Green Overdose

[davidjbillo]

I understand, from reading the forum posts, too much green bias is not an uncommon problem, and ST has ways to deal with it, but I have not typically had images that are too green. Usually, it's the red peak that is furthest to the right, due to some slight light pollution.

Last Saturday night, I imaged two targets, M61+2020jfo, and C/2017 T2 PANSTARRS + M81, M82.

The first, M61, turned out quite well, nothing out of the ordinary, and it had the typical red bias I usually see, easily dealt with.

The other one, comet+M81,M82 had the green peak way out to the right after stacking in DSS. And it's obvious as soon as the Autosave.FITS loads, because many of the stars are green, and the galaxy cores are green. When I get to the colour module in ST, fixing the green always messes up something else...and I get blue highlights where I don't want them, like the comet is blue instead of the cyan expected, and there are blue highlights in M82. If I tone down the blue, then there's pretty much no colour. I can't get that nice yellow galaxy core and blue arms in M81, the the beautiful green/cyan comet.

I'm going to play around with it some more before I resort to posting images or files, but asking if there is some known reason this might happen? I recall that, when stacking M61, I selected the AHD Interpolation, because DSS suggested it, but then when stacking M81,M82 I didn't, but wouldn't think that would cause a green overdose.

Is using masks in the colour module a way to approach it?

Canon T3i on Newt, DSS

[admin]

Hi David,

A green or teal bias is indeed expected from a DSLR or OSC with non-white balanced data.
White balanced data typically has a red or mustard yellow bias.

Firstly, when loading a non-white balanced dataset (recommended!), use the second option. This will enabled Compose mode and re-weight the green channel for a slight signal improvement. Detail (mono) and color will be processed separately (but simultaneously). Color is then introduced in the Color module.

If you follow a standard workflow, there should be no remnants of any bias - Wipe should have taken care of that very early on. Coloring of the remaining detail (with bias subtracted by Wipe) should indeed be corrected in the Color module.

There are many different ways to achieve a good color balance (see here). Using Masks in the Color module is not something that is recommended (except perhaps for the purpose of sampling pixels for white balancing). There is almost never a need to selectively process your coloring. Issues with the Color module a pretty rare - I'd love to have a look at your dataset, as something doesn't sound quite right.

AHD debayering is not recommended - please see here for recommended DSS settings. AHD really has no place in astrophotography as it just introduces artefacts - I'm not sure why AHD would be recommended by anything or anyone...

[davidjbillo]

I think I'm doing, mostly, the right things in ST...I'm using the second option to load, I'm cropping, and binning, and I'm noticing thoughout the development, contrast, and sharpening steps that the image has no colour. I'm sure there are other things I'm missing, or doing wrong in ST, and it's been so long, I probably need to bone up on the standard workflow. I've been going through the modules in the order they are placed in the left side menu, i.e. left to right, top to bottom, skipping what doesn't appear to apply to my data, and mostly using the default settings. I'm sure I don't know how to use Wipe and Banding modules to best effect.

I'm sure this is something that is happening either in the camera, or in DSS. I've been using this camera and DSS for quite a few years, and don't recall ever seeing an image with this green overdose before, and this is apparent before I get to using ST.

As I mentioned, I'm going to try restacking a few more times, work on it with ST, then I will upload some files if I don't seem to be getting anywhere.

[happy-kat]

The image will show the colour when you get to the colour module (though you had a glimpse in the Wipe module)

If using DSS and a DSLR then in settings you could ensure there is no white balance. Then in StarTools colour module select your camera in the matrix if opened the fits file as linear and no white balance.

[davidjbillo]

Quick update: In DSS, started to re-mark the comet to do a re-stack, and noticed the distinct green in the single camera images. So I checked the RAW/FITS/DDP settings, and it was checked "No White Balance Processing". If I check "Use Camera White Balance" then the green overdose goes away and the images look normal.

So, obviously, I have the camera set to some kind of white balance, (Canon t3i), and I must have gone through the DSS settings after doing the M61 set, but before doing the comet,M81,M82 set, and changed to "No White Balance Processing".

