Coma issues again

[RobertI]

Managed my first EAA session in 7 months last night. Whilst it was a trouble free session with some nice results, I had coma-like problems in all fours corners, the same problem I have experienced before. The reducer is a Meade SCT 3.3 operating at around x0.5 on an F9 RC scope giving F4.5 ish and the sensor is a Lodestar. I am at a bit of a loss to understand where the problem lies - I swap the camera in and out regularly, and usually just have slight coma in just one corner - for a brief period once \I had virtually no coma and almost perfect stars to the corners. I have collimated the scope visually but I am sure adding the imaging equipment results in the sensor being off centre slightly, which I would expect to show coma in one corner. If I get coma in all four corners, I wouldn't have thought it is a collimation issue. I don't think it is anything to do with reduction as I adjusted the reduction slightly and it seemed to make no difference, and as mentioned I have had almost no coma previously at this level of reduction.

Does anyone have any ideas or some suggestions as to where the problem lies or how to go about identifying the cause? Thanks.

[Oddsocks]

Hi Rob.

I used to use the Meade series 4000 f3.3 reducer many years ago with an LX200 and Canon 35mm SLR film camera.

The problem you are having is almost certainly due to spacing between the rear lens element of the reducer and the camera.

With the ultra aggressive beam shaping and very acute beam angle of the ray paths leaving the reducer the camera sensor has to be placed at just the right distance from the reducer and the tolerance is very tight, +/- 0.1mm will show a change in coma at the edges of the frame.

My experience with the 35mm format SLR was that coma was impossible to eliminate and only the central area, around 5x5mm was free of coma, and that was with perfect spacing, an imperceptibly small change in distance between reducer and film plane would see coma creeping in further.

I quickly gave up with the f3.3 reducer, particularly because with film you never knew if it was going to be any good until many hours of darkroom processing time had been wasted. For the remainder of the time I had the LX200 i just used the more forgiving Meade f6.3 reducer.

It probably comes down to how you are attaching the camera to the reducer, even a rigid screwed-together assembly will be subject to variations in optical path length due to temperature variations from one session to the next.

If you are able to finely tune the distance between the reducer and sensor “live” then that may offer a solution for you but it won’t be simple as the distances to move are so small.

The above is all that comes to mind given my experience with the f3.3 reducer all of nearly thirty years ago....

William. 

[Oddsocks]

Rob.

One other thought occurred after posting the above reply, are you using a I.R. blocker filter with the camera? 

The Meade series 4000 f3.3 reducer was not well corrected for I.R. and produced quite a large bloat if a blocking filter was not used.

If not using an appropriate blocker I wonder if part of what you experience with variability in coma correction might lie with variability of the stars I.R. content with atmospherics?

William.

[RobertI]

Hi William, many thanks for the detailed information and your experiences, I do suspect that I may never get this consistently right. I have tried using the exact spacings (but perhaps not to the mm) but interestingly the Japanese version I have has a significantly different spacing to the Chinese version I also have, but they both appear to display similar issues. Interesting point about the IR block filter, I was previously using a luminance  filter which has an IR blocker, so shall re-instate this and see what happens. 

The confusing thing is that there seem to be plenty of EAA people who are using C8’s and cameras with Lodestar size chips and operating at @f3.3 with no coma at all, I had even less success with my C8, couldn’t get anywhere near f3.3! 

Regarding your point about needing to get the spacing accurate to 0.1mm, a lot of people say this and it may be right, but again what  confuses me is that the original Meade reducer came with different spacers and instructions on how to get different reductions, kind of implies that spacing is not that critical, but I realise that for VERY small chips which it was designed for, these coma issues are not a problem.

I will try again with the filter though and let you know, thanks again for the help. ?