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  #21  
Old 08-09-2017, 10:19 AM
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Quote:
Originally Posted by Merlin66 View Post
For AP images can we call it "cosmetic noise" rather than refer to the scientific SNR - which is not really used......
Some of us actually do the calculations to optimise data capture, so perhaps we can apply for a license to use the term SNR? Purely for scientific purposes, of course

Quote:
Originally Posted by Camelopardalis View Post
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).
Signal is what's in the pixels, Dunk Take a look at the SubframeSelector script. It calculates a SNRWeight which gives you an idea of the quality of each sub wrt SNR. PI can only estimate noise, not measure it, so it's not perfectly accurate. High SNRWeight isn't always a good thing either. Light pollution reflected off a dirty big cloud will produce a high SNRWeight, and that's why we blink subs as well as measure them.

Don't forget that there's no such thing as a single SNR number for a sub. The SNR will be higher in bright areas and lower in dim areas due to the impact of shot noise.

Cheers,
Rick.
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  #22  
Old 08-09-2017, 10:33 AM
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Quote:
Originally Posted by RickS View Post
Signal is what's in the pixels, Dunk Take a look at the SubframeSelector script. It calculates a SNRWeight which gives you an idea of the quality of each sub wrt SNR. PI can only estimate noise, not measure it, so it's not perfectly accurate. High SNRWeight isn't always a good thing either. Light pollution reflected off a dirty big cloud will produce a high SNRWeight, and that's why we blink subs as well as measure them.

Don't forget that there's no such thing as a single SNR number for a sub. The SNR will be higher in bright areas and lower in dim areas due to the impact of shot noise.
Cheers Rick, yeah I get that the signal is the precious pixel values

I'm assuming the noise estimation is based on the whole sub, and I roll with the premise that it's going to be more or less (within a margin of error) equally inaccurate for all the subs I collect.

Will take a look at SNRweight, I already use Subframe selector, for better or worse...
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  #23  
Old 08-09-2017, 10:41 AM
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ImageJ will give you an SNR figure for the selected area of your image...
AA6 gives similar - average and standard deviation...I would think packages like Maxim would do likewise??

I'm not convinced that any/ many AP imagers actually finalise their image based on a rigorous SNR figure.....
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  #24  
Old 08-09-2017, 05:10 PM
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Thanks Ken.

No rigor intended, more curiosity about when the noise becomes largely unobjectionable. Of course, that's subjective...

I have a few targets in the hit list that are fainter than the usual suspects I've been going for, so I've been looking for a yard stick really. The fainter targets will need more total exposure time to have acceptable noise levels when stretched, so it'd be useful to get a feel for what SNR/noise estimate I'm after.
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  #25  
Old 08-09-2017, 05:11 PM
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My order to use the tools are like this:

AstroImageJ (demosaic, calibrate, align, stack, my version only Canon EOS!):
http://astrofriend.eu/astronomy/tuto...roduction.html

Fitswork (rgb combining, adjust levels, background elimination):
http://astrofriend.eu/astronomy/tuto...-fitswork.html

After that have done the above, a final touch with some photoprogram.


AIJ (AstroImageJ):
If you have an Canon DSLR you don't need the deBayering process because of the demosaicing in my macro for AIJ.

With that process in AIJ you lower the random noise and take away or reduce the static pattern without need of dark and bias frames, works very good.

Noise:
Where to measure the S/N in the image depends of what's important, the strong stars has very high signal and then high S/N. But if you measure the S/N in a nebula or the weak part of a galaxy, then the S/N is interesting to know. Most of the noise comes from background if not narrow band images.

/Lars
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  #26  
Old 08-09-2017, 05:49 PM
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Quote:
Originally Posted by Camelopardalis View Post
No rigor intended, more curiosity about when the noise becomes largely unobjectionable. Of course, that's subjective...

I have a few targets in the hit list that are fainter than the usual suspects I've been going for, so I've been looking for a yard stick really. The fainter targets will need more total exposure time to have acceptable noise levels when stretched, so it'd be useful to get a feel for what SNR/noise estimate I'm after.
Dunk,

Why not make some measurements on a small area of an image you already have that's just around your level of minimum acceptability for noise. If you can figure out how many photons you need to capture in each pixel to get to that point then you can make inferences about future targets on the basis of a few sample subs.

