Author Topic: New Script: Calculate Sky Limited Exposure  (Read 35643 times)

Offline harist

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Re: New Script: Calculate Sky Limited Exposure
« Reply #45 on: 2010 December 07 11:57:16 »
Quote
With his help I have updated the script to use equation (16)

I'm sorry I couldn't follow up the thread in the last couple of days but I am glad that Charles Anstey was so helpful. I also had unclear points in my mind although I tried to work out his formulas with Excell. Thanks to your excellent (and impressively fast)work I will be able to do a lot more experimentation now. :)

One question though: is there a practical purpose for the 4 hrs (14400 sec) total exposure time (I never get anything good below 6 hrs with my level of LP, usually go for 13 hrs).

Best Regards
Tasos

Offline Sean Houghton

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Re: New Script: Calculate Sky Limited Exposure
« Reply #46 on: 2010 December 07 20:31:52 »
Can those of you with crash problems please try the most recent version on GitHub https://github.com/seanhoughton/CalculateSkyLimitedExposure?  If it fixes the crash I'll post another release here.  This might be something that Juan has to look at with a debug build of PixInsight.


Quote
is there a practical purpose for the 4 hrs (14400 sec) total exposure time

This is one of the reasons the Ereadout models are not always great.  If you have a nice dark background then you end up with a ridiculously long exposure in order to get the sky noise up to 95% of total noise.  Charles mentions this in his paper as one of the reasons not to use that model.

I think it's informative, but maybe not practical now that the Anstey limit I provide includes readout noise in the equation.

I've been considering adding some charting to the script to provide a more intuitive look at what's going on as the sub-exposure length changes.
Sean
Carlsbad, CA
cerebiggum.com

Offline Nigel Ball

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Re: New Script: Calculate Sky Limited Exposure
« Reply #47 on: 2010 December 08 02:47:36 »
Can those of you with crash problems please try the most recent version on GitHub https://github.com/seanhoughton/CalculateSkyLimitedExposure?  If it fixes the crash I'll post another release here.  This might be something that Juan has to look at with a debug build of PixInsight.

Sean

I'm still getting the error on close of the image window with this build

Nigel
Nigel Ball
Nantwich, Cheshire, United Kingdom

Takahashi FSQ-106 at f/8, f/5 and f/3.6 on AP900, Nikon 28 mm and 180mm f/2.8
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ST-10XME, Astrodon HaLRGB
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Offline harist

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Re: New Script: Calculate Sky Limited Exposure
« Reply #48 on: 2010 December 08 06:42:00 »
Hi Sean,

I was getting the error with version 2.0 but it stopped with the latest one (windows 7, 64 bit).

Quote
I've been considering adding some charting to the script to provide a more intuitive look at what's going on as the sub-exposure length changes

with respect to the total integration time would be of great value.

BR
Tasos

Offline canstey

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Re: New Script: Calculate Sky Limited Exposure
« Reply #49 on: 2010 December 10 13:08:01 »
This is Charles Anstey, the author of the paper in question.  I have been trying to find a simpler way to explain what is going on and why sub-exposure length matters.

Let's consider each term separately because they follow superposition.

Target signal : Goes up linearly with time so it depends purely on total time Tgt(t).  So 1 100-minute shot or stacking 100 1-minute shots are equal.
Light pollution signal : Goes up linearly with time so it depends purely on total time LP(t).  So 1 100-minute shot or stacking 100 1-minute shots are equal.  However we can simply subtract this out from the image as a bias.
Light pollution noise : Goes up with the SQRT(time) so it depends purely on total time Elp(sqrt(t)).  So 1 100-minute shot or stacking 100 1-minute shots are equal.

At this point if we had a noiseless camera, there is no advantage to shooting longer sub-frames and the implications are pretty astounding.  When advanced amateurs start using noiseless cameras, astro-imaging will take another huge leap forward in quality.  At that point, DSO imagers will be using 10-30 frames per second just like planetary imagers, collecting as many frames as they want, and then stacking only the best of the best until all the desired detail has been attained.  Look up "Lucky camera" to see the results of such a camera.

However, the current real-world "affordable" cameras all have read noise.  The total read noise in an image depends purely on the number of frames stacked and goes up by the SQRT(#frames)
Read Noise : RN*SQRT(# sub-exposures).  So 100 1-minute shots has a lot more read noise than 1 100-minute shot.

The goal of the formula is to balance the number of sub-exposures with respect to read noise and light pollution.  Reducing the sub-exposure length and having more total frames results in more total noise from the increase in read noise with no gain in target signal, fixed by total time.  Increasing sub-exposure length and stacking fewer frames gains basically nothing because the faintest signal discernible above the total noise has already been limited by the total exposure time and dominated by LP noise so lowering total read noise will not help.

From a practical point of view when you don't know how much total time you wish to spend on an object, shooting longer sub-exposures allows you to keep adding more frames if you decide to keep at it with the only penalty being that if the frame is lost because of bad guiding or other error, you lose a larger piece of time.  Use the formula to determine the practical upper limit of the total time for a given sub-exposure and know that if you are thinking of possibly putting in even more total time, you need to use a longer sub-exposure to be safe.

