How much noise is noisy?

Camelopardalis (Dunk) · #1 31-08-2017, 06:30 PM

Folks,

I've been thinking about noise, how much and how little we want in our images.

I figured a good place to start is a low noise camera. Then we want to stack a collection to reduce the noise further. Personally, I don't like noise reduction techniques as featured in popular image processing software as I feel there's always a trade-off with detail.

So, how much noise do you get from your subs, and how much do you accept in your pre-processed stack?

Just curious...I'm interested in "benchmarking" my process so I know where I should stop

All thoughts welcomed...

Cheers,
Dunk

Merlin66 (Ken) · #2 31-08-2017, 06:39 PM

Dunk,
In spectroscopy we have basic limits....
Less than 50 SNR is very low and only usable if there's nothing else to use.
Above 100 SNR is acceptable and >200 SNR is really required to submit data to ProAm campaigns.

RickS (Rick) · #3 31-08-2017, 07:36 PM

1. You can overcome read noise with subs that are long enough regardless of camera read noise
2. Shot noise is unavoidable and is sqrt(n) where n is the number of photons you detected (per pixel)

That's really all you need to know, Dunk

You use sky limited subs and you stop when the dimmest parts of the image that you are interested in look acceptably clean. Noise reduction is acceptable, IMO, when selectively applied so that data that already has acceptable SNR is not adversely affected.

The only time you get SNR that is consistent over a whole image is when you have a very bright source, like when you take flats. With the objects we normally image there will be areas of good SNR and areas of poor SNR. If you want to see the dimmest areas looking clean you'll need to get a very large amount of integration time under dark skies. A short amount of integration time under very bright skies will also get you high SNR, but this is only useful if you like imaging light pollution gradients

cometcatcher (Kevin) · #4 31-08-2017, 08:11 PM

I think the answer is subjective. Everyone has different "acceptable parameters" and in time even they change.

Camelopardalis (Dunk) · #5 01-09-2017, 08:41 AM

Thanks for your thoughts, chaps... I'm looking through some of my AstroFest data and trying to determine if I need more subs

One of my stacks is claiming noise of 5e-05 in PI. I've never really paid that much attention to the numbers before, more just looked at the image and decided. To be fair, I've usually been time-limited before, so I've probably been of the mentality that more would always be better

sil (Steve) · #6 01-09-2017, 01:54 PM

Quote:

Originally Posted by cometcatcher

I think the answer is subjective. Everyone has different "acceptable parameters" and in time even they change.

I agree here, i've seen many ways to measure it but they all have drawbacks it seems with consistency and accuracy. you can measure the SNR in one program twice and get two different values.

Chasing a perfect SNR or noise-free image is a nitpicking exercise. There are ways and means to reduce capturing noise in the first place (eg understand the different types of noise and their sources). By stacking you are NOT removing noise (not everyone understands this), just pushing it fainter from the Signal you actually want so its perceptually less noticable. If you set clipping values in your software you can more aggressively remove noise along with some signal too. Noise reduction filters will effect signal too in different ways depending on the method used. Keep in mind your end goal for your image when processing and how noise reduction will impact it. Also many processes actually add noise (unwanted Signal ) to your image too. Personally I capture very noisy subs and my final images are fairly noisy too, I try to compensate for being unable to capture the way I want or should so I roll with the best I can do with my limitations. I keep looking at different ways though to reduce the obvious noise for me AP is a constant cycle of learning and experimenting, so how much noise is too noisy? for me when you see NO signal at all, otherwise ever shot is still in draft for me, i can still work on ways to improve it. I enjoy the process.

Camelopardalis (Dunk) · #7 01-09-2017, 11:04 PM

Thanks for your input sil, I think I've been tying thoughts up in knots a little worrying about noise, but I guess it's all a matter of perspective.

Part of the problem is having a 16MP image and feeling compelled that the appearance of noise is minimised even at 1:1...in reality, I don't have a 16MP screen

Atmos (Colin) · #8 01-09-2017, 11:12 PM

It all comes down to what you find acceptable, what you can live with and how much noise reduction you're willing to do

Camelopardalis (Dunk) · #9 01-09-2017, 11:17 PM

See above - not a big fan of noise reduction, as the trade-off bothers me...maybe I'm just using it all wrong

The 1600 is a pretty low noise photon collector, but see above...I think I've got into the bad habit of peeping...

sil (Steve) · #10 05-09-2017, 08:43 AM

Doesn't work for everyone but my approach to most things is examine minutely and ask myself "what's wrong/bad here?" and from there I work on why and how to fix/eliminate it. Work on the broken bits and leave the good alone, its fine already.

Bit like the old adage of a mason and a sculptor making a statue of an elephant. The sculptor will keep adding clay to the pile until its the shape of an elephant, while the mason starts with a block and remove everything that doesn't look like an elephant. different points of view, same end result.

Camelopardalis (Dunk) · #11 05-09-2017, 12:25 PM

Also I think one of the traps that easy to fall into is to do a quick stack and apply a STF in PI...always looks bad

RickS (Rick) · #12 05-09-2017, 01:06 PM

Quote:

Originally Posted by Camelopardalis

See above - not a big fan of noise reduction, as the trade-off bothers me...maybe I'm just using it all wrong

It does require a delicate touch, Dunk

Quote:

Originally Posted by Camelopardalis

The 1600 is a pretty low noise photon collector, but see above...I think I've got into the bad habit of peeping...

Even if it was zero noise, perfect QE photon hoover you'd still have shot noise to contend with.

Cheers,
Rick.

Camelopardalis (Dunk) · #13 05-09-2017, 09:53 PM

Subtlety is a strength, huh Rick

RickS (Rick) · #14 05-09-2017, 09:56 PM

Quote:

Originally Posted by Camelopardalis

Subtlety is a strength, huh Rick

Not claiming it is my strength, of course

Camelopardalis (Dunk) · #15 06-09-2017, 12:26 PM

:d

Astrofriend (Lars) · #16 07-09-2017, 09:33 PM

Hi,
I did an Excel sheet about Signal / Noise relation to do some test. I have also implemented how optic vignetting lower the S/N in the corners.

It's very simplyfied but still it give you a good idea how readout, sky background, object strenght, calibration nosie works together.

You can download the Excel sheet on the page:
http://astrofriend.eu/astronomy/tuto...ng.html#part06

/Lars

rcheshire (Rowland) · #17 08-09-2017, 08:41 AM

...as much as you didn't manage to eliminate through acquisition and can't be removed without smudging the image. Unless you like the watercolour / impressionist look.

In an emergency - ImageMagick...

convert img.in -despeckle img.out

...does wonders...

Merlin66 (Ken) · #18 08-09-2017, 09:21 AM

For AP images can we call it "cosmetic noise" rather than refer to the scientific SNR - which is not really used......

Camelopardalis (Dunk) · #19 08-09-2017, 10:45 AM

Thanks all

Maybe we should call it "film grain" then Ken?

Camelopardalis (Dunk) · #20 08-09-2017, 10:46 AM

Which poses an interesting question/curiosity from me...

How do you calculate the SNR of an image? PI gives an indicator of noise but not signal (unless I'm missing something obvious).