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Green inner to red outer gradient with SCT and reducer

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#1 AnthonyQ

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Posted 15 August 2020 - 02:19 PM

I tried searching this and had no luck, but I am sure it’s been discussed before. Feel free to redirect me if you know of a previous discussion.

 

When DSO imaging with my APS-C Pentax with a Starizona .63x Reducer/Corrector II on a Celestron 8SE SCT, (maybe also without but nobody got time for f/10), I always get a red center to green outer gradient, and I think this is normal.

 

I have figured some tricks to remove this from images of galaxies, (small objects against a big flat background) in Photoshop, but I am wondering if anyone has any advice for things like nebulae that take up the whole field? So far my experience with gradient remover actions in Photoshop have just produced mostly colorless images.

 

If anyone has any advice on equipment set up that might address or minimize that, I’d welcome that also.


Edited by AnthonyQ, 15 August 2020 - 05:15 PM.


#2 Michael Covington

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Posted 15 August 2020 - 03:53 PM

A gradient in the background?  Or a color smear in each star image?

If it's a gradient in the background, it has to be stray light.  What lights are in the vicinity of your telescope when you're imaging?  

Can you share a picture?



#3 AnthonyQ

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Posted 15 August 2020 - 05:36 PM

A gradient in the background?  Or a color smear in each star image?

If it's a gradient in the background, it has to be stray light.  What lights are in the vicinity of your telescope when you're imaging?  

Can you share a picture?

Background gradient, not on each star. (I have another issue with that but tiny blue tails on the outer stars with the Starizona FR/CC are normal, but not talked about much, and I have a solution for that.)

Sorry but I got it backwards, it's red inner to green outer. (Edited in my post but I can't figure out how to edit the title of the thread?)

Anyway, I have very little stray light, and this pervades my imaging with only my SCT. I have a Stellarvue SVX080T-3SV and no issues with gradients with images taken with that with location, ambient light, all other parameters identical. The only difference between the setups is that I use an OAG with the SCT, and a guide scope with the refractor, but I am pretty sure there aren't any substantial light leaks with that. I live in a Bortle 2.7 neighborhood and I get my neighbors to turn off their outdoor lights when I am imaging. I did have this same issue on one image shot at a Bortle 2 site with no surrounding light sources at all. Pretty sure this is not a stray light issue.


This is 19.5 hours (+/-) of exposure.

https://drive.google...iew?usp=sharing

This image is processed as follows to specifically highlight the issue:

-Stretched in Curves
-RGB aligned in Levels
-Stretched a bit more in curves
-Saturation turned way up to show issue prominently

(Finished image here to show what I was able to do with the data ultimately... https://www.astrobin.../full/ms0nni/B/ )
 



#4 Michael Covington

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Posted 15 August 2020 - 07:32 PM

I see what you mean.  What is your flat-fielding process?



#5 AnthonyQ

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Posted 15 August 2020 - 08:37 PM

I see what you mean.  What is your flat-fielding process?

Two layers of T shirt on led tracing panel at lowest illumination resting on front of optical tube.

 

Histogram just left of center, exposures typically .5-1 second. 45-50 flats each night.

 

I am using DSS, all frames are registered and stacked  as Median Kappa Sigma Kappa 2.00,  5 iterations 


Edited by AnthonyQ, 15 August 2020 - 08:37 PM.


#6 Michael Covington

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Posted 15 August 2020 - 09:20 PM

I am just wondering if the gradient is present in your flats.  Also if you have an internal reflection of some kind.



#7 AnthonyQ

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Posted 15 August 2020 - 09:26 PM

I am just wondering if the gradient is present in your flats. Also if you have an internal reflection of some kind.


Scope is flocked by Optic Wave Labs.

The flats may play into this, as it seems that the gradient is shaped like vignetting, but I wouldn’t have any idea what to change?

#8 Michael Covington

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Posted 15 August 2020 - 10:13 PM

Next step is to see if there is a color gradient in the flats.



#9 sharkmelley

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Posted 16 August 2020 - 02:38 AM

How are you calibrating i.e. how are applying your flats?  What software are you using? Are you supplying bias frames to the calibration process?

 

Mark


Edited by sharkmelley, 16 August 2020 - 02:39 AM.


#10 AnthonyQ

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Posted 16 August 2020 - 02:47 AM

How are you calibrating i.e. how are applying your flats? What software are you using? Are you supplying bias frames to the calibration process?

Mark


Info on flats posted above. As I mention, I use DSS.

I am using both bias and dark flats.

Edited by AnthonyQ, 16 August 2020 - 02:49 AM.


