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PixInsight OIII Integrated Flat Doesn't Look Good

astrophotography beginner imaging
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23 replies to this topic

#1 BetaDraconis

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Posted 10 August 2018 - 12:06 PM

This is my second attempt at processing my OIII data from the Eagle Nebula and I'm getting heavy vignetting on the master flat, much more than I had when I did the Ha flat. Any ideas?

 

oiii flat
Details:
 
QHY163M
15 flats @ 1.88s
200 Gain 100 Offset -15C
Average, multiplicative normalization, weight don't care, PR1: windsorized, equalize fluxes, PR2: default percentiles
 
Masters:
Bias 50
Darks 15 (calibrated and optimized)
200 Gain 100 Offset -15C

 


Edited by BetaDraconis, 10 August 2018 - 12:06 PM.


#2 jfrech14

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Posted 10 August 2018 - 12:12 PM

If you have an STF applied you need to be careful because it could be incredibly even across the central field and only drop a few percent at the corners and due to scaling it will look nearly black. Use the script that calculates the gradients of your flats or just look at the values to see how much it changes from center to edge. Typically my OIII flats appear much worse than Ha.



#3 cfosterstars

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Posted 10 August 2018 - 12:18 PM

The OIII histogram is usually tighter than the Ha filter. You can compare them with the HistogramTransformation process. That means that the screen stretch will not be equivalent. The other way to compare vignetting is to use something like CCD inspector flats tools. My guess is there is nothing wrong and the STF is just making them look differnent. You can also use the STF settings from one of the filters to compare to the other.

 

Also compare the unstreached flats, severe vignetting is visible without stretching. Even if it is visible without stretching, you should still be able to calibrate that out. That is what the flats are for.


Edited by cfosterstars, 10 August 2018 - 12:19 PM.


#4 pfile

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Posted 10 August 2018 - 02:56 PM

i guess the question is - does that flat correct the light? if so that's all that matters.

 

rob


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#5 dhaval

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Posted 10 August 2018 - 03:02 PM

+1 on what Rob said - did it work? 

 

CS!



#6 BetaDraconis

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Posted 10 August 2018 - 06:05 PM

+1 on what Rob said - did it work? 

 

CS!

We'll see! I didn't go any farther than this, however, the unstretched flat looks pretty even. I'll move forward from here and see how the light comes out. 

 

Thanks, guys!



#7 cfosterstars

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Posted 10 August 2018 - 06:16 PM

What does the unstretched flat look like? Can you see any vignetting without stretching? Even if you can see vignetting without stretch, as long as it is not too severe, it will calibrate out.



#8 BetaDraconis

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Posted 11 August 2018 - 09:24 AM

What does the unstretched flat look like? Can you see any vignetting without stretching? Even if you can see vignetting without stretch, as long as it is not too severe, it will calibrate out.

Like this (bonus histogram included):

 

oiii flat unstretch


#9 cfosterstars

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Posted 11 August 2018 - 09:28 AM

That flat looks fine. There is very little vignetting - I would say less than 5% - so it should calibrate out. You can even have vignetting severe enough to be seen without stretching and still calibrate it out as long as it is smooth. Where you run into issues is with reflections or light leakage where the intensity of the illumination is not linear with the source illumination. When I do stretches of my flats, they all look different with some showing more vignetting and others less, but that is not real. The vignetting is set by the optical system, but how you stretch the histogram changes how it appears on your screen.



#10 BetaDraconis

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Posted 11 August 2018 - 06:08 PM

That flat looks fine. There is very little vignetting - I would say less than 5% - so it should calibrate out. You can even have vignetting severe enough to be seen without stretching and still calibrate it out as long as it is smooth. Where you run into issues is with reflections or light leakage where the intensity of the illumination is not linear with the source illumination. When I do stretches of my flats, they all look different with some showing more vignetting and others less, but that is not real. The vignetting is set by the optical system, but how you stretch the histogram changes how it appears on your screen.

Thanks! Here's the light frame just after integration:

 

oiii Lum Stf stretch


#11 cfosterstars

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Posted 11 August 2018 - 07:04 PM

That looks pretty normal for OIII. DBE should make it nice and flat.



#12 BetaDraconis

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Posted 11 August 2018 - 10:11 PM

That looks pretty normal for OIII. DBE should make it nice and flat.

Looks like I'm on the right track, thanks gain!



#13 pfile

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Posted 13 August 2018 - 12:15 AM

hate to say this but it does look like the flat overcorrected the light a little bit. how were the flat subs calibrated? with bias+calibrated darks? with 1.88s flats you can probably use only a master bias frame to calibrate the flat subs. when using darks (and the default dark optimization turned on) with such short exposures, it's possible the flat calibration is not correct.

