calibrate star colors

Hi Hans @hans_van_overzee,

I think it's safe to say, that the red color is not gone. Probably the initial star color calibration settings need to be adjusted a little bit. Have you played with the settings?

Have you checked enabling the saturation in the preview filter?

If you share your data, I will have a look at it 😉

Mabula

Ha Mabula,

Thanks for your offer, it is most appreciated.

I will send you the stacked, light pollution and star color Tiff files by wetransfer.

The picture is made with 14 lights of 4 minutes,  6 darks, 20 flats and 50 BIAS frames.

Thanks a lot!

Hans

I just noticed something I use een light polution filter on my camera.

I did not use this in the program.

Does this make a big difference for the quality of the end result?

Hans

Hi Hans @hans_van_overzee,

Thank you for providing your data.

A light pollution filter should increase contrast in your data, especially when used in heavily polluted areas of the sky. It accomplishes this by blocking part of the regular visual wavelengths as you know. This means that not all color of all stars and nebulosity are recorded by your camera since it is blocked by the filter.

This then usually means somewhat different settings in the Star Color Calibration tool. The settings to be used in this tool are dependant on both the used camera and the used filters. If you use another filter, the settings will be different.

This is your data that you corrected for gradients, looks great by the way 😉

The data is very red though, this is partly due to the filter. The star colors, especially blue ones, are not correct, they are blue/green, which is called cyan. So a small correction is needed I think.

This is what I get with the star color calibration, the initial red color dissapears, because due to the filter, red is much stronger as expected with regard to the blue and green wavelengths. We need to correct for this in the settings of the tool:

model Black Body and extinction

star rejection kappa 2.00

blue-red: 0.10 (making it redder !)

B-G vs G-R

slope 0.80

constant -0.15 (reduces green cast)

B-R vs G-R

slope 1,60

constant -0.15

Select star areas not too much affected by the Red Nebulosity of the clear Hydrogen alpha emissions clouds 😉

Let zoom in on a blue and yellow star, left is before calibration, right is after:

What do you think of this calibrated result?

I used these preview filter values on the calibrated data, to give you an idea of what I did with the preview filter:

Kind regards,

Mabula

Ha Mabula,

Thanks a lot for your very long answer.

This will bring me a lot further.

Kind regards,

Hans