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.

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6 Responses to “The mini-WASP array – one step forward and three steps back.”
  1. Andrew Smith says:

    Greg – Have you tried collimation usinga cheshire eyepeice? I used to do this when I had a refractor. Andrew

  2. Greg Parker says:

    Hi Andrew,

    No I haven’t although I have seen it mentioned in a few places. How successful were you using it? Was it difficult to set up? Absolutely ANY information would be useful as there seems to be very little on this subject out there.

  3. Hi Greg,

    This is a great pity about your Sky 90 being out of collimation. Have you considered using an artificial star? This makes star test collimation so much easier as you don’t have to have it tracking, you can use a Black and Decker workmate if you want to hold the telescope solid and you can even do it in daytime!!!!

    I have one and find it invaluable. This is the one I have and it is so cheap too:-


    Best wishes,


  4. Greg Parker says:

    Thanks Steve,

    I have just bought one (man they’re cheap!!) from the site you suggested. Hopefully one collimated Sky 90 not too far off now 🙂

    All the best,

  5. Andrew Smith says:

    Greg – It’s very simple. Just put a Cheshire in the focuser and cover the lens. Illuminate the cheshire at the side as normal and you should see rings from the reflection from the various glass surfaces. Adjust so they are concentric. However, I did this in the days before anti-reflection coatings so I am not sure how bright the rings will be.

    If you can’t get them all concentric at once it means the lens elements are not set correctly in the cell.


  6. Greg Parker says:

    Many thanks Andrew – I will try this. I am pretty sure the lens elements are set correctly in the cell (famous last words) and that it can be sorted by fiddling with the collimation adjusters. If this works I will try to sort out the collimation on my 3rd Sky 90, an old one where I hope the 3 screws are collimation adjusters and not simply holding the cell in place (with no adjustment possible).


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