PixInsight Processing tricks, tips and trips...

[RobertM]

A thread for PixInsight processing tips/tips and things that may trip you up.

[MarkS]

How to duplicate the IRIS asinh (colour stretch).
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Open a PixelMath process and put "a,r,g,b,l,ls" in the Symbols box.

Edit the RGB/K box and copy the following lines:
a=.0001;
r = $target[0];
g = $target[1];
b = $target[2];
l=r+g+b;     *see comment below*
ls=ArcSinh(32767*l*a)/a;
$target*ls/l

The parameter "a" is the Stretch Parameter and the scale of 32767 in the ArcSinh function means it will have the same effect as in IRIS.  A good typical value to use is 0.0040   Now run this process against your image.

There is one "gotcha to be careful of - the centres of saturated stars may go dark - the same thing occurs in IRIS.  So one extra step is required in PixInsight:

Run another PixelMath, this time with the formula $target*1.5 for example (adjust the scale factor as required) and untick the "Rescale result" box.  This will brighten the image and undarken the star centres.  However, I bet there's a better way of performing this second step.

[Later edit:  Ivor pointed out to me that the line l = r+g+b really ought to be l = (r + g + b)/3 ]