Trouble with RGB imaging

[billyharris72]

Well, LRGB actually, but the problem is in the RGB bit. I seem to be getting an issue with the sky background having all sorts of odd colour casts, that I don't think are entirely due to light pollution. Part of it in the image below (ED72 and ZWO ASI1600MM) might be due to the blue channel being less focused than the red and green (I did refocus but the FWHM was not as good, a problem I seem to have recurrently with the Blue channel). But is anyone with more experience able to take a look and advise what could be going on?

I was able to salvage some of this by using the AstroArt gradient background removal tool and cropping the image, but it would be nice to be able to take advantage of the full field of the ASI1600.

Thoughts appreciated,

Billy.

[vlaiv]

Did you apply per filter flats properly? Could there be some sort of mix up between those?

[symmetal]

I'd say this is primarily a problem with the blue channel. The areas you've labelled as greenish are more yellow (-blue) and it could be flare/reflections with the blue filter. Are you using the unmounted 31mm ZWO filters by any chance? Does the blue flat show a similar response?

Alan

[billyharris72]

11 hours ago, vlaiv said:

Did you apply per filter flats properly? Could there be some sort of mix up between those?

I did take flats for each filter, but bizarrely that seems to make that paticular issue worse. If I use luminance flats throughout I get overcorrection in the red, but the yellow / blue mottling vanishes.

I might try dropping the flat frame for the blue channel and trying to generate a synthetic flat, just to see if this is a problem with my flats.

Billy.

[billyharris72]

11 hours ago, symmetal said:

The areas you've labelled as greenish are more yellow (-blue) and it could be flare/reflections with the blue filter.

This sounds possible - I'm suspecting it may be the flat for the blue as much as anything. I've always struggled with that particular filter, and wondering if it's time to try another.

11 hours ago, symmetal said:

Are you using the unmounted 31mm ZWO filters by any chance?

Yes. This is a bit worrying as it's the second time I've been asked this. Is there an issue with these filters?

Billy

[symmetal]

5 hours ago, billyharris72 said:

Yes. This is a bit worrying as it's the second time I've been asked this. Is there an issue with these filters?

The early versions of these filters didn't have the edges and bevels blacked out, some just the edges and some none at all. This increased the chance of reflections and halos in your images. In early 2018 (I believe) Zwo brought out a new filter set with the proper areas blacked out. If you have the early versions Zwo made some masks to fit over the filters to cover the edges which they supplied free if you asked them. If you bought your filters from FLO they have sets of the masks to give you. The masks are different for the 5 position filter wheel and the 8 position wheel so do specify which filter wheel you have.

This thread shows a discussion about the filter masks which may help. This may not be the cause of your problem but is worth doing if you have the early filter sets.

The flare/reflection effects are dependent on the number and positions of bright stars in your image and your flats light source may introduce different effects making things worse. 

Alan

[billyharris72]

44 minutes ago, symmetal said:

The flare/reflection effects are dependent on the number and positions of bright stars in your image and your flats light source may introduce different effects making things worse. 

Thanks Alan. I've email FLO - fingers crossed that will at least help.

Billy.

[kens]

What is your imaging sequence? If you do all R, then G then B you can get different sky gradients on each filter as the scope rotates.