Combining a non-linear RGB file to a non-lineat Lum file?

[dpaul]

I've started a new topic based on feedback from RICKS in the recent discussion topic ''Colour images come out black and white''.

Up until now I have done LRGB integration by working on R, G, B and L separately then stretching and saving them ''separately''. I then used LRGB combination process with the 4 separate non-linear files.

Rick - I did as you suggested and combined RGB linear files into a single RGB file using channel combination. Then I did colour calibration whilst still linear before saving as a non-linear RGB file. Then I tried to combine this with a non-linear Luminance file using LRGB combination and it doesn't recognise the RGB file at all (not a comptable image).

Am I doing something wrong?

Regards

David

[RickS]

Hi David,

The "trick" for combining Lum with an RGB image is to select the luminance image as the L channel in LRGBCombine and disable the R, B and G channels.  Then you drop the new instance icon (the little triangle at the bottom left of the LRGBCombine dialog) on the RGB image and voila!

Cheers,
Rick.

[dpaul]

Thanks Rick,

That's exactly what I tried and it didn't work ''BUT'' I've now discovered why.

I mixed up the files with an older luminance processed image which wasn't star aligned exactly to the new RGB stretched image. So I aligned the frames, did a slight dynamic crop and voila - all now perfect.

Best regards

David

[dpaul]

 Hi Rick,

Attached are the two results - Version A is without colour calibration and each R, G, B and L were stretched separately before LRGB combination. Version B is colour calibrating the RGB combined linear image first. certainly different results and I probably haven't matched the images correctly in terms of intensity (probably the wrong word)

Excuse the quality - they were very poor frames, a windy night and only 4 - 6 frames  (had to delete a lot). The coma correction wasn't correct, neither was focus and each filter was manually changed. I used equal exposure for each RGB and L, presumably that's correct methodology?

The true colour is somewhere between these no doubt?

Thanks

David

[dpaul]

Rick

Here is version A (it didn't load in my last reply)

David

[RickS]

Hi David,

Version A looks closer to the colouring I'd expect.  You can get a lot of variation using ColorCalibration depending on what you choose as the white reference, e.g. you could use the stars or you could take the galaxy itself.  I almost always use PhotometricColorCalibration for RGB images these days.  It produces consistently good results for me...

Cheers,
Rick.

[dpaul]

Hi Rick,

I tried photometric colour calibration but on this occasion it had failed. I generally can get this to work about 2/3 of the time but sometimes it doesn't.
However I just persisted again and I got it to work by upping the log sensitivity to mid range (not sure if that's good). Everything was ok including pixel size, focal length, etc. I also added noise in advanced.

Anyway, after it did work the result is as attached which is between the other two I showed before. Incidentally when using colour calibration method I had used a preview frame in a dark starless part of the frame as the white balance??

Thanks for your help Rick

David

[RickS]

Hi David,

That's looking reasonable.  I'd bump up the saturation but I always do that   PCC has worked reliably for me but I always plate solve my registration master image and use that as the source of the position data for PCC.  A background preview probably isn't a great white reference.

Cheers,
Rick.

[dpaul]

Thanks Rick

I did push up the saturation a little but could have done more. As mentioned previously, the raw was quite poor so I'm going to get at least 20 frames per filter in future and use linear fit integration, etc.

Best regards

David