Newb question - Which stretch and how much?

[pakarinen]

How do you choose which stretch function to use and how much stretching should one do? Using JS9 for example, there's a choice of linear, log, asinh, etc. Should I just use whatever looks best for any particular channel? Should the stretched background be as dark as possible?

I tried stretching some RGB files and when I combined them, there was nothing but brighter stars and / or the color balance was way off - too red or blue or green.

Thanks!

[STEVE333]

Hi Wassan - Which image processing software are you using?

Steve

[KathyNS]

I use PixInsight, which has more functions than any human could possibly learn. Sometimes, I will try arcsinh stretch or masked stretch, if I am not having any luck with anything else. Mostly, I just use a standard histogram stretch, followed by careful work with the curves function.

In the linear phase of the histogram stretch (i.e. adjusting the black and white points), i adjust the white point to give 0.001% clipping. I don't generally adjust the black point, since there typically isn't much room for adjustment there. Then, I will adjust the mid point to bring the peak of the histogram to 0.25. I usually prefer my backgrounds to be about 0.125 in the final image, but bringing it up to 0.25 allows me to see faint details that I might otherwise miss. I reduce the background later using curves.

[pakarinen]

Hi Wassan - Which image processing software are you using?

Steve

I've only played with JS9 so far. I do have a copy of FITS Liberator, but haven't really used it.