scope I didn’t have to really bolt down the adjusters to the metalwork on the M26C to get good collimation.
So after just over 5 years I finally come to the conclusion that the collimation of the Sky 90 itself is way out!! Luckily this is the model that has the three collimation adjuster screws on the lens cell. So the offending Sky 90 has now been removed from the mini-WASP framework and is now sitting on its own tripod ready for yours truly to attempt collimating against a star. Not done this before for a refractor, so I’ll let you know how I get on, and if the problem goes away.
Even though collimation on the second camera wasn’t too good (again) last night, I still took 6 x 10-minute subs of the North America nebula using both cameras and scopes. I’ll be looking at the data very shortly.