Light pollution vs nebulosity

[Andy56]

Hi,

Thanks for the previous advice given.

I'm starting to get to understand more, but I have another question because I'm not sure if my challenges are caused by my in/ability or whether the subs are just not good enough.

If I have light pollution and possibly moon light, how can more subs make images better.

My thinking is if background is as light or lighter than the nebulosity how can it be distinguished when processing. Longer/more subs will increase the pollution as much as the target.

I'm considering this after stacking.

Many thanks

Andy.

 

[vlaiv]

Astro imaging is about signal to noise ratio in the image.

You are right that LP adds more signal to the image - but that is signal that is mostly uniform and can be easily removed. What can't be removed is the noise associated with that signal - that is the problem.

Noise associated with any signal (shot noise) is equal to square root of signal itself. This means that it "grows" slower than signal. If signal is amplified by x100 - noise will be only amplified by x10 (which is square root of 100).

With adding more exposures in light pollution - you will increase target signal (good), target signal noise (bad), LP signal (not really an issue since you remove it in processing either by levels or using some sort of gradient removal tool) and LP signal noise (bad). Both target signal noise and LP signal noise accumulate slower than target signal - so more time you spend on target - greater Signal To Noise you'll achieve.

 

[Andy56]

Hi Vlaiv,

Reading you response I think understand the noise better as in your point that the noise accumulated slower than the signal.

Also I've had a eureka moment. If I'm right (ignoring noise) the image will be LP + target so the difficulty with high light pollution is finding the right discrimination level (ie the black level) between the LP and target without losing the fainter parts of the target. 

 

Andy

[vlaiv]

1 hour ago, Andy56 said:

Also I've had a eureka moment. If I'm right (ignoring noise) the image will be LP + target so the difficulty with high light pollution is finding the right discrimination level (ie the black level) between the LP and target without losing the fainter parts of the target. 

Quite right.

If gradient is not strong - then all you need to do is just move black point to proper place. Unfortunately gradient is almost always present so it is better to use some sort of gradient removal tool which does basically the same thing but eliminates gradient as well (different black point in different parts of the image).