[Placidus (Mike and Trish)]

It is much, much, much easier than that. You certainly don't need an iterative, fiddly approach. Ignoring the jargon of any particular program, you just need to:

(1) Subtract the brightness of the black background. This is the same as setting the zero point or the black point. The way I do it is to look (with extreme accuracy) at the histogram, and find the darkest value that has a relative frequency above zero. That is exactly the same as subtracting the brightness that causes the "foothill" of the histogram to touch the y-axis.

The point of this step is that it removes any "astronomically meaningless" brightness from the image, without the loss of any meaningful data.

(2) Use an arcsinh curve to make the image as a whole look about the right brightness.

The point of this step is it makes the midtones brighter without burning out the highlights.

Best,
Mike