Black level adjustment

[BlueAstra]

I'm trying to understand whether i'm doing the right thing with black levels. The first image is a single 3m M42 sub in my LP environment with a Hutech LPF. Next is the histogram of that sub. Then I use DSS on 10 x 3m raw subs. Thats the third image, with its associated histogram.

1 Why are the histograms skewed to the right?

2 Why is the DSS composite image even worse?

3 Is it acceptable to drag the right marker (black level) to the right to meet up with the histogram?

4 Should I do something else?

I would appreciate any advice on the steps to take.

[MartinB]

I don't use DSS but I suspect it is saving your image in a format with a higher bit depth ?32bit floating point. There is no problem with this but the change in format creates a lot of levels to the left of the histogram and makes the background much brighter.

Yes, move the left hand slider over to the right. Leave it to the left of where the histogram starts. It is very tempting to go too fare and cut into the histogram since this increases constrast and can create a more dramatic looking image but this is nearly always a mistake.

[blinky]

'Wot Martin said!

And he is also correct in that Deep Sky Stacker will save in 32bit TIFF format.

[BlueAstra]

OK, thanks for that. Is the overall washed out effect in a sub what you'd expect in a LP environment with a LPF filter? I seem to get a lot for such a short exposure.

[blinky]

I would say that image is more or less what I get.

[roundycat]

the first one looks to me as if it is overexposed. Reducing exposure will automatically move the data to the left. It helps to think of 'dark level' rather than black. You do not want black in your pictures.

Dennis

[BlueAstra]

When you say overexposed, do you mean the exposure is too long in that the LP is dominating or burning through? 3m does not seem a very long exposure time in typical astronomy subs, although M42 is a particularly bright target. What I was aiming at was a range of exposures to get the different areas of the target, and then combine them for an overall correctly exposed image of the whole area of M42. Maybe 15x2m instead of 10x3m?

[sgazer]

I think a LP filter would help. Also, when I in DSS, my images always come out much darker than the original, as long as I don't apply the DSS settings to the saved image (see the check box when saving). Then I process in PS. Also, make sure you are stacking the lights with the average setting, not the max setting.