star repair in PixInsight - part 2

While reprocessing old data, I came across a very instructive incidence of star over exposure. When imaging under light polluted skies, sky glow is added to any sky signal (stars, nebulae, etc). If the pollution is strong enough to over expose stars, the true star colour data is destroyed. When the sky glow is removed during post processing, this will lead to bright stars having the wrong colour.
For example, say a bright star is slightly blue in colour. Without any sky glow, it would register as RGB 0.8, 0.8, 0.95. Sky glow gives an added 0.3 in blue. This will put the star colour on sensor as RGB 0.8, 0.8, 1.0, since the maximum value for a pixel is 1.0.
When the sky glow is removed during background extraction, the value 0.2 will be subtracted, and we end up with a star's RGB values of 0.8, 0.8, 0.7. The star core has suddenly turned yellow. However, away from the core, the star will still be blue, since these areas weren't over exposed. Therefore the need to correct star colours.
During normal stretching of an image, most of the bright stars will be maximised, and have a final colour of RGB 1.0, 1.0, 1.0. However, if masked stretch is used, the stars will not be saturated, and will have a funny looking core.
Here's an example of an unstretched star, with (left) and without (right) colour repair.

When should star repair be implemented in the workflow?
Of course, before any stretch is applied, the colour must be corrected. However, there is one earlier instance where star colour matters in processing, and that is during colour calibration.
Colour calibration tries to set a white point by looking at all the stars in an image, and apply a calibration scheme that is determined by the average star colour. When the star colour is wrong due to over exposure and background extraction, the white balance after colour calibration will be off. Therefore, there is an argument for applying HSV repair prior to colour calibration. The workflow then becomes as follows:

• cropping of edges
• background extraction (ABE or DBE)
• background neutralisation
• HSV repair
• colour calibration

This image shows the effect of doing HSV repair before (left) or after (right) colour calibration. Due to the skyglow adding mainly blue to the image, several star cores had a warmer colour after DBE. This resulted in the colour calibration routine making the blue stars more intense blue. By doing the HSV repair process prior to colour calibration, stars got a more natural blue colour after stretching.

star repair in PixInsight

When stars start to get over exposed during data collection, their core may become a different colour than the normally exposed outer halo. When the stacked image is stretched using MaskedStretch in PixInsight, stars are stretched less than dimmer parts. This means that stars in the stretched image are not saturated, and can display an odd colour. PixInsight has a script that can correct star cores of partially saturated stars. It is called "Repaired HSV Separation", and can be found under Scripts -> Utilities. The script should be applied just before the first stretch. It will decompose the colour image into H, Sv, and V colour components, and repair the colour values of the stars.
The different components are then to be combined using the ChannelCombination process (which is under ColorSpaces).

The upper part of the dialog box is used to determine which images are to be created, while the lower part is used for repair of the H, Sv, and V channels.For best results, mainly the Repair level parameter needs to be adjusted.
Alejandro Tombolini (who brought this script to my attention), recommends to use the unrepaired V channel when combining the channels. But experiment to find out if the repaired or unrepaired V channel works best. Be sure to select the HSV colour space.

Here's how it affected one of my most recent images, a before and after shot of the Pleiades (M45). Look at the core of the brightes stars. In the original image, the cores arepink in colour, while in the repaired image, they are blue, the same colour as the outer haloes.