Huge green bias

[patraip1]

Hello everybody.
I am a new member of the startools community. I am a beginner in astrophotography but have been enjoying the ride so far. I (try to) image from southwestern Greece from a bortle 5 backyard. Here is my story:

My setup is the following:

-Skywatcher heritage 130p

-Skywatcher AZ GTI mount in alt az mode

-Nikon D5500 prime focus

I imaged M31 and got 100subs of 13 sec each, 25 flats, 25bias, 25darks at iso800 (too high...I know now)

I stacked in DSS with its original settings (camera white balance checked and background calibration checked) and it stacked 79 subs producing an autostack.fts file which I imported originally to gimp and got a not so good result, as I am a newbie at processing

IMG_20211016_100134.jpg (237.15 KiB) Viewed 6543 times

then I came across startools....

After a few tries and having read a lot about the workflow and watching youtube tutorial workflows, using the exact same stacked file as above, I came up with this image and purchased the license.

IMG_20211016_100134.jpg (237.15 KiB) Viewed 6543 times

For this I used a star mask to create a sample in the colour module.

At this point I was quite pleased with the result....

I then came across the suggested DSS settings and decided to restack using all of the suggestions in DSS...

I came up with a stacked image in which the histogram at the bottom showed a huge shift of the green channel to the right. I saved without embeding changes to file as a 32 integer fts file and imported that to Startools.

I followed the same workflow as before..

Autodev, bin, crop, mask out m31 and wipe(not hiting colour) with agressiveness 95 and daf 6, developdev, contrast, Hdr, sharpen, deconv, superstucture, Mask-star mask, colour-sample-mask-clear-invert-keep AND the image flooded with green...

Max RGB was a green screen. The luminance was great with a lot of fine detail but even maxing out the cap green and green bias reduce buttons did not do much. (I obviously skipped the remaining workflow : shrink and denoise)

If anybody has any ideas I would be grateful.

PS : I will try to upload the original files (a raw sub sample, the stacked 32bit fts file and some screenshots) as soon as I get to my laptop tomorrow. Btw how can I upload huge files?

[admin]

Hi Tryfon,

I also replied via email;

A big green bias is normal for many DSLRs (it is a good sign, and usually means that the stacker is not interfering with the coloring any more).

That green bias, however, should/will be removed once you use the Wipe module. You listed it in your workflow, but everything points to this step having been skipped... Can you confirm this step was completed successfully?

If this does not help, would you be able to upload the stack somewhere?

Good file sharing services are WeTransfer, Dropbox, Google Drive or Microsoft OneDrive.

Looking forward to the uploads,

[patraip1]

Ok
Sorry for the delay but was away for the weekend without my laptop....
Here is the stack of 70 subs 13 sec, 15bias and 15 flats at iso 800 with the recommended dss settings (as per this site)

https://drive.google.com/file/d/1njAJEk ... sp=sharing

[admin]

No problem, and thank you for uploading.

I can indeed easily replicate the issue.

It appears something has gone really wrong in DSS, as the red and blue channels contain barely any data. For example, this is the red channel extracted by StarTools' Compose module;

Selection_721.jpg (23.28 KiB) Viewed 6471 times

All pixels are clipped to 0, except the very brightest of stars.

It also appears the little data that is there (only some of the very brightest stars) in the red and blue channels have been misaligned by DSS;

Selection_720.jpg (56.83 KiB) Viewed 6471 times

I have never seen this type of output before from DSS, and I am thinking either the flats or bias frames were loaded incorrectly or are not suitable. You can try to verify this theory by just stacking the lights and see if the color problem goes away.

Do let us know how you go!

[Mike in Rancho]

That is totally weird. I also used DSS and Gimp to look at it. Even DSS lets you know something is wrong, with the way the R and B look in the histogram (a single spike which usually means nothingness, and that is in fact what was there).

Maybe upload and link a few samples of each of your lights and the calibration frames, with which we could perhaps replicate and remedy what is happening?

On the alignment, it's possible some field-shifted subs might need to be culled, or perhaps they didn't align properly due to the weird color channel issue.

I see you are using lights, flats, and bias. Should be fine, I often stack my D5300 the exact same way, unless I need to take darks due to edge glow on the sensor (summertime thing). Whereas your other stacks apparently used darks.

Your DSS settings may also be of help to figure out what happened.

But, two things off the top of my head -- first, are you using the individual calibration subs, or masters that were generated previously? If you changed things, there's a possibility those masters are no longer applicable to the new situation. Second, in RAW settings have you checked off the infamous set black point to zero (SBPTZ)? This is an oddball issue that seems to be persistent, but the DSS guidance is: check it if you are using bias frames, and uncheck it if you are not.

A real mystery here...

[patraip1]

Ok guys thanks...
I confirm that stacking just the lighs eliminates the issue and something is very wrong with my master flat

I am using the master flat and bias frames of the same session. Sorry I erased the individual calibration frames, as I have a small hard drive and need the space. I am using all of the settings ivo recomends on dss. The previous stack was with dss standard settings. I will try to upload everything

https://drive.google.com/file/d/1aI2lcP ... sp=sharing
https://drive.google.com/file/d/159Amov ... sp=sharing
https://drive.google.com/file/d/17tjGrb ... sp=sharing
https://drive.google.com/file/d/1kJlHLO ... sp=sharing




https://drive.google.com/file/d/1TRWsXL ... sp=sharing
https://drive.google.com/file/d/1Csq2nZ ... sp=sharing
https://drive.google.com/file/d/1MlvUxD ... sp=sharing

My rig is untouched , can I take new calibration frames?

