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Weird semicircle in my images using the Antlia Tri-band filter

Steffen Hirtle GraXpert ZWO ASIAIR Plus Celestron C6 Equipment Troubleshooting
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Anthony (Tony) Johnson avatar
Anthony (Tony) Johnson avatar
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two images from my Nikon 810a modified full frame DSLR. Both are shot with the Antlia tri-band filter, the Jellyfish is shot with the Skywatcher 82ed evostar with the reducer/flattener, the other is shot with the Celestron C6 at f10, no reducer. All shot and guided with the ASIAir+ on the AM5 mount. All shot from my backyard in bortle 6 skies, no streetlights or house light to interfere. Both were cropped to just remove stacking artifacts, and then GraXpert was used to remove the gradient. Background extraction was the same at all setting and with various background extracting scripts or software used. I’m not using this camera any longer, but thinking of selling it. I’d like to know exactly what was going on here. Also all possible problems with light leaks were explored and ruled out before these photos were taken. To me, they it looks like some sort of weird reflection, but I’m at a loss and been at one since I’ve been using the camera. Problem is I never shot using this camera without the triband filter so I have no comparison image to rule out the camera. Anyone have any experience with anything like this.
Oskari Nikkinen avatar
Oskari Nikkinen avatar
Did you take and apply flats? Trying to brute-force gradient correct an uncalibrated image will look more or less like this.
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andrea tasselli avatar
andrea tasselli avatar
1. Your backspacing is wrong and/or the flattener isn't working for you

2. As @Oskari Nikkinen  above said, did you calibrate you light frames with flats (and were themselves calibrated)?
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Got an example we can see?
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
My bad, I missed the link. I've been something similar with mounted filters. I've since replaced all of my filters to unmounted and don't have this issue anymore.
Anthony (Tony) Johnson avatar
Anthony (Tony) Johnson avatar
Oskari Nikkinen:
Did you take and apply flats? Trying to brute-force gradient correct an uncalibrated image will look more or less like this.


andrea tasselli:
1. Your backspacing is wrong and/or the flattener isn't working for you

2. As @Oskari Nikkinen  above said, did you calibrate you light frames with flats (and were themselves calibrated)?

Telescope and flats, biases and darks have all been ruled out, along with reducer and back focus, this problem persists no matter what technique I tried to mitigate it. Along with light leaks. Backfocus was adjusted, different flats were taken at various different exposures, along with darks and biases. These images have this problem before anything is done, straight out of the stacking software, which has been them all. Absolutely nothing I’ve did, and this has persisted for the year I’ve used the camera, has helped any. Like I said in my original post, I’ve not taken an image without the filter so I have no comparison there. Also the C6 image is at f/10, with no other glass to cause a backfocus issue, camera directly on the back of the scope. Not sure what other info I can provide.
Oskari Nikkinen avatar
Oskari Nikkinen avatar
Just saying, the filter has nothing to do with the issue here. I am 99% sure you have an issue with your flats, or how they are applied.

I do have the Triband and have used it many times, never had issues with flats. Do take note that the flats exposure time can take a bit of trial and error to dial in properly as the filter has a blue-green bias in the light that it passes. If you expose improperly you can clip one of the bandpasses and flats are guaranteed to not work.

As an example here is what the histogram of a flat i took with the Triband filter looks like (just picked one at random):

Notice the detached peak to the left, which in this case is the red bandpass. Green is in the middle and blue is to the right, as my panel at the time had a blueish white LED panel. If you expose so that the leftmost peak is clipped to black, or the rightmost peak is clipped to white your flats fail. This example works fine.
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andrea tasselli avatar
andrea tasselli avatar
Telescope and flats, biases and darks have all been ruled out, along with reducer and back focus, this problem persists no matter what technique I tried to mitigate it. Along with light leaks. Backfocus was adjusted, different flats were taken at various different exposures, along with darks and biases. These images have this problem before anything is done, straight out of the stacking software, which has been them all. Absolutely nothing I’ve did, and this has persisted for the year I’ve used the camera, has helped any. Like I said in my original post, I’ve not taken an image without the filter so I have no comparison there. Also the C6 image is at f/10, with no other glass to cause a backfocus issue, camera directly on the back of the scope. Not sure what other info I can provide.


*I'm pretty positive that the 2nd example isn't calibrated as the dust motes are still there. You still have a light leak or a bad flat. Maybe providing us with a raw light (or more) with the attendant calibration files would help.
Anthony (Tony) Johnson avatar
Anthony (Tony) Johnson avatar
andrea tasselli:
Telescope and flats, biases and darks have all been ruled out, along with reducer and back focus, this problem persists no matter what technique I tried to mitigate it. Along with light leaks. Backfocus was adjusted, different flats were taken at various different exposures, along with darks and biases. These images have this problem before anything is done, straight out of the stacking software, which has been them all. Absolutely nothing I’ve did, and this has persisted for the year I’ve used the camera, has helped any. Like I said in my original post, I’ve not taken an image without the filter so I have no comparison there. Also the C6 image is at f/10, with no other glass to cause a backfocus issue, camera directly on the back of the scope. Not sure what other info I can provide.


*I'm pretty positive that the 2nd example isn't calibrated as the dust motes are still there. You still have a light leak or a bad flat. Maybe providing us with a raw light (or more) with the attendant calibration files would help.

I truly appreciate what you are saying, and if this was an isolated image I’d probably agree about the flats, but every image I’ve taken with this camera, and that’s all last year I had the exact same issue, flats have been taken with a couple of different flat panels using the auto setting on the ASIAir and then trying to do them manually with various exposures, I’ve also checked the flats with the flat contour script in PI and from what I’ve watched and read they seem dead on.
As far as the dust molts are concerned, I’ve had issues with dust molts appearing after the flip and then disappearing during the flats. As I said in my original post, I’ve since quit using the camera, but I’m wanting to sell it and trying to rule out a problem with the camera. One other note, I’m using a canon 60D now, with the exact same configuration with the scopes and filter, along with the same technique for the flats, and the problem has disappeared, it’s only present with the Nikon. Another reason I don’t believe it’s the flats. I’m just hoping actually it is the filter in combination with the mod on the Nikon. Not sure what that’d be though, but the images I get with the Canon are clean. Of course it’s not modded. Also to address light leaks, actually that’s not possible, the camera is totally enclosed, explored that issue for a few months last year.
No, the problem seems to be with the camera sensor, but I wonder if it isn’t in combined with the filter. Basically when you’ve exhausted all other possibilities the only thing that’s left probably is the reason. I’ll use it one last time without the filter and see what I get, thanks for your help, but this problem has persisted all last year with every image, taken with a refractor and 2 SCTs, a 6in and a 12in, and in every single image the problem was there, and all avenues have been tried and failed. I just thought I’d put it out there again to see if any one had a new take on it since I’m wanting to sell it.