Hint: previews, the Statistics process and PixelMath will do everything you need along with the understanding that shot noise is n^0.5 if you capture n photons (in a single sub or a whole stack) and SNR is n/n^0.5 = n^0.5. You can ignore other sources of noise so long as you're doing subs that are sky limited (and if you're not you should be!)

Ken might not believe me but I do take a fairly rigorous approach to SNR when I'm chasing very faint features. The jets of NGC 1097 would be a good example.

Cheers,
Rick.
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  #27  
Old 08-09-2017, 06:02 PM
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Rick,
I believe you.....
So,what's the SNR in the faint stuff you see as acceptable....SNR>20?
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  #28  
Old 08-09-2017, 06:20 PM
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Quote:
Originally Posted by Merlin66 View Post
Rick,
I believe you.....
So,what's the SNR in the faint stuff you see as acceptable....SNR>20?
Sorry for poking you, Ken, partly tongue in cheek

The incredibly dim R4 jet in NGC 1097 I got to a SNR of just over 9 and convinced myself and a few others that it was visible.

I have a half completed image of IC5148 that I really should finish up. It has a very faint Oiii halo that I have at a SNR of around 14 after 46 hours of 1800s subs. It's quite visible but not pretty.

SNR>20 sounds quite luxurious
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  #29  
Old 09-09-2017, 08:42 AM
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Quote:
Originally Posted by RickS View Post
Dunk,

Why not make some measurements on a small area of an image you already have that's just around your level of minimum acceptability for noise. If you can figure out how many photons you need to capture in each pixel to get to that point then you can make inferences about future targets on the basis of a few sample subs.

Hint: previews, the Statistics process and PixelMath will do everything you need along with the understanding that shot noise is n^0.5 if you capture n photons (in a single sub or a whole stack) and SNR is n/n^0.5 = n^0.5. You can ignore other sources of noise so long as you're doing subs that are sky limited (and if you're not you should be!)

Ken might not believe me but I do take a fairly rigorous approach to SNR when I'm chasing very faint features. The jets of NGC 1097 would be a good example.

Cheers,
Rick.
Will give it a try, thanks Rick
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  #30  
Old 09-09-2017, 04:11 PM
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Originally Posted by Camelopardalis View Post
Will give it a try, thanks Rick
And report back, pls Dunk
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  #31  
Old 09-09-2017, 05:41 PM
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Very interesting discussion.

I tried to follow Rick’s instructions to measure SNR in a 2.5 hour test image of NGC 6744 taken from Paddington.
I measured mean background signal in a few featureless areas in the image and subtracted it from mean signal in a bright, medium and dim areas of the galaxy. Then I multiplied the resulting ADU values by gain so that in theory (I hope) should give me the number of photons in a 2.5 hour stack. Signal is still quite dim in 2.5hr exposure; it was only 27 photons in the bright area, 16 photons in medium-bright and only 1.4 photons in the dimmest parts that I could recognise, giving me SNR of 5, 4 and 1.2 respectively. I need more exposure!
Is the above workflow correct?
Suavi

A link to the image in question (although this stack had a touch of noise reduction applied): http://www.astrobin.com/full/305157/C/?nc=user

It clearly looks very noisy…
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  #32  
Old 09-09-2017, 06:03 PM
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Interesting methodology....
In spectroscopy all we do is to remove the background from the image and then measure the SNR in the result.
I have to say at a SNR <10 then the signal is very very low and difficult to differentiate from the actual data signal.
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  #33  
Old 09-09-2017, 11:51 PM
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FWIW, it looks like many of us have a similar view of what constitutes an acceptable amount of noise in a finished "pretty pic" type astro image. I just did a quick check on 5 good quality images from IIS and a couple of my own better ones - in all of them, the noise level in the darker regions was consistently around 300-500DN RMS in a 16 bit representation. Also interestingly, the same sort of noise level seems to be acceptable in brighter regions - the extra signal gives better SNR, but the acceptable noise still seems to around 400DN(16bit). Would be nice if that could be checked by others on their true 16 bit data, rather than me using JPEGs off the web.

