3 replies to this topic

[Michiel94]

I recently started imaging with a mono setup using an SV605MC with Optolong SHO NB filters (7nm) on a WO ZS61II. The first night I imaged, I noticed I had some issues with Ha and Sii flats because the EL panel I use does not emit much light in those bands. I ended up not using flats for Ha, and the Oiii flat was fine, so I was able to make a decent HOO image.

 

Yesterday, there was finally a bit of clear sky again and I was able to gather 2h of Sii. While waiting for the clouds to disappear, I shot flats and dark flats (35x 3.5s, used automated calibration in ekos at 10.000 ADU). I noticed the flat frames came out a bit weird since the Oiii flats and flat with my DSLR always just had vignetting and the odd dust spot, but the Sii flats show a weird pattern (right image below). I didn't think much of it, and today I tried preprocessing the data using WBPP.

 

The result is as follows: I initially stacked the data with the flats, resulting in the middle image. I was surprised that there was such strong vignetting, so I tried processing the data without flats (just using a master dark, nothing else) and the result can be seen on the left.
It seems to me like incorporating the flats caused a strong decrease in SNR, and instead of dividing by the master flat, WBPP actually multiplied it, making the pattern of the flat very visible in the middle 'calibrated' image.

 

I have not had any issues with WBPP before, leaving everything in it's default settings, so I am unsure what caused this or where to start debugging it. I know the basics of what goes on behind the screen, but can't analyze it step-by-step (yet).

 

Does anyone have an idea of what is going on here?

 

I'm using the latest version of PI (1.8.9-3) on Ubuntu 22.04.5 LTS.

 

 

Attached Thumbnails

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[whwang]

I suggest to try twilight sky for SII instead of the EL panel.  In my observatory, we also had some issues with Ha flat with EL panel, and the problem appears similar to yours.  Our twilight Ha flat works fine.

 

That being said, I have no idea why WBPP did the multiplication instead of division.  Weird. 

[Michiel94]

I suggest to try twilight sky for SII instead of the EL panel.  In my observatory, we also had some issues with Ha flat with EL panel, and the problem appears similar to yours.  Our twilight Ha flat works fine.

 

I used to make flats on a white wall indoors. Logistically, taking twilight flats is a bit difficult since twilight is during working hours currently... 
But it's a fair point, I might try taking flats indoors again and see if the pattern persists

Edited by Michiel94, 11 December 2024 - 01:34 PM.

[Michiel94]

The issue is solved!

Apparently, since I'm still new to AP, I forgot to pay attention to the camera offset. The darks were shot at an offset of 10, while the flats and light were shot at 0 offset (which ekos defaults to). 
I managed to get a normal master light using only flats, without any other calibration. Unfortunately, the data is somewhat compromised since there are some zeroes in the light frames.

 

Lesson learned!