[AdamJL]

Originally Posted by Sunfish

I think as long as the histogram is not hard up against the left hand side so data is cut then that is fine. Perhaps more details about the camera used would make things clearer. With faint astro the histogram is always going to be over to the left or some parts will be blown out or noisy.

How do you stretch the image though, if you have a tiny gap on the left?
I guess this is where I have to learn processing for astro.

My camera is a 5D Mark IV.

Originally Posted by bojan

Have a look at this link... It might be helpful to you.

Very helpful, thank you!

Originally Posted by middy

Hi Adam,

Your histograms are perfectly fine. You are probably making the mistake of thinking the histogram represents your signal, and I made the same mistake for a long time until I came across an article on the internet that explained it all clearly. The peak of the histogram does not represent your good signal (the stars and nebula), it represents the background. If you think about it the vertical axis shows the count of pixels in the image that have a particular luminosity value. The typical astro image is predominantly dark with a sprinkling of stars and the nebula/galaxy. It makes sense that the histogram is over to the left always because the number of dark pixels far outweighs the number of light pixels. The good bits of the signal, that we are interested in, are all down the right side of the histogram and out along the tail.

Comparing your two histograms, you can see the peak shifts slightly left on the image taken in the darker skies. This is expected because the sky has less light pollution so the background is darker, hence the histogram peaks at slightly lower luminosity values. If you look at the right hand tail of the two histograms you can see the longer exposure in the dark skies now extends out past 64 now because the nebula is a lot brighter and has a lot more data to it. This is all the good, juicy data that we want.

If you plotted the histograms using a log scale for the y-axis you would see it even more clearly. This is the reason they say to leave the right slider (white point) alone when adjusting levels because you can easily drag it too far to the left resulting in much of your good data being set to luminosity 255 (i.e saturated).

Cheers,
Andrew

Hi Andrew. Terrific post, thank you very much for that. I have trouble stretching the histogram in PS with Levels because it's so close to the left, but I'll try Star Tools soon, see if that can help.

Thanks again!
Adam