GHSv2 vid2v2

A second tutorial on v2 of the Generalised Hyperbolic Stretch (GHS) script by Mike Cranfield for Pixinsight users. In this tutorial, an emission nebula image that fills the frame and spans a large degree of dynamic range is stretched. Starting with the initial stretch of a linear image and continuing with intermediate stretches, I show how to zero in on the parameters needed for the transform equations can be determined. Along the way, the importance of contrast placement is emphasized and how to display dim nebulosity while protecting stars is illustrated. Finally, the ability of the script to stretch saturation to restore or enhance a colour image is shown.
Mike has created a fantastic environment in which to apply the hyperbolic stretch transforms, with many features that are described in these follow-up videos, as well as other videos hosted on the ghsastro.co.uk/ website. You can obtain the script from this website, and can easily install it into Pixinsight. Mike and I think this is an excellent addition to your image processing tool kit - and it's free.

[@عليالحواس-ن9م]

Great videos David and they are really comprehensive and helpful. What I like the most is I get more understanding of how does the histogram work and how to manipulate in a correct way.. Keep going m8.

[@janelubenskyi1177]

GHS rocks!❤ color and contrast and specific localized stretch….a game changer.

[@davidpayne8646]

Thanks Jane.

[@robertbates4892]

Nicely done David. Very easy to follow.

[@davidpayne8646]

Thanks Robert. Mike has added many shortcuts in v2.1 that minimize numerical typing too, that I failed to show. Mike and I are also scheduled for TAIC youtube/webcast on May 29, 2022 if you are interested.

[@rickbria8420]

Very helpful video. Especially the demonstration of the second stretch and using the logarithmic display. Thanks!

[@davidpayne8646]

Thanks Rick

[@desmcmorrow2978]

Another very useful video. Keep them coming! Cheers, Des

[@davidpayne8646]

Thanks again, Des

[@NewAdventuresChannel]

Excellent!

[@davidpayne8646]

Thank you! Cheers!

[@brianvalente]

Hi David - thanks for putting these videos together. Without these, i'm not sure i would have investigated GHS. Do you have any tips (or In the future would you consider doing a tutorial on this) for a galaxy target to tame the bright core and bring up the faint nebulosity structures?

[@davidpayne8646]

Hi Brian, video 3 has your galaxy(ies). M51 is the image. Hope this helps.

[@cliffordmarcus6683]

Have you done something to the colour balance to your original image? my SHO images are overwhelmingly green at that stage....

[@davidpayne8646]

HI Clifford.
Funny, but I never actually finished processing this one...and I am not sure what I had done to calibrate the colours. My normal practice is a three steps.
1) Linear Stage: My normal practice (now) is to use GHS to line up the histogram peaks - in a SHO image that generally means stretching red/blue channel - this still leaves the image quite green, and essentially still linear. The Green channel likely has a larger tail so I also try and smear out the red/blue by stretching out with a substantial b, and bring it them back to the green peak using HP. (Even though technically this makes the image non-linear - I don't worry about it) - If I am using an extracted luminance, after this step is when I extract it.
2) I remove the stars typically and then do an initial stretch to non-linear with GHS using RGB, but you can also leave the stars in. - the resulting image will still be green. However, after the initial stretch I will then stretch the red and blue more, individually. This will bring more colours into the image reds, yellows, aquas, and blues - but still leave some green. It takes a little practice, but you don't want to spoil the colour balances, so I employ HP and LP along with D, SP, and B at this stage.
3) Finally I use the method of Cuiv the lazy Geek ruclips.net/video/UzL_UwcqkfI/видео.html to remove as much remaining green as I want via LRGB combination and reducing green contribution to the colour, and the stretched L I extracted before. I also remove any magentas by invert-->SCNR-->invert method as CUIV outlines.
The other method, is of course, just to use SCNR to remove green, but this is far less satisfactory.
Hope this helps and sorry I can't remember what I did with the NA nebula.
Dave