Green/purple patches all over my stacked image.

A

atamuli

Member
Hello Experts,

I have a total exposures of 4 nights, totalling 4 hours of good exposures(after discarding 150+ bad frames) stretched over 4 nights. I have collected 40 darks and 40 flats for each night. I have 200 bias frames.

I have followed this tutorial from start to finish: https://www.lightvortexastronomy.co...rating-and-stacking-images-in-pixinsight.html I assure there is nothing wrong in the pre-processing.

I understand that seeing a green/purple cast is normal when I do a STF, but what I think is not normal is the purple and green blotches in the background. DBE cannot solve it, SCNR cannot solve it, "Markarian's Chain with DSLR " tutorial does not seem to work. I am literally lost :)

Please help me understand what I am seeing and how it can be corrected.

Thanks in Advance,
AT
 

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can you post the .xisf of the integration to a cloud file hosting service and post the link here? hard to say much from a screenshot/jpeg.

rob
 
Hello Expert,

Below is the link for the .xisf file. It is 2x drizzled.

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I am unable to remove the green/purple blotch/moth in the background with eiter DBE(ran over 20+ different combinations) or PCC(with background neutralization). I have tried ABE over DBE. All possible combinations that I have tried have failed.

You will be able to see the red cast with the attached STF stretch. I have been trying to solve this isssue for days now wandering over cloudynights and Pixinsight for a possible solution.

I have followed line by line of the pre-processing tutorial in https://www.lightvortexastronomy.co...rating-and-stacking-images-in-pixinsight.html.

For debayer I have used RGGB and VNG.

For your information, I am using a Nikon D7500 and a William Optics Zenithstar 61.

Please assist as I am clueless at this point now.

-Abhijit.
 

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Last edited:
hm, i am not sure what is going on here. there is a repeating blotchy pattern in the integration that seems to me like it could be the result of a calibration problem. can you post one of the subexposures? there seem to be two pointings as well so can you post one of each?
 
Hi Expert,

Please find attached the link for the 21 subs, spread over 4 nights.

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I also do suspect that this may the process of calibration that might have crept in. I am attaching the stacked image from DSS, it does not have that much of a red/green tint in the backgrouond as the one stacked with PI.

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I want to drizzle, but I do not have the RAM,CPU resource to allocate to my VM running windows and want to move away from DSS.

Please let me know if there is some setting that needs to be changed while pre-processing as per the tutorial https://www.lightvortexastronomy.co...rating-and-stacking-images-in-pixinsight.html

Kindly assist.

Thanks and Regards,
AT
 
Hello,

Wishing you a happy new year :)

I did a rerun of the image integration and calibration process. I was doing two mistakes:

1) After following the lightvortexastronomy tutorial, I was subtracting the bias frames from my dark frames subs first. This is not recommended.

In general, I prefer to not bias subtract my darks because the dark current in my sensor is very low and this can lead to underflow issues; meaning any given pixel in my bias frame could have a higher value than the corresponding pixel in my dark frame so when I subtract them I would get a negative value. Since most programs clip negative data this leads to incorrect information in the calibrated and master darks. This isn’t a problem with flats and lights since the background level is generally well above the bias level (this isn’t always the case especially if you are doing narrow band imaging so you still have to be careful).

Credits: http://trappedphotons.com/blog/?p=693

2) I was shooting with a very low ISO of 320, the dark current and readout noise for my Nikon D7500 DSLR is the highest at ISO320. I did runn a CCD Sensor analysis. please find attached the report for the same. https://drive.google.com/file/d/15pqjbpPYehO_XZqienA6GbwmKDhSB-p5/view?usp=sharing

Now, the issue is that still will an ISO of 800, I see the greenish tint in my image. Reflection Nebula M45 is green :). The Attaching a snapshot for your reference https://drive.google.com/file/d/1FU-EVbW1FhqnaCLgD1wsuc3XdtkVHP-9/view?usp=sharing

Is it due to the light pollution or due to the RGGB bayer matrix of my DSLR?

Thanks in Advance,
AT.
 
ok good to hear you figured it out. i think the color looks pretty normal for an OSC image. OSC images usually have a green cast because of the higher SNR in the green channel - there are 2x the number of green pixels as the others and the two green pixels in each bayer site get averaged with one another so it's a little like each frame is 2 green frames from an SNR perspective.

color calibration should take care of the green cast.

rob
 
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