Stretching

[valleyman]

What is stretching a image and how do you do it?

[ollypenrice]

An 'unstretched' image is in linear form. The faint stuff is faint, the bright stuff is bright, as recorded. This means that lots of interesting faint stuff does not show, though it is there in the data. Stretching means changing the recorded brightness values so as to lift the dark stuff up into the light, so to speak, in order to make it visible. Usually the bright stuff is bright enough so that gets left alone.

I do this in Photoshop at the moment. A tool called Curves gives you a linear graph from bottom left (faint stuff) to top right (bright stuff.) You can grab this striaght 'curve' near the bottom left and lift it steeply, rounding it off near the top and keeping it straight again as it heads up to the bright corner. This is usually done in several iterations. You are aiming to get the faint data up to the noise limit and then calling a halt. Between iterations you can cut back the noise in Levels. It appears to the left of the main peak of the histogram. Here you must be careful not to clip off real faint data, though.

It can be useful to forget the bright stuff when stretching the faint. You can let it become overexposed. Then you open a second copy of the linear image and stretch this one less aggressively, concentrating now on getting the bright stuff just right. You can then combine the two layers controlling which parts of each image will form part of the final product.

Stretching tools are found in various software packages like AstroArt, Maxim, PixInsight (sophisticated) etc.

Olly

[brianb]

Olly has described this just about perfectly. Increasing gamma is a useful way of pulling up "shadows" without burning out "highlights" ... Just remains to add that level stretching can have a very ugly effect on the background if your flat fielding isn't done properly!