#11 Tapio

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Posted 16 August 2020 - 02:54 AM

It looks like a flat issue.
Either it's the tracing panel color balance issue (test of sky flats or better flat panel) or it's a processing issue (what calibration frames are used etc).
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#12 Michael Covington

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Posted 16 August 2020 - 08:52 AM

Yes, I'm thinking there's a color gradient in the flats somehow, or maybe the flats are overexposed, so that in the brightest part of the image, one of the three color channels is maxing out or going nonlinear.



#13 AnthonyQ

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Posted 16 August 2020 - 07:18 PM

Sorry for the delay in response, and thanks to everyone for your thoughts and willingness to help.

I am pretty sure flats are color neutral? Certainly the stacked Master Flats from DSS are only grey scale...

My flat sub's colors vary depending upon how many t-shirt folds or focal length between my lenses. (Done to keep exposure times around .5-1 second.) The SCT reduced to f/7.2 (that's what I actually plate solve at with the .63x reducer) is blueish green, (Example linked below), the 80mm f/6 refractor reduced to f/4.44 is yellow and I recently shot C/2020 F3 NEOWISE with a 200mm Asahi SMC Pentax lens at f/5.6 and those flats were orange-ish. There are no color gradients in the flat subs, only brightness gradients. I checked by over saturating in Photoshop, and it's all one color, just varying brightness. I attribute the color to the t-shirt not being perfectly white. BUT, I reiterate, the Master Flats show no color data, and it has been my understanding that flats are color neutral.

THIS IS A KEY FACT- I do not have this issue with any lens except the SCT with the Starizona reducer corrector! (I have not tested the SCT without the FR/CC.)

Single flat sub with SCT and FR/CC: https://drive.google...iew?usp=sharing

 

Master Flat from DSS, brightened in a basic WIndows photo program to show image better: https://drive.google...iew?usp=sharing

 



#14 AnthonyQ

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Posted 16 August 2020 - 07:22 PM

In-camera histogram in response to thought that flats are over exposed...

https://drive.google...iew?usp=sharing



#15 AnthonyQ

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Posted 16 August 2020 - 07:28 PM

And in camera histogram by channel, showing it meets DSS's criteria for acceptable flats...

https://drive.google...iew?usp=sharing

 

"Is it possible to use colored flat frames?

The short answer is yes.
The overall tint of your flat frames is not really important because DeepSkyStacker is processing each channel separately and is applying the flat frames to each channel accordingly.
Of course if your flat frames are fully red only the red channel of your light frames will be properly calibrated but otherwise as long as as the peak in each channel is between 1/3 and 2/3 of the maximum you should have no problem using slightly colored flat frames."



#16 Michael Covington

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Posted 17 August 2020 - 08:20 AM

OK, it's not overexposed flats.  I'm puzzled.

 

If it's confined to one lens (the Starizona reducer), maybe an internal reflection in that lens.  I don't know.


Edited by Michael Covington, 17 August 2020 - 08:21 AM.


#17 sharkmelley

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Posted 17 August 2020 - 08:57 AM

I tried searching this and had no luck, but I am sure it’s been discussed before. Feel free to redirect me if you know of a previous discussion.

Magenta to green gradients - it's beginning to look like it may be a similar problem to what I've experienced with both my Sony A7S and Nikon Z6:

Sony A7S Purple/Green Swathes

Colour Matched Flats - A Case Study

 

I'm not 100% sure of the cause but flats matched to the hue and intensity of the night-time sky seem to solve it.

 

Mark


Edited by sharkmelley, 17 August 2020 - 09:07 AM.


#18 Michael Covington

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Posted 17 August 2020 - 04:40 PM

Directional color sensitivity of the camera pixels/Bayer matrix?



#19 AnthonyQ

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Posted 18 August 2020 - 12:41 PM

I called Starizona to research whether the FF/CC could hsve anything to do with this.

After a bunch of discussion and experimenting with Steve there I think we have this narrowed to a combination of vignetting and amp glow that is not being well handled by DSS.

My theory as to why I am not seeing this on my other scopes/lenses is a combination of shorter exposure times (faster optics/brighter objects) and colder weather reducing amp glow, and a bigger image circle on the non SCT lenses.

#20 AnthonyQ

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Posted 18 August 2020 - 12:42 PM

And I must note...

Steve with Starizona has gone WAY above and beyond in trying to help me!

Please send these guys your business, I know I will be sending them more in the future.

#21 t-ara-fan

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Posted 18 August 2020 - 05:29 PM

Do you get this with the reducer but not using flats? No ==> bad flats. Yes ==> maybe the reducer, try shooting at f/10. 




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