Deep sky stacker, DSLR and astro colour problems

[jbdave (David)]

So following my last thread with frustration of getting the images due to poor polar alignment (I did get up to 1 min exposures), I am now going through frustration of post processing.

When I stack my RAW files, the image looks great in DSS. The output file however produces a horrible green tinted image which I can't seem to draw much colour out of.

I then tried to stack in jpeg which resulted in a better looking output file than the raw stacked one.

I have been told the blues are due to the dslr camera, but the whole image isn't turning blue. The green is doing my head in when stacking in raw though. The whole image is green stars and nebula on a dark sky. I have downloaded a plugin for photoshop which removes the green tinge, but then I cant really get as much of the colour info out of the picture as I want to.

I also question whether it could be the camera (Canon 80D), as I don't recall having this problem on the couple of shots I took with the 600D. The blues were always present, but i never recall the heavy greens (mind you I was using the older version of DSS then which from memory only stacked jpegs).

Anyway, here are a couple of test shots, using jpeg stacks (as the raw file stacks are terrible).
The first one is with an ED80, the second with an 8" newt (I desaturated the blues in the 2nd image).

Taken from my backyard in suburban light polluted Heidelberg, Melbourne.

[Imme (Jon)]

Are you using the correct bayer matrix for the camera?

[jbdave (David)]

I'm not sure what to do there. I googled about this and read comments about the bayer matrix. I then changed the camera selection from the generic RBG to a Canon 50D (had a guess as they didn't have my camera in it). This was under the FITS file menu however, not the TIFF file, which only gave me the number options.

Do I need to change the numbers in the TIFF section, or alter the output file to FITS so it picks up the different camera ?

If you can't tell, I am confused

[sil (Steve)]

Basically a camera records a pixel in a photo from four sensors with colour filters on them (1x red, 2x green, 1x blue typically), so there is more green being recorded in raw by the chip as the green typically is the more sensistive of the threethe camera processes these four colour values to produce "one colour pixel" in your camera image. This filter is a bayer matrix and usually the four sensor are arranged in a square pattern to make one pixel. The pattern is usually something like RG/GB in a square so the two greens are in opposite corners. There is no standard bayer pattern for all cameras. The four sensors (or subpixels ) each capture at full bit range of the camera sensor, often 12 or 14 bit per pixel these days and the data combined to produce a 12 or 14bit value for that one pixel. and then stored in the raw file or to a 16bit container format. So this means colour cameras are loosing some data due to the bayer matrix when taking regular photos and because twice as much green filtered data is recorded when processing astrophotography you are stretching data a lot so the green colour cast comes forth because its half the data.

As an aside a monochrome camera does not have a bayer matrix so you can get all four sensors recording directly as pixels or used in producing a pixel without discarding any. Which is why they can produce much more detailed and sensitive signal data.

So in your case if the software doesn't have your camera listed, you can try to find online information about the bayer matrix used for your camera and some listed in the software and try to find a match, if you can find the sensor model in your camera and another camera using the same sensor (not merely brand like canon or sony but actual model) and give that a try. Also try searching for your processing software and green cast as most have a tool for removing the green cast for this very reason.

[astro_nutt]

Hi David. I'm learning about DSS, and, when I'm able to do images longer than a minute, spend more time working on it. The first time I tried stacking 5 x 40 second images, it came out greeny\bluey, and it took a while to work out the colour. But I then found out that you can stack the images to collect as much data then process the colours, brightness, etc, on say MSN Photo Gallery.

This is an early image of Carina using a Nikon D7000 on a 10 inch reflector taken from Heathcote.
Cheers!

[rustigsmed (Russell)]

maybe post a copy of the stacked file in dropbox so we can take a look

[Jasp05 (Aaron)]

As a fellow DSLR user (1100d) I too have the issue with the green tint to everything.

After stacking in DSS the images will have a green hue to them. And I too have struggled to remove it in post processing with Photoshop / Lightroom.

Just thought I'd throw that out there that your not alone in your struggle JBdave.

and I will be following along to see if there is an answer to the problem. I personally had assumed it had something to do with the RGGB bayer Matrix of the sensor.

[OzEclipse (Joe Cali)]

I think of DSS as an app for stacking large numbers of images. If you're only stacking a handful of images, up to 10, I suggest you use Photoshop.

The attached image is a stack of 17 images all stacked and processed in Photoshop.

Joe