Boosting star colour - Repaired HSV Separation - Barry Wilson

Boosting star colour - Repaired HSV Separation

Whenever I am processing a broadband RGB, LRGB or combining Ha data with an RGB or LRGB dataset, I use the Repaired HSV Separation script.  This script is found under Script>Utilities.  Information on the script can be found here on the PixInsight forum, forum page.

You will need some experimentation, trial and error, to set the values for the characteristics of your particular ccd and typical dataset so that you can maximise the 'repair' of saturated pixels within the cores of the stars across your image.

The processing step is carried out in the linear stage, after combining the individual R, G and B channels, cropping, carrying a background extraction to remove excessive gradients (eg with DBE) and colour calibration.

It is important to carefully select a small preview area of background sky that is free from gradients or nebulosity or stars.

Linear RGB image

Before commencing the Repaired HSV Separation processing step I find it beneficial to boost the colour saturation by a small amount using Curves.

This step is not critical and you can omit it or choose to boost by a larger degree or iteratively with small steps - experiment.

Repaired HSV Separation script

After carrying out an optional boost of colour saturation, open the Repaired HSV Separation script.

Seelct the 'V - no repairs' option, then set the values for 'Repair level' (this does not need to be exact, set the value you wish for the for the first two decimal places) and 'Max Repair Radius'. You can see in the image the values I set using a QSI683wsg-8 imaging at 2.1"/px.

Then click 'OK'. This will execute the script and new image windows will open.

Script output windows

Opening the ChannelCombination process, select 'HSV' and then select the image windows accordingly, ie for 'H' the window named "your chosen image identifier ending in _H".  Note the final window to select for 'V' is the one ending in ". . ._Unrepaired_V".

The linear Repaired RGB image

To demonstrate the effectiveness of 'pushing' colour into the pixels of the otherwise saturated star core, I have chosen two preview windows of the bright orange star in the bottom right quadrant of the image.

The left hand image above shows the non-STF preview window before carrying out the Repaired HSV Separation step.  The right hand image shows the non-STF preview window after executing the script.

You can clearly see the star colour right across the star profile.

The next step is to stretch the linear RGB image using techniques that will preserve this repaired colour.

ArcSinh and Masked Stretch - two stage stretch process

Step one of this two stage stretch process is an application of the ArcSinhStretch process.  You will need to experiment to find a value of the 'Stretch factor' that suits your taste and data.  For myself I have chosen 15 as a good starting point.  Select the two options 'Protect highlights' and 'Use RGB working space'.

De-activate the STF instance and then apply the ArcSinh stretch by dragging the instance triangle onto the image.

The image above shows the result.

Masked Stretch

The second stage is to carry out a Masked Stretch using a small background preview as a background sky reference: select this in the 'Background reference' box.

I use the default settings in the process.  The result is shown above.

SCNR to remove green gradient

The final step is to use SCNR set to 'Green' to remove any residual green gradient.