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JWalk
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Reged: 06/06/10

Loc: San Antonio, TX
A Comparison of Exposure Lengths... XMas Tree
      #5667753 - 02/07/13 07:56 PM

I read a very long thread the other day on here and I wanted to see if I could make some type of headway on the subject. I had some great skies again last night and I re-shot the Xmas tree area. I had previously shot it at 93X300sec. I shot it last night with 20min exposures. 9X1200sec. I tried to process them as good as I could.

Here are the results....

9X1200sec -40C

20min exposures

93X300sec

300sec exposures

I hope this might help a bit. I sure enjoy being able to shoot the 20min exposures on certain stuff.

Jimmy


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rainycityastro
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Reged: 03/29/10

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5667764 - 02/07/13 08:08 PM

The difference is quite astonishing. I would have thought they would be very close if not indistinguishable. With 93 subs I would have given the edge to 300 sec exposures.

Are you sure you processed them similarly? :-)

Regards,
--Ram


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JWalk
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Reged: 06/06/10

Loc: San Antonio, TX
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: rainycityastro]
      #5667776 - 02/07/13 08:15 PM

I gave the 93X300 a lot of tries before I posted it. I went round and round with it. The 20min one from this morning was very easy to process. I worked it with my morning cup of Joe before I played.

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neptun2
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Reged: 03/04/07

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5667874 - 02/07/13 09:12 PM

Well both examples are really good but the longer exposures really show more detail. You should have really dark skies to be able to make such long exposures. I can rarely go over 5 minutes without the background to become too bright.

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korborh
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Reged: 01/29/11

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5667898 - 02/07/13 09:29 PM

The 20min one is definitely better (deeper). Thannks for doing this. Do you know the read noise of your camera and the sky flux ?

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mcarroll
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Reged: 08/28/10

Loc: Greensboro, NC
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5667904 - 02/07/13 09:34 PM

Nice comparison. Personally, I'd be thrilled with either image. I think it makes sense that the 1200s image would be deeper. My simple understanding is you would need at least 16x more subs to get similar signal to noise with 4x shorter exposure time per sub.

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neptun2
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Reged: 03/04/07

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: mcarroll]
      #5667911 - 02/07/13 09:40 PM

I think that the problem is not only the SNR. There is faint detail which is simply missing in the shorter exposures and no matter how many of them you stack it just can't be pulled out.

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Rick J
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5667995 - 02/07/13 10:34 PM

Just what I expected. Reading your post before opening the images I thought "This won't be even close" and it wasn't. Your skies are very dark so you need a lot of time to drown read noise in the low sky noise. OSC is considerably worse this way than mono cameras of the same read noise level. The 300 second exposures wouldn't be close to photo(sky)limited in your dark skies so noise would cause weak data. It's always far better to stack images close to being photon limited than those a long way from it when going for something as weak as this area.

Have you measured what times it would take to reach sky limit? It likely will vary depending on which color you measure. You'd want to expose for the one needing the longest time as long as saturation wasn't a major issue.

Rick


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neutronman
Carpal Tunnel


Reged: 01/10/08

Loc: Dallas, Texas
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Rick J]
      #5668114 - 02/07/13 11:32 PM

Thanks for posting this, Jimmy. Yup, about what I expected. I ain't no technical genius, but I have observed that even given the same total exposure, the longer subs will always net more signal and fainter detail. If your skies be good (and yours are), go for as long as your tracking (and saturation) allows.

Now, one might say "gee, then why not do 3 x 60 min exposures instead of 9 x 20 min exposures"? Well, you need a decent number of subs to allow data rejection to work and to get cleaner combined subs.

BTW -- I'm coveting your ASA


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Warhen
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neutronman]
      #5668157 - 02/08/13 12:03 AM

Well done JWalk. As I said to you here in December '...I'm going to make a suggestion- longer exposures, fewer subs. Most who know me well, know a serious math guy I ain't, but I believe the evil 'law of diminishing return' rears its ugly head at ~16 frames, and really flatlines after ~24. With OSC, even at your very fast f/ratio, I'd suggest 600s... I'd have no problem w/ you going to 900s and I think 600s is a good move back. It's more about the lack of significant improvement with a huge number of frames, so we achieve greater impact gathering more signal in the individual exposures.'

Right on RickJ, Neutronman, etc. We simply cannot gain dimmest detail with a gazillion short exposures. And as John Smith pointed out at AIC 2012, which has been reinforced by VPcirc and RickJ on the forum, this is especially true in dark sky where overwhelming read noise is an added necessity.

Edited by Warhen (02/08/13 12:04 AM)


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JWalk
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Reged: 06/06/10

Loc: San Antonio, TX
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Warhen]
      #5668165 - 02/08/13 12:12 AM

Trial and Error. I'm still new to this and it is all about getting to know your equipment. I have experimented with exposures and I had the data from some experimentation with exposures and just wanted to give some raw data on the current subject that has been hot on the forum. I have learned that longer seemed to be better.

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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5668274 - 02/08/13 02:41 AM

Quote:

I read a very long thread the other day on here and I wanted to see if I could make some type of headway on the subject.




One of the main points I tried to make in that thread was that the benefit of longer exposures depends on the parameters of the imaging session. I see your camera listed so I can guess the gain and read noise - but I don't know the background sky flux in e/s. If that is small due to dark skies and the use of OSC Bayer filters, there will be greater win from longer exposures.

