Background calibration tips

[Switch10]

Hi - I'm struggling to find the sweet spot for getting a consistent background value in my images that makes it look natural and not too dark (which to me looks over processed). Also if you put my images side by side the background sky values are always different so im after a consistent RGB pixel value to target.

Does anyone have a target pixel value range for the background sky rhat they aim for when tonal stretching L + RGB images and if so, what is it? Or do you do it what looks good image by image?

I recently processed M81 with a background sky RGB values of around 20-30 and it looked more natural than when ive had it around 10 on other images but wonder how others tackle this?

Thanks

Edited April 16, 2021 by Switch10

[Adreneline]

1 hour ago, Switch10 said:

Does anyone have a target pixel value range for the background sky rhat they aim for

I was advised to aim for 23:23:23 - not sure why 23. I guess it provides adequate ‘blackness’ without suppressing feint nebulosity too much.

HTH

[Switch10]

Thanks - ive always winged it to some degree so that's helpful

[wimvb]

1 hour ago, Adreneline said:

I was advised to aim for 23:23:23 - not sure why 23

This is about 9% of maximum brightness on a computer screen. It is dark but not so dark that you can't seen any subtle details. If you make the background much darker, you risk clipping the data.

 

3 hours ago, Switch10 said:

I recently processed M81 with a background sky RGB values of around 20-30 and it looked more natural than when ive had it around 10 on other images but wonder how others tackle this?

I use levels for the first stretch (histogram transformation in PixInsight), then adjust with curves. For the initial stretch, I aim at a background value of about 10% (25), making sure not to clip any pixels, but in the final stretch, I usually lower this to about 6 - 8% (20) by using an S-curve stretch. A darker background tends to give more punch to the target.

[Switch10]

9 hours ago, wimvb said:

For the initial stretch, I aim at a background value of about 10% (25), making sure not to clip any pixels, but in the final stretch, I usually lower this to about 6 - 8% (20) by using an S-curve stretch.

I follow the same workflow with linear stretching first followed by non linear but normally ended with pixel values of around 10 which just looked unnatural.

Good to hear a value of 20 (roughly) come up twice, that did look way better on my last image.  

Edited April 17, 2021 by Switch10

[michael8554]

I agree that zero is totally wrong, and that 23 is more like it,  but so much depends on monitor settings.

I use 30 to 35.

Michael