Collecting  Photons

1) A detail from the stretched luminance

Images were acquired with my ASA 10" astrograph through Baader Luminance filter. Calibration, alignment, data rejection and sum were performed in CCDstack and Registar. This image is the result of several stages of stretching: the haloes, that were not visible at the time of the initial noise reduction, are now quite apparent: rolling on the image with the mouse will display an inverted and stretched image that will show the artefact quite well.   

2) Creating a mask for the selective surgery

The general strategy I opted for can be summarized in two points: 1) creation of a mask that selects for the haloes around the bright stars. 2) application of the appropriate level corrections to the chroma and luminance channels.

The decision of separating the processing on luminance and chroma is quite important because it allows for a far wider freedom of regulation. It also help in controlling star colour and saturation.

The first step in the generation of the mask, consisted in extracting the star field from the original luminance. This is performed by wavelet filtering removing all the spatial components larger than 11 pixels. This procedure leaves only the stars: if small residue of the extended objects are still visible they can be removed in CS. Then this image is clipped in order to suppress the weaker stars. The threshold is decided in a way that only the stars surrounded by an halo are visible. This mask is expanded with a dilation operator and low pass filtered. The parameters for these operations must be decided based upon the size of the haloes. Finally, the core of the star images must be protected by subtracting a properly scaled star mask. On the left is shown this complex mask. 

3) Application of the corrections.

The correction levels consists in histogram corrections applied through the above mask. I determined that it was sufficient to modify the histogram midpoint to produce a satisfactory halo cancellation. Obviously, some experimenting helps, here. The screen copy on the left shows the various levels applied in Photoshop CS.

The background contains the RGB image. Its correction level is very subtle and helps in desaturating the residual halo artefact and in controlling the star colour. Indeed, a second regulation is applied to desaturate a bit the haloes itself. On top of the RGB layer sits the Luminance layer and its correction level that is applied as a clipping mask (that means that the correction only applies on the level immediately below). This regulation has a strong effect and must be negotiated carefully.  

4) Final result

The original image is shown on the left. If the mouse is rolled over, the corrected image will now appear. It can be noted that the star haloes are far less obvious and that the star themselves are less prominent, while the background nebula is left unaltered. Final adjustments can now be applied to this image.