All of my color shots using ZWO ASI585MC Pro have a Green cast

Skye (@skysong)

It would be helpful to know whether this is a new problem with a camera you have used before, or it may be your first time with this camera. It will affect what follows.

Assuming this is the first use of the camera, I would say that it is completely normal for the individual shots to have a green cast, as there are twice as many green filtered photosites in a normal colour sensor. By the time the shots have been debayered, calibrated, normalised and integrated this should be mostly removed. Using 'remove light pollution' and 'calibrate star colors' tools should correct any minor remaining colour issues.

If this is a new problem with a camera you have used before, then ignore everything I have said above. Its a much trickier issue. Worse case is a fried circuit board in the camera. This has happened to me and after much fruitless searching for human errors, software settings, etc, I had to replace the camera and the problem disappeared. Hopefully this is not the case with your camera.

JC

@connor231 ASI585MC Pro, no I used the ASI585MC and Pro for many years, and this particular camera.

@skysong

As you are an experienced user who has used that camera for years, I'd say it doesn't sound promising for your camera. In my case it was a 2600mc that I'd been using for a couple of years, and suddenly everything was green. I've heard that this is a rare but not unknown problem with ZWO colour cameras. I have also heard that the camera can be sent back and the circuit board can be replaced, but I don't know this first hand.

I'm assuming you are using the default tab(0) settings. Otherwise I have no other suggestions.

JC

That could be it, but its just green cast, just slightly too much green in the result.   Some say its the 2 GG in the bayer matrix, some say its the camera is more sensitive in green.  But i think the demosaicing algorithm could be off, or its (NGC2403) actually this color, which I don't think so. 

If its only a bit green, have you tried using the 'calibrate star colors tool' ? I have found that it sometimes dramatically improves colour in my galaxy shots, although sometimes it can seem a bit too blue. But that can be tweaked.

JC

Here's what I mean, the pic on the left was taken with color camera (both ASI585), both processed in APP, and right with Mono.  This is ngc2403.   I think its APP doing this since i've never had an issue with stills or just live stacking. 

Skye

Now that I have seen the image I can see that there is some colour data there, so fortunately my speculation about a fried circuit board is probably not right. However, there isn't much colour data and it does look overstretched (no offence intended). If you wish to pursue this further I think we need some basic information to start.

• what scope was used - aperture and focal length (or focal ratio and focal length if it was a dslr lens)
• how many exposures and what exposure time
• was there much light pollution
• were any filters used

JC

One thing I noticed today as well is ASIAIR has a "white balance" view for its previews and live stacking and without it they are the same greenish tinge, but with it on its white.   And i tried the APP white balance, but it doesn't seem to do anything.  So I think its definitely a white balance issue. 

The scope is an FF107 (same as 107PHQ).   749mm FL, 107 mm objective, f/7 FR, exposures are 60 sec each, and we're doing 200 overall in these images.  for Color images it was Uv/ir filter, for mono is LRGB (80%l in this one).  camera sensor was imx585 in both cases, one color, one mono, both cooled to about -15. 

I'm sure every color camera has this same issue, since it has two G pixels in the bayer matrix (this one is the ASI585MC Pro).  How are those normally balanced. 

Skye

As I understand it the white balance control in APP only applies to dslr raw files, and should have no effect on an astro camera. That's not the issue.

I am still unsure how many exposures were taken with the colour camera, as I read your reply to mean than there were 200 exposures for both cameras combined. It may be helpful to know how many 60s exposures were taken with the colour camera, as per your original question.

At this stage I would be more concerned about the small amount of colour saturation rather than a slight green cast. Is it possible that when saving the stretched image you may have saved a desaturated image by not ticking the 'saturation' box on the right hand panel ?

JC

@connor231 Both were 60s sec/ 200 frame per camera.   Another thing i noticed is in ASiAIR preview photo with white balance off, gives same green cast (on OSC camera only).   So even the initial preview is like this.   Since wb cleans it up, i don't see it as a camera hardware issue, but rather a software that needs to account for the increased green sensitivity and/or double G in the Bayer Matrix, though every camera would have this same issue as well likely.

Skye

As you say - most colour cameras have the same issue of green bias in the bayer matrix. APP normally handles this perfectly. I have used four or five colour cameras over the years and APP was fine - no problem with green bias (except that one time when my camera was fried).

200x60s frames should be adequate to provide reasonable colour, even with a colour camera at f7. The image is lacking in colour saturation, but you did not answer my question about whether the saturation box was ticked when saving the stretched image. At this stage that would be my guess as to the cause. However, with only about 3 hours and a relatively slow scope the image will probably stilll require a saturation boost (either by the saturation slider or in post processing) before any colour bias can be assessed.

If there is a remaining colour bias it can be corrected with the 'calibrate star colors' tool, or by a number of other means.

