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Old 07-09-2018, 11:26 PM
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PRejto (Peter)
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Join Date: Jan 2011
Location: Sydney, Australia
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Blue Filter Issue (CCDI Aspect)

A while back you may recall that I started a thread involving the mounting of a Gemini rotator and the resulting tilt and concentricity issues. Happily with Martin Pugh's efforts the tilt issues are essentially resolved and the concentricity issue is improved though not perfect.



Naturally a new problem has surfaced. If you look at a series of photos I took this evening it jumps out that the blue filter has far worse aspect (i.e., not round) than L, R, G filters. I've been trying to figure this out for a few days so I have tried re-focusing before every blue filter sub. This has not helped at all. Tonight I took 9 x 10 sec images through each filter and then used CCDI to produce maps of curvature (i.e., focus) and aspect (where the 9 images are averaged together). I would conclude from the curvature maps that I have very little tilt in the system. But, the aspect maps are all different with green looking excellent and blue quite wonky. The L filter shows best aspects generally in the center of the image, but the B filter shows the best aspects in the extreme left edge. The R filter shares a bit of the best area moving towards the left edge, but surprisingly the R filter images show decent aspect. Based on B results and the way the aspect map looks I would have expected R not to be as good as it is. My first thought is that perhaps the blue filter is tilted....but these are Astronomik Deep Sky 2" mounted (screw in) filters. Martin has looked at the filters and cannot see how they could be tilted. My previous camera (Moravian G2-8300) with Astronomik 31mm unmounted filters did not have this problem on my TEC180.



Anyone have an idea of what might be going on here?



Thanks!


Peter
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Click for full-size image (Blue.jpg)
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Last edited by PRejto; 08-09-2018 at 12:05 PM.
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