Of course, the signal level in any part of an image can be set at any desired value for aesthetic purposes, so measuring SNR is only of value for comparative testing or for validating theoretical approaches to system design - however, since there is some evidence that "good quality" images all have similar noise levels, measuring just the final noise may well be a useful thing to do as one part of "pretty pic" image quality assessment. Good thinking Dunk . cheers Ray

Last edited by Shiraz; 10-09-2017 at 01:47 AM.
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  #34  
Old 10-09-2017, 04:31 PM
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And report back, pls Dunk
Will do, as soon as I stop being a beach bum and knuckle down at the keyboard
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  #35  
Old 10-09-2017, 04:37 PM
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Ray, yeah the end-result noise is what we perceive in the end, but obviously doesn't factor in the noise introduced and/or reduced in processing which can't readily be reversed to reveal the SNR of the original linear, single sub or stack.

I figure there's going to need to be a baseline for a pre-processed image to ensure decent results. If, say, I stack only 4 subs, it's only good for a sneak peek and not worth processing (no doubt obvious!), but for a given target I'm going to want to figure out a rough number of subs to aim for.

And this is without even considering proportions of L to RGB
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  #36  
Old 12-09-2017, 07:58 AM
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Hi all,

While investigating plausibility of galaxy imaging from our heavily light polluted site I looked more closely at acceptable level of noise in featureless background in a master luminance.

It turns out that for sky-limited subs, measured level of noise follows very closely theoretical predictions, in spite of significant variations in sky glare from night to night in my location. The number of subs can be easily estimated with the following simple formula:

Number of subs = [ St dev (background) / St dev (desirable level of noise) ] ^ 2

I propose that an acceptable level of background noise in a master luminance is at or below 10 ADUs for galaxy imaging – or any imaging when we want to pull out faint bits that are just above the background noise.
In my location and with my 4” f/6 refractor at 1.18” pp, during the darkest nights I get St Dev for background signal of about 220 ADUs in a 5-minute Luminance sub. Therefore, to bring background noise down to about 10 ADUs, I need…

Number of subs = [ 220 / 10 ] ^ 2 = 500 5-minute Lum subs, which is about 40 hours of integration. To arrive at this result more quickly, I either need a faster setup or move to a darker location (or both).
Attached Thumbnails
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Last edited by Slawomir; 12-09-2017 at 08:11 AM.
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  #37  
Old 12-09-2017, 12:47 PM
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Quote:
Originally Posted by Slawomir View Post
Number of subs = [ 220 / 10 ] ^ 2 = 500 5-minute Lum subs, which is about 40 hours of integration. To arrive at this result more quickly, I either need a faster setup or move to a darker location (or both).
And if you had a 5 scope Dragonfly setup, all Lum filters, you might be able to squeeze that in one night. Probably be better to go for a 10 scope Dragonfly instead to make it a more acceptable 4 hours, right?
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  #38  
Old 12-09-2017, 01:48 PM
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Must say I'm loving this discussion. Excellent work Suavi. Might have to come back and ask some questions about how to actually measure all this stuff to apply it to my own gear.
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  #39  
Old 12-09-2017, 02:40 PM
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Quote:
Originally Posted by lazjen View Post
And if you had a 5 scope Dragonfly setup, all Lum filters, you might be able to squeeze that in one night. Probably be better to go for a 10 scope Dragonfly instead to make it a more acceptable 4 hours, right?
I also have a formula for such multi-scope set-ups Chris

actual imaging time = ( amount of money spent on the gear * time spent tweaking ) / (number of telescopes^2 * real amount of money needed to be invested to make it work)


Quote:
Originally Posted by troypiggo View Post
Must say I'm loving this discussion. Excellent work Suavi. Might have to come back and ask some questions about how to actually measure all this stuff to apply it to my own gear.
Glad you find it useful Troy
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  #40  
Old 12-09-2017, 03:06 PM
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You're way ahead of me Suavi I've got a couple of subs open in PI for analysis...
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