It looks like if I just re-stack this set using "Use Camera White Balance" will fix it, but what is the correct camera setting to use for future? I will have to look at what I can control there.

[EDIT]: Checked the camera, and it does not appear there is any way to turn off white balance, only adjust it. It is set 0,0 for "white balance shift" and "neutral" for picture style, although I don't know what that is. (am I gonna have to RTFM?)

[EDIT2]: Although I've used DSS for years, I think only now a version which doesn't automatically white balance, if I recall correctly.

[Burly]

check no white balance in dss , you sample the white in colour module - Mask -auto -stars -shrink -grow twice-keep - sample image flashs three times - mask clear invert -keep ,colour module will balance whole image.

[davidjbillo]

OK, thanks Burley. "No White Balance" in my first stack was what precipitated this post, so I'll load that one again and try the white sample as you describe. I think I tried it once before, but ended up with coloured fringes on stars, so probably did not do the mask correctly.

So, what do I use as the white sample? I must be something I know for sure is white, like the core of a fat star?

I also tried a stack with "Use camera white balance", but then I'm not sure which option to choose when loading that into ST...would it be the first option, "Linear (only)"? since it would have been white balanced, then I shouldn't use option 2, which is the typical DSLR, "Linear, and has NOT been white balanced"...or do I use that one anyhow?

FWIW, "Use camera white balance" seems better, i.e. not the green overdose, but still, the colours aren't right.

[Burly]

When you go into the mask and use -auto _stars -Do -shrink-grow twice what you are doing is choosing the stars for colour module to sample the White point so when you go back into colour module the image will blink three times you then click sample once the white circle bottom right of screen stops spinning then sample taken ,go back to mask press clear then invert mask, that will cause screen to go green so whole image is sampled ,now press keep, back in the colour module the whole image will blink three times and apply the colour balance to whole image , now there,s is a few tweaks you can do top right there is a button ,Max RGB pressing that you can see where the colours are you should have an equal mix of colour you can adjust sliders to suit if say there is to much green then up the slider to reduce green in the green channel etc ,there is also Cap green bottom left of screen ,i usually whack that up to 100% probobaly not so on a comet image maybe ivo could comment on adjusting colour when there is a green comet in an image ,as for choosing correct button on a dslr then second button osc not the linear button first button ,there is on startools website best settings for DSS https://www.startools.org/links--tutori ... r-settings, i have used dss but prefer to use Siril using scripts ,though i haven't used this on a comet image though probobaly can do though Dss has an easy way of doing this HTH and i am sure ivo will comment if there is anything wrong in what i have said.

Regards Dave

[admin]

Color calibration of the non-whitebalanced dataset (opened using the second option in the pop-up) in the StarTools Color module should - at the very least - be able to give you identical colors to the ones DSS came up with. The difference is that the StarTools result should yield less noise, while you also have far more freedom to tweak the colors without impacting the luminance (detail) portion. As of 1.6, you are able to choose your camera's manufacturer-prescribed color matrix using the Matrix parameter in the Color module.

StarTools' Color module typically does a good job coming up with decent default setting, though if your dataset has misaligned channels or suffers from chromatic aberration, this will confuse it.

Perhaps you can post some (scaled down) images that show the issue? The dataset itself may also help.

[BrendanC]

Just to say, this thread might have provided me with a major breakthrough.

I've been choosing the first option when opening files. Auto dev would give me a hot, green mess, even with high quality sample files such as those provided by Jerry Lodriguss at http://www.astropix.com/html/i_astrop/p ... files.html. It would then just get worse with wipe etc.

However, if I choose the second option, it seems to be working better.

Ivo, I think more on-screen guidance on this very first, critical step would be very useful. I've been on the verge of abandoning ST, especially when I couldn't even get the Lodriguss files to work, because all I could see was that ST seemed to make a mess of perfectly good data. If it really is as simple as making the right choice at the beginning, it would help out a lot.

Sorry, didn't mean to hijack this thread, but as soon as I saw the title 'Green Overdose' it rang a very loud bell in my head...