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Old 26-09-2015, 05:30 PM
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Originally Posted by codemonkey View Post
I'll have a play later and see what I can come up with :-)



Yep, SNR was measured on unstretched data.

I combined 40x1min and 8x5min and found that the former had better SNR. That's not what I expected to see either. Not sure which looked better, I had difficulty eyeballing them. Might post up some samples a bit later.

.
suggest you have a close look at exactly what you are measuring Lee - there is no way that you will get better SNR from more reads and the same overall exposure. The theory has stood the test of 40+ years of EO system design and eval - it is not likely to be wrong now.
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Old 26-09-2015, 05:42 PM
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What Rick wrote makes sense to me. I have found that with my camera (4e read noise) there is not much benefit in going longer than 15 minutes (with narrowband imaging) for the fainter DSOs, also due to shallow wells and sky glow. For bright DSOs, such as Carina, 1 to 2 minute subs with narrowband filters is really plenty with my camera.

Maybe the next generation of sensors for low light applications (that will also be affordable) will have near zero read noise? That would be very nice.
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Old 26-09-2015, 05:56 PM
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I imagine on bright objects where the SNR is immediately far higher you can go with much shorter sub times. The only reason to do so might be if your exposure times are short enough to allow you to pick through the seeing - basically as the planetary guys now do with fast sensors.

It wasn't that long ago the stacking video frames for planetary work was cutting edge mind you.....

Last edited by RobF; 26-09-2015 at 07:48 PM.
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Old 26-09-2015, 06:05 PM
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Yeah, the original person I saw doing the tiny exposures was working with low read noise uncooled cmos cameras. I saw another guy who got a good result (only saw the one) using a camera which, according to what I could find during a google search, had in the area of 10e- of read noise, which made me wonder what I could do with a 5.4e- cooled ccd.

So it looks like Ray was on the money and this was a measurement error. Craig Stark calculates SNR in his article by mean / stddev, which is exactly what I did. However, just then I stacked without any normalisation, and without any rejection (so a straight average) to see what that would do to the SNR, and at that point I saw a stddev that was 4x the mean... so I don't think I can trust these numbers.

So now what I don't understand is, when read noise limited, why would fewer, longer (but still read noise limited) exposures always be better than more, shorter exposures?

The whole reason we stack images is because noise in random but the signal is "more constant" (obviously poisson noise comes into it) so we can "average out" the noise, so I don't find it that intuitive.
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Old 26-09-2015, 06:11 PM
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Originally Posted by Slawomir View Post
What Rick wrote makes sense to me. I have found that with my camera (4e read noise) there is not much benefit in going longer than 15 minutes (with narrowband imaging) for the fainter DSOs, also due to shallow wells and sky glow. For bright DSOs, such as Carina, 1 to 2 minute subs with narrowband filters is really plenty with my camera.

Maybe the next generation of sensors for low light applications (that will also be affordable) will have near zero read noise? That would be very nice.
I've got pretty dark skies, so for me, I think PI's CalculateSkyLimitedExposure tool gave me very high numbers... might have been 30 or 40mins for NB. I should check that.

ZWO already have a CMOS camera that's apparently producing images with 0.75 - 1.5e- read noise. According to a post I saw on CN they're also bringing out cooled versions, I believe in the next few months. Interesting times ahead!
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Old 26-09-2015, 07:39 PM
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Hi Lee,

Thank you for mentioning the CalculateSkyLimitedExposure tool. - I have not used it before. Below I have collated suggested optimum sub exposure time from various sources. Relevant data was extracted from a 15-minute 3nm Halpha sub collected with QSI 690 in Milton near Brisbane's CBD.

1. PI CalculateSkyLimitedExposure tool: E-readout limit pointed to 80 minutes, while Anstey limit suggested 15-20 minutes.
2. A method from this article (http://www.hiddenloft.com/notes/SubExposures.pdf)
resulted in a 40-minute optimal exposure time.
3. SGP suggested around 15 minutes.

So anything between 15 minutes up to 80 minutes...wow...

Well, with my reliable yet imperfect mount, with the planes overhead and occasional clouds I think I will stick with what I have been doing so far
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Old 27-09-2015, 06:46 AM
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Glad to have helped :-)

Hard to know which one to trust, isn't it? I'd probably be inclined to favour the one that inspected my data, but still...

As you say, there's tradeoffs here; a lot can go wrong in 80mins. If 15 works for you I say keep doing 15.
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Old 27-09-2015, 07:51 AM
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You could also try this one:

http://www.stark-labs.com/craig/reso...Calculator.xls

It doesn't point to a single optimum sub length but presents a series of charts that you can use to decide based on your circumstances. This is what I used in the charts I posted above.
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Old 27-09-2015, 12:28 PM
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Best of all, it is quite simple to measure your subs to check if they are read noise limited or not:

http://www.iceinspace.com.au/forum/s...d.php?t=117010

Cheers,
Rick.
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Old 27-09-2015, 02:49 PM
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Quote:
Originally Posted by RickS View Post
Best of all, it is quite simple to measure your subs to check if they are read noise limited or not:

http://www.iceinspace.com.au/forum/s...d.php?t=117010

Cheers,
Rick.
Many thanks for reposting about that thread Rick.
I was guilty of ignoring the first time around, and realise now just how helpful Ray's suggested TargetADU approach is!