JC

It's my belief that this green cast that dogs most astrophotography processing is due to not using the colour conversion matrix (CCM) that is a property of the colour sensor. It compensates for the different sensitivities of the colour filters and the fact that each colour filter lets pass some of the other colours, meanwhile an IR filter limits the red end of the spectrum. All colour cameras for photography have their own CCM which can be supplied in the metadata or externally in RAW converter software like Photoshop or Lightroom. I believe that most astro cameras dispense with the CCM as it is invalidated by the custom filters use in AP - for example no IR filter on the sensor. (However, APP was updated some time ago to use the CCM for DSLR type cameras.) So the RGB colour from an astrocam will not be calibrated. The camera driver is usually designed to give a somewhat normal colour balance for the respective astrocam by adjusting the gain of the red and blue pixels.

So I think your sudden appearance of green cast may be due to the camera driver settings being changed somehow. Maybe you updated the driver and it has a problem and needs to be updated again. Anyway, I would make sure the driver is working correctly.

Well I think you've hit it on the head then, the color matrix is supported for DSLR but not for Astro-color cameras.  

One solution might be to move the RGB sliders on the the 0) Color Settings, though I don't see to have the right "mix" on it.  What are you using to get your color camera (not DSLR) white balance? 

My colour is usually reasonable so I finish with star calibration but another setting I forgot about is the "neutralize background" in Normalize tab - or perhaps you have clicked "disable normalization". APP remembers your settings usually and so if you were playing around and forgot to put things back to the way they were, you will have problems you didn't see before. Best thing to do is reset APP to default and start over - I'm pretty sure "neutralize background" is on by default.

EDIT: use the license and settings button on the top left of APP window to reset the settings.

I was finally able to make some progress on this by applying custom color levels from GIMP (by taking output of APP after integration) for this specific camera in GIMP, or R=1, G=.94, and B=1.1, I calculated these via using some basic algebra and looking at the provided color QE charts on Zwo Astro.  Using Red as a base (=1.0), i calculated the levels of adjustments needed by looking at the graphs provided, where G = 90%, R=85%, and B= 77% on the graphs.   So solving for this, the adjustments needed (to leave R=1) were G=1/90 = x/85, x=.94, and B=1/77= x/85, x =1.1, then I used these values in the White Balance adjustment for Gimp.  These might work for APP too, I'll have to try it. 

Don't worry about the lights for the moment. If you load up a flat in APP does it look completely green? If so when you tick the 'neutralise-BG' box at the top right of the main screen does most of the green disappear? If so, this is perfectly normal behaviour and your green cast is simply because that check box, which controls what is shown in the preview window (and exported), was not ticked for some reason.

This is totally normal, you need to either do an unlinked stretch, not sure APP has this option, (although it should) or run the background neutralisation 

As I said above - white balance on load is not the issue. Many of us have used APP with colour cameras, and APP handles the green bias in these sensors very well.

JC

Hi Skye @skysong and others in this topic,

Thank you all for advising Skye here.

The most important reasons for getting green casts in results with OSC cameras that need to be debayered/demosaiced is simply twofold.

It is both the fact that the Color Filter Array on the sensor has twice mroe green than Blue and Red pixels. So the quality in the green channel will always be higher ...

And almost all consumer and regular astronomical camera's are simply more sensitive in the green wavelengths.

To correct for this, there are many ways. You can initially correct for this with a preset color white balance, like the R,G,B sliders in 0) Color Settings.

But you can also simply not do this and trust that colors become correct with calibrating the sky background and then performing Star Color Calibration if it is broadband RGB data.

Sky, i will look at your data still and will get back to you on this 😉

Mabula

I'm sure these are some of the reasons; interesting that 2600MC comes out balanced from what I hear.  Would be nice to have an auto-white balance feature we can turn on. 

Hi Skye, @skysong

Another reason for have a color cast in your images can simply be the sky conditions, a night with slight moon glow for instance will also give different cast then a moon-less sky. There are many reasons for color casts, really. I used to shoot many years ago with a H-alpha modified Nikon D5100. Most of the time, I had magenta cast instead of green cast, but after background calibration, this was always solved fine.

I am working at the moment on an improvement in 5) Normalize, I think that will improve the sky background neutralization/calibration for  all images after the normalization phase. That really should become the auto-white balance feature  for your integrations as well.

I will also try to implement this improvement in the preview filter. The Neutralize-BG option below the histogram on the right side should work better then as well.

Mabula

@mabula-admin Well I hear you, though using "ASI585MM mono (with wheel)" vs "ASI585MC color" on same skies reveals "perfect" blacks and color balance (for MONO) WHILE "color camera" needs a lot more help color balance and backgrounds.  VERY excited about the NORMALIZE calibration feature you mentioned.  I've used the Background Calibration tool with the "rectangles" but it still not enough to get you there (i.e., to get you close MONO result).   Yeah the CBG tool can make the 'background' BLACK which is important, but the OBJECTS are still green.