[happy-kat]

If the rig is untouched you can take new calibration frames if they are not temperature sensitive. Flats and dark flats are fine to do and so are BIAS files I think. Darks are temperature sensitive.

[patraip1]

Ok guys...
I know my m31 subs are not the greatest as I had tracking issues with my mount. I have since placed a counterweight on my az gti and updated its firmware which has helped tremendously
I just imaged M33 last night, under a 75%moon which was at the time high south. m33 was at 62deg altitude when I started, which in theory is a nogo for altaz imaging, but still it was the only target available through the clouds....
I took 227 subs of 10 secs at ISO 200.
I used the default backround RGB calibration and camera white balance on DSS (I have my white balance on direct sunlight in my camera).
I also used the new calibration frames I took at the end of the session
36 bias @ 1/4000 iso200,
26 flats 1/60 iso 200 with the histogram peaking right at the middle
26 dark flats @ 1/60 iso200.
DSS stacked (interpolation, kappa sigma clipping) 197 images and then I saved to file without applying changes as a 32bit integet fts file.
I processed with ST and apart from not being able to remove the entire light pollution gradient I liked what I got.
I used autodev, bin, crop, wipe (vingetting with 90 agressiveness and 6 daf), filmdev, contrast, hdr, mask - starmask, colour, sample, mask-clear-invert-keep, colour incrase saturation to 270, superstructure, denoise, save as jpeg

m33.jpeg (300.76 KiB) Viewed 6414 times

I am currently restacking with all calibration frames (not masters) with no white balance and no background calibration and will update with both stacks, along with specimens of light and calibration files....

Here is the stack of 203 lights out of 227 @iso200 10sec with flats, bias,and dark flats
with no white balance and no rgb background calibration:

https://drive.google.com/file/d/1wVt843 ... sp=sharing

here are some lights

https://drive.google.com/drive/folders/ ... sp=sharing

bias:
https://drive.google.com/drive/folders/ ... sp=sharing

flats:
https://drive.google.com/drive/folders/ ... sp=sharing

darkflats:
https://drive.google.com/drive/folders/ ... sp=sharing

Here is what I got after processing this stack:

m33iso200sub10secnowhite.jpeg (171.03 KiB) Viewed 6393 times

to be honest it was a quick attempt but I like the one from the original stack better ....AT Least the colour issue seems to be fixed!!!!
Here is the original stack from DSS with camera white balance and rgb backround align:

https://drive.google.com/file/d/1vqklcp ... sp=sharing


Any help with the workflow would be greatly appreciated....
Anyway everyone feel free to take a stab at both.

I also stacked my iso800 m31 13 sec subs with the new calibration frames with no white balance and no rgb background align to see if it will fix the green bias....Here are 64 subs (selected the best) iso800 stacked with bias,flats and darkflats:

https://drive.google.com/file/d/1nTE3RH ... sp=sharing

After processing the green bias is gone but I have problem achieving good colour whereas in my original stack just sampling stars produced very nice colours...

M31NEWCALnowhite.jpeg (494.34 KiB) Viewed 6386 times

Now that I solved the bias problem I would love some guidance with processing the liner stacks because I honestly feel I was doing better with processing the default dss stacked data
Thanks for all the help!!!

[Mike in Rancho]

Hi Patraip1,

Or is it Tryfon? I dunno...

So far I've only had the chance to look at just the last link, the restacked M31. Will look at the other stuff when I can later though for sure.

A few things... I looked up the scope, so a 130mm dob eh? A light bucket.

Even with the short subs, I think you are blowing out many star cores. The ISO 800 is shortening up your well. With that Nikon you are probably going to want to use ISO 200 or 400, which is seems you have already taken steps to correct.

And a little terminology/definitions - it wasn't that you had a green bias, really (which is normal), but rather that somehow your red and blue channels had been mostly erased. Possibly from the use of those master files. Until you gain some experience and learn exactly when you can and can't reuse DSS master files, I suggest keeping your calibration subs and using them in your lists. And if you want to archive them, save all the subs, not just masters. A little USB3 multi-TB external HDD might be a good thing to get for saving stuff.

There are two types of settings in DSS that sound similar but aren't. One is background calibration, of which one option is RGB channels. That is not recommended for ST. Maybe only useful for very extreme situations such as drastic background brightness variations across imaging sessions. The other is Align RGB Channels, which again is something completely different. That option is for aligning the color channels spatially if they don't match, which can happen with things like atmospheric dispersion (stars end up different colors on different sides).

Anyway I did a super quick and heavily binned down version of that M31, and did find some colors at least where stars weren't blown out. At less than 14 minutes, it is awfully short integration, for sure.

Finally, though I cropped them off, the flats didn't seem to be fully working, as there was still vignetting in the corners.

So...a few things to work on, but all good, that's what it's about. I'm not sure this is good enough to be a helpful workflow to post, but I can if you want.

patraip1 M31NEWCALnowhite.jpg (482.24 KiB) Viewed 6374 times

[patraip1]

Hi Mike
Thanks a lot for your effort!!!
That looks way better than mine lol!!
At this point I am just experinenting with ST....
I woukd appreciate some help with wipe
I think the tablet I am using for taking flats may be small so I am not illuminating evenlemy the entire entrance of the scope. Will try again

Thanks for the help