[RickS (Rick)]

Originally Posted by PKay

I am keen to understand why the chap that writes the code for PI, says that there is a limit.

Hi Peter, are you referring to this comment?

“There is, however, a law of diminishing returns. Statistically, the limit from which you can obtain significant benefit is thirty images.” – Inside PixInsight, Warren Keller

Warren knows his way around PI and produces some lovely images, but in this case I think he's promulgating a myth that originated in the days when astro cameras all had terrible read noise. I even gave a presentation debunking this myth to my astro club

Originally Posted by PKay

Maybe there is a link with the brightness of the target, as compared to the level of background noise?

There is a link to brightness. To get good SNR you need to collect lots of photons. A bright area doesn't need a lot of integration time to get to an adequate SNR because the photons are coming thick and fast!

The problem is that our images usually contain dim features as well. If we want to get good SNR in areas of faint nebulosity or the halo of a galaxy then a lot of integration time is required. This won't make much improvement to the bright features (which do suffer from diminishing returns) but it does matter for the faint stuff.

Originally Posted by PKay

I can feel some nasty maths coming on, something I was trying to avoid.

It is tough to understand without the mathematical basics. Our intuitive ideas about how this stuff should work are often wrong. The maths isn't very difficult but it would be tough to figure out without at least a little background in statistics and algebra.