Flats don't work right all of a sudden

Max

I don't have a good answer for you, but I can offer some comments.

First - if you look closely, the first image has the exact same issue as the second - its just a lot fainter. So there's probably something wrong with both the old and the new flats - its not a new problem.

I have seen something similar before, and I never completely got to the bottom of it. But my working theory is that it is caused by internal reflections on the interior wall of the scope. I suspect that it is because the light from the flat panel is not coming in parallel. Instead, the light from one side of the panel bounces off the other side of the scope and creates a circular distortion that is different to the light path of a distant object (stars).  In support of this - you can see the the effect of the spider support for the secondary. Its clear that most telescope geometries do not significantly suffer from this. Perhaps your rc6 is an exception.

If this idea is right then a solution may involve changing the light source for the flats so that the light comes in in a more parallel way. Perhaps move the panel further from the scope (maybe with a dew shield), or try sky flats. If you choose to try this and it does work I'd be interested in hearing back from you.

Remember, this idea is unproven and I may be completely wrong. If anyone has a better idea I'd like to hear it.

regards

JC

I noticed that it doesn't work to do your flats at the end of a dew shield - maybe more chance for reflections. Your flat panel should be directly applied to the OTA entrance.

@astrogee

You may well be right. I'm certainly not confident that my idea is correct - but it does give Max (@maxthebuilder) something he can test. I hope he can let us know if it succeeds or fails - either way that may be useful information.

JC

Yes, those are reflections inside the tube and also from the secondary mirror spider.

That's exactly what flats are supposed to take care of (besides dust motes, etc.).

I don't use a dew shield.
I place the flat panel exactly the same way every time.
Use same brightness and about  the same exposure time every time. For years.

I actually suspect that my acquisition software (INDI/Ekos) might not handle 2x2 binning 100% correctly.
Once I switched back to 1x1 binning - everything was back to normal.

Thanks guys for your replies!

Hi Max @maxthebuilder,

I read the entire topic, to me this seems likely to be a software capture issue with camera control somehow on shooting 2x2 binning, maybe something goes wrong with the sensor bias level. The bad or worse result with the bin2x2 data looks like, in my eyes, a sensor offset issue where the flats, bias, flat darks are not compatible with the lights.

The report that the issue goes away when using bin1x1 leads me to think that there is a problem here in INDI/Ekos, but it can be useful for me to have a look at the bin2x2 data as well. Maybe there is something in the data or fits header that we can use to adjust the scaling/normalization of the flats and then the problem might be solved from APP's end.

Can you upload the data here:

https://upload.astropixelprocessor.com/

username: uploadData

password: uploadTestData

Please make a folder with your name and issue like:  Max-Flat-bin2-issue

and upload the data there and let me know once done.

@connor231 & @astrogee thanks for your assistance !

Thanks,

Mabula

Mabula - thanks!

Added few frames to> https://upload.astropixelprocessor.com/

Thanks Max,

I will have a look and will get back to you when I know more 😉

Mabula