Concentric bands when using Dual Band Filter

[Priesters]

Hi all,

Hoping someone can help me with this.

I've just tried out my new Altair 26C (IMX571 sensor) and an Optolong Ultimate Filter with my Meade 8" LX200 and Optec 0.63 reducer and have some weird concentric circular banding on my stacked images.   The flat panel I've used has always been ok in the past and I can see the same issue but to a lesser degree when using an Optolong Enhance filter.  I can't see the issue when I'm not using either filter.  This makes me think its a problem with how I'm taking the flats as opposed to being equipment related.  I've attached the stack, the master flat and a single raw frame so you can see what I mean.  These were captured using SharpCap.  I've also attached the camera settings used when taking the flats.

Any help much appreciated/

Andrew 

Stack_32bits_7frames_2100s.fits MasterFlat_21_51_29.fits frame_00001.fits MasterFlat_21_51_29.fits.CameraSettings.txt

[Elp]

Strange coincidence this, I just got the same exact problem this week with my Lextreme mounted onto the front of my SY135. At first I thought it was due to internal reflections due to using a non native lens.

From experience though, all the Optolong OSC filters I've used cause strange green and red colour patterns (more like a large random swirl pattern across the image) and I've never really found a solution. I've tried dimming my panel down with acrylic sheets (4) and two of them are dark grey and it still happens even with long 10s flats. I've made images with the filters but by not stretching them too much but ultimately this is a restriction we don't want.

I'm going to try my last session with the longer flats to see if the short ones are overcorrecting. I also pre process the flats by applying the relevant dark flats to them first before calibrating the lights.

[ONIKKINEN]

Flat looks ok to me, the light frame looks underexposed, or at the very least not optimally exposed. If you have used the exposure optimization calculations in Sharpcap you should know that those are a bare minimum viable exposure, not the best possible one (at least in my opinion). But that i dont think has anything to do with the flat calibration issue.

What do your darks and darkflats look like? Post them if you want to, there may have been light leaks or something else in them that affects the whole process. Although not quite sure how they would create the concentric rings but maybe it makes sense in context with the darks.

[Priesters]

Darks attached.  Didn't take and Dark Flats or Bias.  Next time out I may turn the brightness of the Flat Panel down as if I remember rightly I had it on full power - just in case its some sort of reflection problem.

MasterDark_30_frames_-9.9C_2023-04-17T18_35_56.fits

[ollypenrice]

It looks as if the flats are the problem, but why? Have you tried stacking and calibrating these files in a different software?

Olly

[Priesters]

12 minutes ago, ollypenrice said:

Have you tried stacking and calibrating these files in a different software?

Not yet.  I'll give that a try though when I get chance.

[Priesters]

Managed to get this without the Ha from the filter tho.   A long way to go compared to some of the quality images I see on SGL but it was a very breezy couple of nights and I’m happy with it 🙂.

[ONIKKINEN]

4 hours ago, Priesters said:

Darks attached.  Didn't take and Dark Flats or Bias.  Next time out I may turn the brightness of the Flat Panel down as if I remember rightly I had it on full power - just in case its some sort of reflection problem.

MasterDark_30_frames_-9.9C_2023-04-17T18_35_56.fits 12.37 MB · 0 downloads

Im out imaging at the moment so cant check the dark, but just so you know the offset has to be removed somehow from both the light frame and the flatframe or calibration cannot happen. You can also subtract offset in some stacking software instead of with an actual bias/darkflat image. If you did not do this then it is likely this was your issue.

[Elp]

18 hours ago, Priesters said:

Managed to get this without the Ha from the filter tho.   A long way to go compared to some of the quality images I see on SGL but it was a very breezy couple of nights and I’m happy with it 🙂.

Looks like a result. So was the issue the HA? I tried mine with the longer (5s) flats and DFs and the issue was still there. Looking like I can't use my data (only 1.5h or so anyway).

Edited April 23, 2023 by Elp

[Priesters]

Yep, only seem to have the problem when the Optolong filters are in.  Hoping to have another session tomorrow night so I’ll try the longer flats and use other calibration frames and see what I get.  

[ONIKKINEN]

Tried to do a bit of pixelmath with your dark and the concentric rings seem to remain, although i think they are weaker. But on a stacked image they should still come out.

Dark looks OK otherwise, not the source of the rings at least. Maybe a bit of light leak on one side but could just be a gradient on the sensor (its normal, these cameras are not 100.0% ampglow free).

Something about the flats will probably be the cause. Do you take them right after imaging each night?

[Priesters]

3 hours ago, ONIKKINEN said:

Do you take them right after imaging each night?

Thanks.  Because I’m using SharpCap and watching the image develop in LiveStack I take them before the imaging session. I suspect the problem is because the flat panel is on full power and maybe something to do with the reducer and reflections- but I’m just guessing. Hoping to experiment again tomorrow night 🤞