Vignetting Over-cor...
 
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(@mountainair)
Brown Dwarf Customer
Joined: 6 months ago
Posts: 8
March 4, 2021 04:07  

Hello!  I have a new full-frame camera (ASI6200MM-Pro) for my RASA 11.  I've never had to deal with vignetting before (my image circle falls off about 23% towards the edges of this large sensor).  This is also the first time I'm using filters with this camera and telescope.  No matter what I do, the flats seem to over-correct, causing bright corners.  I must be doing something wrong, but what?  I've never had problems using flats before.

Bias:  250 x 0.1s, Gain 100, Offset 50, Temp -15

Lights:  17 x 300s, Gain 100, Offset 50, Temp -15

Darks:  50 x 300s, Gain 100, Offset 50, Temp -15 (combined into a master dark a month ago)

Flats:  43 x 6.868s (ADU 25k, but also tried 30k and 35k), Gain 100, Offset 50, Temp -15

DarkFlats:  30 x 6.868s (ADU 25k, but also tried 30k and 35k), Gain 100, Offset 50, Temp -15

I have tried again and again to get this to work.  I've re-captured lights and flats multiple times.  I've tried 8 different ADU levels to see if I was somehow over-exposing the flats, even though they all appear to not clip in the histogram.  I've tried with and without bias files.  I'm wondering if this is something I have configured wrong in APP, or if it's with my data (which is 122mb per file, so not easy for me to post for analysis).

The processed result:

Screen Shot 2021 03 03 at 5.29.40 PM

Here is the flat:

Screen Shot 2021 03 03 at 6.55.30 PM

And the flat's histogram:

Screen Shot 2021 03 03 at 6.55.48 PM

In APP, I load my master bad pixel map and master dark, then load all the individual lights, flats, dark flats.  I calibrate and run through the integration step only to be presented with the vignetting at the top.  I have tried advanced normalization, LNC 1/3 and MBB 10% at various points, to no avail.  I have also tried adding a master bias into the calibration, without and even with dark flats.  Each attempt essentially comes out the same.

Any ideas what I may be doing wrong?  As of now, I have two very expensive cameras that have not produced any images without horrible vignetting that destroys much of the value of these large sensors.


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(@mountainair)
Brown Dwarf Customer
Joined: 6 months ago
Posts: 8
March 4, 2021 08:53  

I have a solution, though it doesn't address the root cause.

Even at 35k ADU, apparently my flats were under-exposed.  Using PixInsight's PixelMath feature, I added a base value of .3 ($T + 0.3) to the Master Flat file created by APP.  Then I calibrated/registered/normalized/integrated using that modified master flat and... it worked!  I'm going to adjust it a bit more (probably to .35 or so) and try again, but .3 is pretty close:

Screen Shot 2021 03 03 at 11.48.46 PM

The next time I capture flats, I'll try to go to 40, 45k ADU to see if it helps.  I've never taken flats so bright before; aside from clipping, I worry about pushing the signal into the zone where the sensor isn't linear anymore.  If that doesn't work, PixelMath is an option (though a bit involved; I'd like to see a similar flat adjustment tool in APP - maybe I missed it).

 


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 4, 2021 11:24  

I see the same behavior with my ASI6200MM and a fast telescope: the flats need to be really bright. At F/5 I can use around 15k ADU and at F/4.4 I need to push that to over 30k. I can only imagine that at F/3 and faster the flats need to be even brighter. 


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(@mountainair)
Brown Dwarf Customer
Joined: 6 months ago
Posts: 8
March 4, 2021 16:54  

That's interesting about the F speed (this is f/2.22)... going much brighter isn't much of an option on my other full-frame, the ASI6200MC-Pro.  It's much more sensitive in blue, so my flats turn out like this at 32k ADU:

Image

I would love a way in APP to be able to add a fixed value to every pixel, ala Pixel Math in PI.  I will need to do this with every stack I process in APP; I see it in every filter, with both cameras.


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(@imnewhere)
Main Sequence Star Customer
Joined: 4 years ago
Posts: 36
March 5, 2021 14:33  

Believe it or not, but I also see the same issue with my full frame DSLRs. I had to run the exposure up to get around it.


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 5, 2021 16:02  

@imnewhere Thanks for that additional comment. I would have been surprised if it had been an ASI6200 issue only. Full frame cameras easily suffer from a lot of vignetting and more so at low focal ratios.

What F-ratio did you notice this with?


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(@elgol)
Red Giant Customer
Joined: 4 years ago
Posts: 73
March 10, 2021 12:28  

so is there a solution on the way? I constantly have this issue with 6200mm and 2600mc, with f/5 or f/2, no matter what. An adaption subroutine implemented would be nice. I cannot go up with exposure any further


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 10, 2021 12:32  

@elgol I have asked Mabula if APP can do anything about this. For now the solution seems to be to make sure to create flats with a high center ADU so the corners can be corrected properly.