 

further, sometimes calibrating the dark subs with a master bias can lead to trouble (pixels clamped to 0 in the calibrated dark subs) when the dark signal is very low. that leads to a bad dark master - that could also have something to do with it.

 

rob



#14 BetaDraconis

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Posted 13 August 2018 - 09:29 AM

hate to say this but it does look like the flat overcorrected the light a little bit. how were the flat subs calibrated? with bias+calibrated darks? with 1.88s flats you can probably use only a master bias frame to calibrate the flat subs. when using darks (and the default dark optimization turned on) with such short exposures, it's possible the flat calibration is not correct.

 

further, sometimes calibrating the dark subs with a master bias can lead to trouble (pixels clamped to 0 in the calibrated dark subs) when the dark signal is very low. that leads to a bad dark master - that could also have something to do with it.

 

rob

Rob,

 

I used a superbias plus master dark. Would the alternative be not using any bias at all? Maybe Dark flats instead? By the way, here is the finished image, as best as my newbie self can do: Eagle Nebula smile.gif



#15 pfile

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Posted 13 August 2018 - 10:19 AM

i would try a master bias alone first, then if that doesn't seem to work right, a master dark flat. superbias is an interesting concept but i think making a proper one is kind of involved. if you have a large number of bias subs you shouldn't need to make a superbias anyway.

 

the image looks pretty good though so maybe DBE fixed the OIII up for you?

 

rob



#16 BetaDraconis

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Posted 13 August 2018 - 10:34 AM

i would try a master bias alone first, then if that doesn't seem to work right, a master dark flat. superbias is an interesting concept but i think making a proper one is kind of involved. if you have a large number of bias subs you shouldn't need to make a superbias anyway.

 

the image looks pretty good though so maybe DBE fixed the OIII up for you?

 

rob

Yes, I ran DBE to take care of those corners and it helped a lot. Since I'm still learning I have no problem trying to process this again, this time with a master bias instead like you mentioned.

 

Thanks for the advice!



#17 cfosterstars

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Posted 13 August 2018 - 10:45 AM

I do not use bias frames at all. I only use dark flats with ASI cameras.



#18 BetaDraconis

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Posted 13 August 2018 - 11:40 AM

I do not use bias frames at all. I only use dark flats with ASI cameras.

I have used dark flats in place of bias in Deep Sky Stacker in the past. With regards to PixInsignt, do the dark flats fill the bias slot in terms of calibration and integration?



#19 pfile

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Posted 13 August 2018 - 12:10 PM

are you using BPP? it doesn't handle multiple darks that well.

 

if you do it all by hand, then all you need to do is select your master flat dark as the dark in ImageCalibration, and untick the master bias field (and also untick "optimize" on the master dark.). or, you could just select the master flat dark as the master bias frame. in either situation all that should end up happening is that the master flat dark should be subtracted from the flat subs.

 

rob



#20 BetaDraconis

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Posted 13 August 2018 - 12:43 PM

are you using BPP? it doesn't handle multiple darks that well.

 

if you do it all by hand, then all you need to do is select your master flat dark as the dark in ImageCalibration, and untick the master bias field (and also untick "optimize" on the master dark.). or, you could just select the master flat dark as the master bias frame. in either situation all that should end up happening is that the master flat dark should be subtracted from the flat subs.

 

rob

No, I've been doing it by hand so far. I'll try what you're recommending after I make some flat darks. Thanks!



#21 cfosterstars

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Posted 13 August 2018 - 12:52 PM

I have used dark flats in place of bias in Deep Sky Stacker in the past. With regards to PixInsignt, do the dark flats fill the bias slot in terms of calibration and integration?

For dark flats and no bias, you calibrate your flats with the dark flat only. For the masterdark, you integrate and do not calibrate for the Masterdark. For the lights, you use that calibrated masterflat and the master dark that is it. The bias is not used at all.



#22 BetaDraconis

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Posted 13 August 2018 - 01:55 PM

For dark flats and no bias, you calibrate your flats with the dark flat only. For the masterdark, you integrate and do not calibrate for the Masterdark. For the lights, you use that calibrated masterflat and the master dark that is it. The bias is not used at all.

I'll try this next time. I also printed out the workflow you posted to use as a reference/cheat sheet.



#23 cfosterstars

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Posted 13 August 2018 - 03:11 PM

I'll try this next time. I also printed out the workflow you posted to use as a reference/cheat sheet.

Make sure you look at the later post. there was an update that I posted as a reply to the post.



#24 BetaDraconis

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Posted 13 August 2018 - 04:55 PM

Make sure you look at the later post. there was an update that I posted as a reply to the post.

Got it!




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