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Flat reflections shadows?


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The donut shaped dim shadows would be dust motes somewhere close to the sensor, probably on the sensor window, filter if any was used or the last lens of your corrector. Any further away from the sensor than that and dust on surfaces becomes mostly meaningless.

I assume that this is a stretched image and the corners are not actually 0-value? That would be really weird if they were and you have some serious obstructions to the lightpath (or use a sensor too big for the corrector).

If stretched, its just vignetting which appears to be slightly off center, but i wouldn't worry about that. With a newtonian you have 2 mirrors to align and possible mechanical issues that can bring the most illuminated area off center slightly even with tiny errors in the alignment. If star shapes are good enough, i would say dont bother trying to fix the off-center vignetting and live with it.

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Hi Onikkinen 

Yeah its stretched, I suspect that collimation is out possibly causing the off centre vignetting also there is an issue with tilt in the coma corrector which is visible in top right most of all, not sure if this causes and issue with flats though. 

The donut although its difficult to tell is  more like a nuclear sign i dont think this picture pics it up ill try to exaggerate it  

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Yep, dust motes. They calibrate out and will not show up in the image afterwards, so nothing to worry about in that front.

The uneven vignetting thing is indicative of collimation error, or at least some kind of de-centering (focuser also often the culprit) of the sensor. Up to you to decide whether to tackle that, but if stars are small and round i would not bother.

BTW here is one of my flats, which also has plenty of prominent dust donuts none of which remained in the calibrated lightframe afterwards:


You can try to clean your glass surfaces thoroughly, but its more difficult than you would imagine and ultimately i would say pointless, since they all calibrate out.

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If your flats were perfectly flat you wouldn't need them.

These flats are not particularly remarkable in my view. The vignetting is, indeed, off centre but so it is in many high end refractor flats. Adjust it by all means but this is not outrageously bad as it is. I'd be wanting to measure the linear ADU values in the corners and compare them with those at the centre. If the drop-off in the corners is more than 25% I'd be concerned about their ability to correct the final image properly. However, I had a 23% drop off in my Tak FSQ/full frame camera and all was well in the final image.


Edited by ollypenrice
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