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Why Sii frames have this ?

ZWO ASI294MM Pro Acquisition SII Deep Sky Artifacts
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Ahmed Alqallaf avatar
Ahmed Alqallaf avatar
Hi,
Why my Sii master frame have this weird black ring and how I can remove it? is it because of light pollution or issues with flat frames?

this is total integration of 270x180" I'm using Antlia filters 3nm, the frames were calibrated with 30x darks and 30x2" flats average ADU 30000
Gears I'm using: ZN81, ASI294MM Pro Bortle 9.
Tim Hawkes avatar
Tim Hawkes avatar
Hi Ahmed,   that looks like a problem with the flats and/or  the darks?  Did the brightness setting of the lights exactly match that of the darks?

Bortle 9 never helps of course but SII is more tolerant of light pollution than lower wavelengths.
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Jonny Bravo avatar
Jonny Bravo avatar
That's a very standard looking 294MM integration of Sii / Ha data without proper calibration. You need to take and use proper calibration frames (darks, flats, flat darks).
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Arun H avatar
Arun H avatar
You have a problem with your flats. Pixel response non uniformity is very evident in the 294MM with H-alpha and SII frames. That looks exactly like my SII flat does.

Increase your flat exposure time to 4 seconds, using either a dimmer or diffuser to reduce the light intensity in your flat panel. Don't use a bias, instead calibrate your flats with flat darks of the same exposure time as your flats. And needless to say, make sure you are using flats taken with the specific filter in place. A flat taken with an SII filter will look very different than one with an OIII or L filter.
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Dan H. M. avatar
Dan H. M. avatar
You also might want to check to make sure that the correct side of the filter is facing the sensor if using an unthreaded filter.
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John Hayes avatar
John Hayes avatar
You also might want to check to make sure that the correct side of the filter is facing the sensor if using an unthreaded filter.

The orientation of the filter does not matter.  The OP has a problem with flat calibration.

John
Ahmed Alqallaf avatar
Ahmed Alqallaf avatar
Tim Hawkes:
Hi Ahmed,   that looks like a problem with the flats and/or  the darks?  Did the brightness setting of the lights exactly match that of the darks?

Bortle 9 never helps of course but SII is more tolerant of light pollution than lower wavelengths.

 
Jonny Bravo:
That's a very standard looking 294MM integration of Sii / Ha data without proper calibration. You need to take and use proper calibration frames (darks, flats, flat darks).

 
Arun H:
You have a problem with your flats. Pixel response non uniformity is very evident in the 294MM with H-alpha and SII frames. That looks exactly like my SII flat does.

Increase your flat exposure time to 4 seconds, using either a dimmer or diffuser to reduce the light intensity in your flat panel. Don't use a bias, instead calibrate your flats with flat darks of the same exposure time as your flats. And needless to say, make sure you are using flats taken with the specific filter in place. A flat taken with an SII filter will look very different than one with an OIII or L filter.

Thank you for your reply,
last night I took another flats using flat wizard in Nina with 4.5 sec %50 ADU for Ha subframes (I lost my Sii subs) you were right, I have noticed that the weird ring also shows in Ha and its been calibrated properly. tonight I'll have Sii session and see how it works.
I have here Ha flats,Ha uncalibrated and Ha calibrated.

Themis Karteris avatar
Themis Karteris avatar
If the filters is unmounted, you want filter masks! I have the same problem and i solved it with my 3d printer!
Ahmed Alqallaf avatar
Ahmed Alqallaf avatar
Themis Karteris:
If the filters is unmounted, you want filter masks! I have the same problem and i solved it with my 3d printer!

Hi Themis,
I'm using masks coming with ZWO EFW are they enough or should I use the one you are using ?
Jonny Bravo avatar
Jonny Bravo avatar
The patterning you're seeing is not caused by filter masks. The ZWO-provided ones, while a bit flimsy, are not your problem. For what it's worth, I don't use the ZWO masks, though I _did_ when I first got my filters. The mottled pattern was seen in 1.25" mounted filters, 31mm unmounted filters with the ZWO masks and still seen with the 31mm unmounted filters using non-ZWO masks.

The solution to the problem, like you already discovered, is proper calibration smile.
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Arun H avatar
Arun H avatar
Ahmed Alqallaf:
Hi Themis,
I'm using masks coming with ZWO EFW are they enough or should I use the one you are usin


You have already solved your problem! It isn't filter masks, it is simply that you originally didn't take flats correctly. You still have a gradient in your stacked image, but is originating from the background light pollution and is very easily removed in PixInsight using DBE.
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