Results in same ballpark as the Pixinsight SkyLimitedExposure script, but great rule of thumb to be working off to get a feel for any given sky conditions for a given OTA/camera combination.

Last edited by RobF; 27-09-2015 at 04:49 PM.
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Old 27-09-2015, 04:41 PM
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Thanks Rick for the suggestion.

I have also followed Ray's method that resulted in recommended 50-minute subs for my 3nm Ha filter...maybe when I get a better mount

So it looks like we really should be aiming for significantly longer exposures if we want the highest practically possible SNR, or for greater number of shorter subs if we want smooth images, but at the expense of detail...

I think that skilful use of TGV Denoise and the like can help to lower noise from long subs with high SNR if needed, but extracting detail from short and smooth subs might be a completely different challenge.
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Old 27-09-2015, 04:54 PM
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The other major practicality in all this of course is trying to get a decent number of subs for stacking. Most stacking rejection algorithms get better as you have more subs. I don't like having less than 7, but preferably many more, for each filter.

So if you're shooting LRGB with a mono camera, that means you need the object to be reliably visible for 28 times your chosen sub length for the period you have available. Most of us with portable rigs are often scrounging 1 to 2 nights maximum at a dark sky site. Nothing worse than setting out with best intentions then having cloud or weather intervene. You may have maximal SNR L,R and G but it ain't going to amount to a nice image until next year perhaps when you manage to get the long Blue subs you missed out on

Does anyone set out with a target number of subs you feel you need to collect, in addition to the important requirements of SNR?
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Old 27-09-2015, 05:14 PM
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Good point Rob,

I do set a targeted number of subs, usually these days I aim for at least twenty 15-minute subs per filter, but ideally go for 30 subs or more (narrowband).

With the weather and setting up the gear, I would collect first about 15 subs per filter, so I have something to play with, and then will be adding more subs weather and time permitting.
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Old 27-09-2015, 05:14 PM
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The other major practicality in all this of course is trying to get a decent number of subs for stacking. Most stacking rejection algorithms get better as you have more subs. I don't like having less than 7, but preferably many more, for each filter.

So if you're shooting LRGB with a mono camera, that means you need the object to be reliably visible for 28 times your chosen sub length for the period you have available. Most of us with portable rigs are often scrounging 1 to 2 nights maximum at a dark sky site. Nothing worse than setting out with best intentions then having cloud or weather intervene. You may have maximal SNR L,R and G but it ain't going to amount to a nice image until next year perhaps when you manage to get the long Blue subs you missed out on

Does anyone set out with a target number of subs you feel you need to collect, in addition to the important requirements of SNR?
that's a good point. Lately I have been using an incremental approach - baseline is 30 subs lum and 15 each colour then add subs in increments of about 15 lum and 5 for each colour until the image is deep enough - although lum takes extra precedence if the seeing is good. SGP makes this sort of approach dead easy, by automating everything and keeping track of where the process has got to.

edit: Slawomir just beat me to it - snap!
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Old 27-09-2015, 05:31 PM
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Agree totally about the benefits of SGP tracking how many subs you have on each filter/object. While blessed with a week plus of great weather at Astro Fest this year was so great to set the counters for goal number of subs then watch the counters increment each night.

Particularly since they've added the option to manually increase/decrease collected subs. I find this a big advantage over CCD commander and using manual spreadsheets. Nothing worse than sitting down to do final processing and finding you're short on a subs/exposure for a filter.
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Old 27-09-2015, 09:10 PM
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I have just come across this one: http://www.gibastrosoc.org/sections/...res-calculator

Might be worth giving it a go.
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Old 28-09-2015, 07:06 AM
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Originally Posted by Slawomir View Post
I have just come across this one: http://www.gibastrosoc.org/sections/...res-calculator

Might be worth giving it a go.
I'll be interested to hear how it goes for you, Suavi.

If I had more time I'd dig into the maths of the different models... but I'd rather use my limited spare time for processing
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Old 28-09-2015, 07:51 AM
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Yes, will have to look at that one a bit more closely thanks Slawomir.
I can't help wondering if there should be a field for "is your christian name Rolf"? (if so, multiply required exposure time by 50 )
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Old 28-09-2015, 09:49 AM
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Just reporting back in regards to the calculator.

It returned a value of circa 16 minutes for my setup, so it must be using a similar algorithm to the one that is incorporated in SGP (SGP also usually suggests around 15-minute exposures).

See the attached screenshot for reference.

On a personal note, I have been experimenting with subs ranging from seconds to one-hour long, and eventually I settled for 15-minute subs, unless imaging really bright nebulae. So it is a nice confirmation that some algorithms suggest a similar length of exposure.

EDIT: I have just realised that I did not use calibrated images for calculations

So using correct data - the algorithm suggests 45-minute exposures (not 15-minute ones), confirming the results obtained from the Pixinsight SkyLimitedExposure script and from Ray's method.

I think SGP does not take into account the bias, thus it gives a different result.
Attached Thumbnails
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Last edited by Slawomir; 28-09-2015 at 09:59 AM.
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  #40  
Old 28-09-2015, 11:27 AM
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I had a look at that calculator about 6 months ago, I'd never seen the recommended number of exposures change from 27. I was just putting in random numbers however as I didn't have a CCD camera at that stage.
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