Black level.. what does it do?

[SimonWhitfield]

Hi,

I am genuinely curious as to what black level does and when it should be used?

I have an Altair Hypercam 183C, which I am having fun experimenting with, however most of my images have a "multi-coloured" background once I have finished pre-processing and got them stacked - fine grain noise that is coloured. As I have no idea what the "black level" setting does, I'm hoping someone will tell me I can use it to control where the imaging system believes "black" starts...

Here is an example of the "colourful background" that I don't think should be there..



Thanks in advance

Simon

[admin]

Hi,

the black level just shifts the values of all the camera pixels up a bit (moves the whole histogram shape to the right without changing the shape). This is mainly useful when using higher gains. If you look at the histogram shape with the sensor covered and slowly turn up the gain you will see that what started out as a very narrow peak near zero gradually gets wider.

If you are stacking or dark subtracting then you should increase the black level to make sure that the widened black peak is not hitting the left hand end of the histogram.

Aim for a histogram that is detached from the LHS like this

good.PNG (15.98 KiB) Viewed 7107 times

Not like this

bad.PNG (12.02 KiB) Viewed 7107 times

The reasoning behind this is that if you are cutting off the histogram at the left hand side, you are throwing away pixel values that should really be less than zero, but you get zero instead. This biases the pixel values away from their true mean and causes problems with post-processing.

cheers,

Robin

[mAnKiNd]

Hi Robin, thanks for the guidance above, but I have an outstanding question.

If one sets their parameters to get a good histogram (like your image above) without dark subtraction, when they then add matched dark subtraction (as in gain, exposure, temperature and offset being the same in lights and darks), then the histogram turns into the bad example above as it now hugs the left wall.

Are you saying that one should then increase the black level/offset (with dark subtraction on) to make the histogram look like the good example again?

If that's the case, then won't the lights and darks be 'out of sync' and not matched with each other, or is one just offsetting the lights+darks as a combo together?

Hope this makes sense!
Minos

P.S. I'm referring to live stacking mode.

[admin]

Live stacking - as of SharpCap 3 and above - allows for negative pixel values when calculating the stack. This means that live stack histogram being against the left hand side is much less critical, as values less than zero are not being thrown away. There is possibly an issue if your histogram peak is precisely on zero when saving the stack as viewed, but that's unlikely unless you have fantastically dark skys as dark subtraction does not remove the background sky glow!

It's perfectly acceptable to use lights with a higher offset/black level than the darks - that will just increase the floor level of the dark subtracted frames by a constant amount which can then be removed post-stacking in the stretch.

cheers,

Robin

[mAnKiNd]

Thank you for that clarification Robin, much obliged.

Minos