The problem with John Smith's formula is that it is only valid if you are stacking exactly two frames.  Eventually if you stack enough frames, read noise will become significant enough that for the same total time you would get better results for the faintest signals shooting fewer frames for longer time.  Also the implication is that shooting in LP or during the full moon somehow limits how deep you can image compared to dark skies.  Completely incorrect and an imager can create an identical result under the full moon compared to darkest skies if they image with enough total time.  The increase in total time may be a factor of 10+ but what else are you going to do for that night, nothing?  Seeing and transparency determine the absolute limits of image quality and depth.  You can't stack a bunch of 3.5" FWHM images and wind up with a 2.0" FWHM image no matter how much total time.  Don't skip great nights of seeing just because the moon is out.  The only caveat is the camera has to have enough well depth to cover the LP signal and have plenty of bits left over for the stretch to an 8-bit display.  Cameras with only 8 or 12 bits are likely to be limited by excessive or even moderate LP.

Offline harist

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Re: New Script: Calculate Sky Limited Exposure
« Reply #50 on: 2010 December 13 14:44:11 »
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Use the formula to determine the practical upper limit of the total time for a given sub-exposure and know that if you are thinking of possibly putting in even more total time, you need to use a longer sub-exposure to be safe.

That would require an additional frame in the script UI where subexposure length would be set by a slider and total integration time would be a result (from eq. 17). Sean has already done an excellent job, I don't know if we can ask for more  :-[

Tasos

Offline Sean Houghton

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Re: New Script: Calculate Sky Limited Exposure
« Reply #51 on: 2010 December 16 21:50:19 »
I'll take another peek at the script and see if I can put a couple more tools in there.
Sean
Carlsbad, CA
cerebiggum.com

Offline Simon Hicks

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Re: New Script: Calculate Sky Limited Exposure
« Reply #52 on: 2010 December 17 00:48:49 »
Hi Charles,

Many thanks for coming on the forum and contributing to it. I liked your explanation and it has got me thinking.

I think I have got a couple of points from it. Firstly, the faintest DSO signal is there in the sky superimposed on whatever light polution signal there is, and in fact its also superimposed on the light polution noise. Its not actually swamped by it in some sence that it is washed out and cannot be recovered....its still there, its just a case of subtracting the light polution signal, and reducing the light polution noise (more total exposure time) to a level where it is smaller than the DSO signal. This is where the longer exposures and more of them comes in.

So actually we could all image DSOs during the day as long as we took a huge number of extremely long exposures with a camera that had extremely huge well depth. An extreme case, but it seems to illustrate the point and seems to follow from what you were saying.

Just thinking out loud.  8)

Cheers
         Simon

Offline Nigel Ball

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Re: New Script: Calculate Sky Limited Exposure
« Reply #53 on: 2011 January 01 04:12:19 »
Excellent work Sean   8)

I find the script very useful, I'm using version 2.1

Am I correcting in assuming my ideal exposure should be between E limit (I) and Elimit (II) ?

BTW There is a spelling mistake in the options section Tolerance has only one 'l'  ;)
Nigel Ball
Nantwich, Cheshire, United Kingdom

Takahashi FSQ-106 at f/8, f/5 and f/3.6 on AP900, Nikon 28 mm and 180mm f/2.8
SBIG STL-11000M, Astrodon LRGB, 5nm Ha
ST-10XME, Astrodon HaLRGB
www.nigelaball.com

Offline DarrenS

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Re: New Script: Calculate Sky Limited Exposure
« Reply #54 on: 2011 January 31 11:31:21 »
Excellent work Sean

any chance of adding a few ATIK cameras, 383L 314 ?

I've tried working out the figures and tbh its beyond me.

Darren

Offline Sean Houghton

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Re: New Script: Calculate Sky Limited Exposure
« Reply #55 on: 2011 February 01 20:26:03 »
Hi Darren,

I've added both cameras.  If you add my PixInsight update site to your update repositories you'll get updates automatically - including this one.

Let me know if you have any problems with the update.

http://pixinsight.cerebiggum.com/
Sean
Carlsbad, CA
cerebiggum.com

Offline avastro

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Re: New Script: Calculate Sky Limited Exposure
« Reply #56 on: 2011 February 02 00:27:11 »
Hi Sean,
Excellent work, I added your site in my repositories update and that worked flawlessly.
Your script works fine but when I close it I get this message.
"At address 0000000077AA76FD with exception code C0000005 :
Access violation: invalid memory read operation at address FFFFFFFFFFFFFFFF"
Vista 64 bit.
By the way thanks also to Charles for the very useful explanation about his paper.
Cheers

Antoine
http://astrosurf.com/avastro/
Antoine
Lentin Observatory
http://www.astrosurf.com/avastro/

Offline Sean Houghton

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Re: New Script: Calculate Sky Limited Exposure
« Reply #57 on: 2011 February 02 11:02:39 »
Hmm, that's not good.  I'll double check my references in the script, but I think someone with a debug build of PixInsight might have to take a look at that.  I haven't been able to get my license working on my work machine (a 64bit Windows7 machine) to test it, but I'll give it another try tonight.
Sean
Carlsbad, CA
cerebiggum.com

Offline skoop

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Re: New Script: Calculate Sky Limited Exposure
« Reply #58 on: 2011 November 26 12:20:57 »
Excellent script.
I like to request a small update :)
Any change you can add a "save camera as default" option and remember last exposure? 

Thank you.

Offline blinky

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Re: New Script: Calculate Sky Limited Exposure
« Reply #59 on: 2012 September 30 13:18:31 »
So..... Do I just use a preview from a single un-debayered sub and unstretched sub?  If I do that I get a max exposure of around 2.5mins which is not good :-[  Also how/what is the pedestal value?
I have been taking 600s images in the hope that I can bring out some faint detail but it now seems that 2mins is it and any more is pointless :-\