I expect the benefit you got from longer exposures will be consistent with the simple noise model once all the parameters are known - but I doubt you really needed to go 20m - though it wouldn't hurt as long as you don't lose subexposures. Longer is always better - to some extent - as long as you don't saturate, lose exposures, reveal flexure or field rotation, have too few subs for a good combine - etc.

You also went from 5m to 20m exposures, which is a factor of 4. A more common comparison would be going from 10m to 20m - i.e. doubling. People tend to expose an amount they are pretty comfortable with and then ask - should I double it. If they can comfortably quadruple it - then there is no real reason not to do 10m exposures in the first place.

Frank


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bilgebayModerator
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5668282 - 02/08/13 02:46 AM

Beautiful Jimmy.

I guess well depth is also important with the longer subs. Your stars are not washed out and they have very nice colors.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5668322 - 02/08/13 04:01 AM

Quote:

I have experimented with exposures and I had the data from some experimentation with exposures and just wanted to give some raw data on the current subject that has been hot on the forum. I have learned that longer seemed to be better.




Stacking is a very powerful tool. You just have to know when to use it. When your subs don’t meet the minimum exposure time requirement, then stacking will not work very well. Your images have demonstrated that simple fact very well. It is all about S/N and it has nothing to do with drowning ‘other’ noise in the sky background or another noise. Do not think for a minute that a dark sky makes your images noisy, quite opposite.


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vpcirc
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Reged: 12/09/09

Loc: Merced CA
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Alph]
      #5668329 - 02/08/13 04:13 AM

Please see above as Warren said. We heard it from several sources. Those experts in CCD's, John Smith, Kevin Nelson, are not sea lawyers giving bad information, one manufactures CCD Cameras and is an engineer, the other writes the software to run them and is all an expert in ccd's, not sure if he's an engineer. Please don't restart this disagreement on JWalks excellent post that is very useful to others.

John has a great set of rules for life

http://www.hiddenloft.com/rule_42.htm


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Dean
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5668383 - 02/08/13 06:37 AM

Quote:

Well both examples are really good but the longer exposures really show more detail. You should have really dark skies to be able to make such long exposures. I can rarely go over 5 minutes without the background to become too bright.



One thing not often mentioned is that shorter exposures don't have as much range as longer exposures making for less subtle detail that is harder to bring out. An interesting test would be to take the 300 sec subs and do sum combines with sets of 4subs and then median combining those. That way the sets will have the same effective exposure length as the 1200 sec subs, albeit with the extra read noise.


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Alph
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Reged: 11/23/06

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: vpcirc]
      #5668909 - 02/08/13 12:27 PM

Quote:

John has a great set of rules for life



Great set of rules. I would only add one more.
Even the greatest teachers fail. Some people never learn.


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blueman
Photon Catcher
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Reged: 07/20/07

Loc: California
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5669017 - 02/08/13 01:20 PM

I did not see what FLI camera you were using. But well depth certainly has something to do with exposure time. If you can do 20 minutes without completely saturating all the stars and cores of objects, then that is great. But you do have to make sure that you get the exposure and saturation levels worked out.

I find that with with a faster scope, I have f/5.4 that I use and a ML8300, 10 minutes is about all I can do before saturation becomes a problem. Of course a dimmer object might benefit a bit, but even 12 minutes can be too long for most bright objects.
Floyd
Quote:

I read a very long thread the other day on here and I wanted to see if I could make some type of headway on the subject. I had some great skies again last night and I re-shot the Xmas tree area. I had previously shot it at 93X300sec. I shot it last night with 20min exposures. 9X1200sec. I tried to process them as good as I could.

Here are the results....

9X1200sec -40C

20min exposures

93X300sec

300sec exposures

I hope this might help a bit. I sure enjoy being able to shoot the 20min exposures on certain stuff.

Jimmy




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Peter in Reno
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Reged: 07/15/08

Loc: Reno, NV
Re: A Comparison of Exposure Lengths... XMas Tree new [Re: blueman]
      #5669077 - 02/08/13 01:39 PM

From JWalk web site, it appears he is using FLI Microline 11002 OSC.

http://www.flicamera.com/spec_sheets/ML11002.pdf

From OPT web site, the QE is a little low for color:

Mono: 50% - Color (RGB): 34%, 37%, 42%

Could low QE be one of the reasons for increasing sub-exposure times?

Thanks for posting comparison images. It was useful.

Peter

Edited by Peter in Reno (02/08/13 01:48 PM)


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cn register 5
scholastic sledgehammer


Reged: 12/26/12

Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Peter in Reno]
      #5669346 - 02/08/13 04:22 PM

All I see are two images that were acquired differently, processed differently and, as a consequence, look different.

There is no scientific data here that differentiates between the exposure length. The difference in processing will mask any difference that exposure time could make.

Just to be clear. I'm not suggesting that, all other things being equal, shorter exposures will make a better image, I don't think they will. All I think is that the difference will be small. This is what the science tells us.

Would the difference be enough to notice? That's the question I originally asked and this doesn't answer it, especially for me under my skies. No amount of tests under pristine skies will answer that of course.

The impression I get is that people want to operate their astro photography on faith and arguments from authority. That seems to me like a religion, not a science.

That's not for me, I'll trust science and practicality.

Chris


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