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(@elgol)
Red Giant Customer
Joined: 4 years ago
Posts: 73
March 10, 2021 12:34  

@wvreeven 35.000 is quite high I'say ...


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 10, 2021 12:38  

@elgol I agree, however if it works it works, doesn't it? Honestly I don't think this is an issue with APP but a general issue with flats where strong vignetting occurs. And like I said, I asked Mabula about this so please give him some time to answer.


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(@elgol)
Red Giant Customer
Joined: 4 years ago
Posts: 73
March 10, 2021 12:47  

I meant I use 35.000 and the issue is still there


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 10, 2021 13:09  

@elgol You write that you cannot go up with the exposure time. For both cameras (ASI6200MM and ASI2600MC) 35000 ADU is just over half of what's possible. You should be able to bump this to 40000 and even higher. Why can't you do that?


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(@elgol)
Red Giant Customer
Joined: 4 years ago
Posts: 73
March 10, 2021 13:42  

not if you use gain 100. I could of course try and set it to gain 0 and thus higher, leaving another dark flat round


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(@mabula-admin)
Universe Admin
Joined: 4 years ago
Posts: 2542
March 10, 2021 13:43  

Hi all,

These new CMOS camera's (even the ones from about 5 years old) are really good in terms of linearity. The old CCD camera's and DSLRs of more than 10 years old, they can suffer a bit with linearity on high ADU values.

If you can only get correct flat-field correction by adding an artificial pedestal to your flats, it means that they are simply still under exposed 😉 So the artificial pedestal can solve that but it is only a workaround and not the best/ideal way to solve the issue.

The best solution is simply to expose the flats longer and use a proper flat-field panel with white light when you are using such fast optics with very strong vignetting.

All the fuss about ideal ADU value for flats is a bit nonsense (if you would ask me) with todays camera's. Simply make sure that the histogram is not clipped on the left or right side. Aiming for higher ADU values of the peak in the histogram will simply give you better flats (better Signal to Noise Ratio) with the camera's that you can buy these days 😉 because the are very linear in response on the entire ADU range.

So I would easily aim for ADU values of 50.000 in 16bits if the histogram does not clip on the right side 😉 (and left side) for all channels (mono or R,G,B).

Oh and personally, I alwasys shoot flats at the camera's highest dynamic range, so at gain 0, the benefit is that the quality again will be higher of your flats and you can still get good exposure times even with such fast focal ratios. Shooting flats with exposure times less than 0,1 seconds can give rise to complications for sure, so try to prevent that...

Mabula

This post was modified 1 month ago by Mabula-Admin

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(@mabula-admin)
Universe Admin
Joined: 4 years ago
Posts: 2542
March 10, 2021 13:56  

Hi all,

In addition, in APP 1.082 you can easily add an artifical pedestal with the Batch Modify tool to all of your flats.

In APP 1.083 you will be able to do this with a new tool : Batch Add/Multiply Tool 🙂

Mabula


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 10, 2021 14:04  
Posted by: @elgol

not if you use gain 100. I could of course try and set it to gain 0 and thus higher, leaving another dark flat round

Can you explain that please? As long as the histogram is not clipped on the right, you can expose longer.

 


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(@elgol)
Red Giant Customer
Joined: 4 years ago
Posts: 73
March 11, 2021 09:21  
2021 03 11 09 19 26 Greenshot

you said it, clipping. anyway, I did a 50.000 ADU test, same thing: over correction it seems ...


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(@wvreeven)
Galaxy Admin
Joined: 3 years ago
Posts: 731
March 11, 2021 13:23  

@elgol Hmm yes there is some overcorrection there. Is there a difference with when using the 35000 ADU flat? Is the amount of overcorrection the same, or less, or perhaps even more? And did you take the flats at gain 100 in the 50000 ADU case?


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(@elgol)
Red Giant Customer
Joined: 4 years ago
Posts: 73
March 11, 2021 15:09  

wanted to push it so yes with gain 100. and clipped. it got less but not enough it seems. hope the announced pedestrial in 1.083 will come soon hahaha. I am aware of this discussions. still ....


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(@ippiu)
Red Giant Customer
Joined: 2 years ago
Posts: 60
March 11, 2021 15:34  

I have the same problem too, since i changed my focal ratio from 5,1 (apex starizona 0,65X reducer) to f7,8 (no reducer or flattener).

Doublet Apo 125 at its full native focal length of 975 mm at f7,8, with no reducer/flattener: small Asi533 sensor.

As you can see in my attached screenshot, where i pumped up stretching and saturarion just for this purpose to show better artifacts, i tried all different ADU flats with 2s exposure, gain 0 and 0,5 second exposure as well: practically it doesn't change anything.

The strange fact is that as soon as i apply the "remove light pollution" tool, almost every sort of vignetting evidence disappears... 🙄 😲

Immagine

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