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I have a Skywatcher 130 PDS, and for imaging I have a TS GPU coma corrector, 5 position filter wheel and ASI533MM. For my flats I use a fairly thick white opaque acrylic plate and this has been working well - it calibrates out dust and vignetting. But one thing keeps bothering me: it looks as if the central obstruction of my newtonian is visible in the flats that I create. I don't have any experience with other newtonians, so my question is "is this a common issue" or is this something specific in my setup? Extra information to triage this problem:
Following image shows the problem (the center is darker):  Following image shows what I get when I move my imaging equipment further out of the focuser:  |
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| I would just attribute it to vignetting, however your vignette is not centered which leads me to believe your collimation and or offset are off. Might want to work that collimation a little better. When it's proper, your vignette will be centered with equal light falloff on all corners. |
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I have the same problem with my 130PDS, when my light source or the sky is to bright. Than the secondary is visible with its shadow and that doesn't fit with the flat from pure optical path. Using less light always worked for me. CS Erik |
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As I saw you wrote that you moved the focusser, please make sure that you do you flats while the telescope is in focus. Here's an example for my PDS 130, please also note that the brightest spot is (roughly) in the center... I am using the Baader Coma corrector... |
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| Nothing wrong with it, after all the secondary *does* shadow the primary thus both the inlet and exit pupil of the scope and having a small light drop-off is only to be expected and the contrastier the image is the easier is to see it. As you move out everything fuzzes up a bit as you move out in the caustic cone. |
