Why are my colour images coming out in black and white?

[alacant]

21 minutes ago, vlaiv said:

Could you post your processing

Again?!

21 minutes ago, vlaiv said:

this time background is not wiped.

And still your stars are all white.

Let's agree to disagree. You tell me that the stars are all white. I say they're not:)

Cheers

edit: Just to finish, Here is my version. With my data. Complete with coloured stars!

 

 

Edited April 22, 2021 by alacant

[vlaiv]

11 minutes ago, alacant said:

Again?!

Sorry, I seem to have missed that, if you mean one of these posts:

https://stargazerslounge.com/topic/376040-why-are-my-colour-images-coming-out-in-black-and-white/?do=findComment&comment=4075548

https://stargazerslounge.com/topic/376040-why-are-my-colour-images-coming-out-in-black-and-white/?do=findComment&comment=4075742

In that case, I'll just point out following:

first image:

Second image:

22 minutes ago, alacant said:

Let's agree to disagree. You tell me that the stars are all white. I say they're not:)

I don't maintain this to be the case, and, yes let's agree to disagree.

Similar analysis on my calibration:

As you can see - that is not perfect match either - bluish stars again look too greenish. In order to fix that we need to do 3x3 matrix instead of simple weights.

 

[rojach78]

On 21/04/2021 at 16:12, vlaiv said:

Here is step by step guide and result:

1. As a first step, I opened TIF in Gimp and separated channels into mono images with Colors / Components / Decompose (use RGB model and turn off decompose to layers)
2. I then export these as fits since I like working with fits files
3. I fire up ImageJ (or rather Fiji - which is distribution of ImageJ with preloaded plugins) and load fits files
4. First step here is to crop edges off and remove stacking artifacts (Images to stack, rectangle select, right click and make a copy - whole stack. Close old one)
5. Next step is to bin x2 as image is from OSC sensor and it has been interpolated to this resolution anyway. I like working with smaller but sharper stars. (image / transform / bin - X and Y set to 2 and method average)
6. Next step is to remove background. I use my own plugin for this that removes gradients as well, but you can just make rectangular selection on piece of background and run stats on it. Then use Process/Math/Subract and subtract median value of background. Do this for each image (each channel)

Here are two rounds of my plugin on Red channel - left is gradient and right is what plugin considers background / foreground:

7. Fire up stellarium and find a star that has B-V index of 0.32 (or 0.35) to be your reference star. These stars are roughly white in color and will help us to do simple RGB weighing

8. Select said star using elipse tool in ImageJ and do Image / Stacks / Measure stack

9. Now we have relative weights for our channels - or rather inverse of those. You need to divide each channel with corresponding mean value (use Process / Math / Divide on each sub - remember to remove selection first),
10. If we now again measure that star - we should get roughly the same values. Don't worry if you don't get exactly 1:1:1 as we have different selection and noise is going to skew values somewhat

11. Now we have to "equalize" the subs. This is due to Gimp and how it reads subs. We need to make sure that we have the same max and min values on all three color subs for it to match them when it loads them (to apply same 0-1 stretch on them). Undo selection on all three files and measure them. Set the largest of three minima to be minimum on each image - here -0.1096 and set lowest of the three maxima to be maximum on each image - here 57.4

12. Save each fits file

13. Now we open each of them in Gimp back again. Note that Gimp says it will scale fits values - and since we are opening each channel independently - it would apply different scaling had we not made sure each of them has the same min and max value

14. Do channel compose again (opposite of channel decompose we did in first step)
15. Extract luminance information by now decomposing that RGB image in LAB model. Keep L component and discard a and b

16. Stretch and process luminance to your liking. I'll do basic three step stretch:
- step 1: use levels and bring down top slider so that galaxy core starts to show (apply levels)

- step 2: move middle slider so that galaxy is nicely visible (again apply levels)

- step 3: move bottom slider up to the foot of histogram (again apply levels)

17. You can also apply denoising at this stage or whatever you want - but I won't as this is tutorial for color processing.
18. Switch back to original image and apply one round of levels - but making sure you enter value of 2.2 in middle box. This simulates gamma of 2.2 for sRGB color space

19. Copy stretched image and paste it as layer over original. Set layer mode to luminance.

Here is the final result of this operation:

Now, this is approximate method as it uses only single star (and B-V index in Stellarium is not very reliable). For best results - data should be first corrected with derived color matrix and then calibrated against multiple stars of known stellar class - but that is much more complicated procedure.

 

Thanks for this, I am learning a lot Thanks to you, much appreciated!

[jager945]

3 hours ago, rojach78 said:

Thanks for this, I am learning a lot Thanks to you, much appreciated!

Sadly you are learning many incorrect things and, frankly, abject nonsense. I urge you to get some second opinions before accepting any of this into your workflow.

[vlaiv]

8 hours ago, rojach78 said:

Thanks for this, I am learning a lot Thanks to you, much appreciated!

You are welcome, and yes, by all means - do your own research to confirm what is correct and what is incorrect as has been suggested.

Look at actual examples and data and theory backing those.

That is always the best approach.

[alacant]

15 hours ago, rojach78 said:

I am learning a lot

Regrettably, how not to do it!

Real data from actual cameras and telescopes rarely take any notice of what theory predicts, so it's a big +1 from us for looking elsewhere.

We neither recommend following the method given in this thread. It is misleading at best. Try instead some the myriad of first rate modern software dedicated specifically to astro-image processing. They all have free trials.

If you wish to pursue the methods here correctly, there are many up to date demonstrations, those of special note being Doug German's excellent hands on guides.

Cheers

Edited April 23, 2021 by alacant