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JWalk
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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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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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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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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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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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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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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
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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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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
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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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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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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
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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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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
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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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Mike Wiles
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: cn register 5]
      #5669389 - 02/08/13 04:42 PM

Ugh.....

Jimmy, before this devolves into ridiculousness.......thank you for doing this. I thought about these two images all the way down the highway on a long drive today. I think it's very instructive and I appreciate that you took the time to do it.

A few months ago I did a comparison taking two images with exactly the same equipment/settings from a dark site and a light polluted site one day apart. I took advantage of PixInsight's object serialization to process the two images with exactly the same parameters. Two images that were exactly the same in every respect except that they were taken in two different locations one day apart. People complained that it wasn't a good demonstration of light pollution's effect on imaging because I didn't process each image differently to try and compensate for the differences in sky background. Clearly, you're doing something right and I look forward to each new image from you.

Both images are really good - but the 20 minute subs are awesome, especially considering it's just 3 hours of total integration. The color depth alone is an obvious reason to shoot as long as is reasonably possible under your given sky conditions.

Mike


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JWalk
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Mike Wiles]
      #5669656 - 02/08/13 07:33 PM

Thanks Mike and others, All I know is that the 9X20min was considerably easier to process than the 93X5min picture. I'll stick with 20min on objects like this.

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Mr_T
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5669659 - 02/08/13 07:36 PM

Very Good result i would stick with 20 looks great

Les


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blueman
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5669694 - 02/08/13 08:08 PM

Because your camera has a fairly deep well, 60,000 and QE of 37% average color, it will definitely do better with long exposures.
Rule of thumb is take the longest exposure possible without causing undue saturation (hitting 65535 adu on all stars)and without the mount causing issues.
So, if you can do 20 minute exposures with these restrictions, go for it!
Blueman


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vpcirc
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: blueman]
      #5670117 - 02/09/13 03:48 AM

Mike, you can lead a horse to water.....

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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5670143 - 02/09/13 04:33 AM

Quote:

All I know is that the 9X20min was considerably easier to process than the 93X5min picture. I'll stick with 20min on objects like this.




Yes - if you can and there is no downside - you probably should. But I see you use HDR on objects like the Orion nebula. So - I assume you realize that "longer subs result in much better images" is not something you apply universally even to your own setup - let alone others with a range of f/ratios, read noise, and sky quality.

The point of the other thread is to provide quantitative guidelines that apply to anyone's setup, and the model used is the same one used by professionals. Both professionals and amateurs appreciate the benefit of stacking many subs, and realize that although the improvement from each added sub is less noticeable - it continues to improve the SNR.

Note that the other thread had a comparison of a narrow band shot using two different subexposures, and the end result showed little difference. Both that result and your result are consistent with predictions - assuming his exposure is sky limited and yours is read noise limited.

If you want to provide more insight into how your result is or is not consistent with the noise model - please provide data on your gain/read noise/sky background. You refer above to providing "raw" data to help people - but two independently processed images are not "raw" - and a general conclusion derived from them could in fact hurt people if they feel they need long sub exposures when they are sky limited.

Frank


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vpcirc
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670241 - 02/09/13 07:42 AM

It's funny how so many of the arm chair experts never post an image themselves and yet criticize someone who is trying to help others with actual results. Thanks for your work and test Jimmy. I'm sure many have benefited from seeing the difference it makes without getting into a quickly degenerating conversation.

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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: vpcirc]
      #5670300 - 02/09/13 08:42 AM

I posted an image in the other thread and described how it fit with the noise model. I also cited an image in the other thread that complements this one - and is somehow being ignored since it does not support the benefit of longer images in all situations. I also cited images by the OP of this thread that show the benefit of short exposures in a bright object (Orion nebula HDR). Other images were posted in the other thread, including one by the OP of that thread, to support his conclusion.

What I have noticed in these threads is that the people who disagree with the noise model also confess that they don't understand the math behind it. It's as if they don't want to be bothered by it, or are suspicious of science itself - yet they are confident that the model is not suited for realistic imaging situations. Again, despite its being used by professional astronomers in their work.

So - is there anyone who *does* feel comfortable with the model, and *does* understand how it works - who also disagrees with it? The math is pretty simple - only involving a square root.

I think there are people involved in imaging who would be perfectly comfortable with the math and they would be "reachable" by the technical aspects of this discussion. I hope that for them the conclusions are clear and they would recognize good advice from bad in these discussions.

Frank


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orion69
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670345 - 02/09/13 09:23 AM

Frank (and others of course), I have question that has been already addressed in this and other posts but to make things more clear (at least for me): suppose I shoot somewhat dim object and I'm using NB filters. After 10x30 min subs I've got my new stacked picture. Now I'm shooting same object, with same gear, same sky quality (completely same conditions as before) only now I'm shooting 5 min subs x times.

Question: is there some detail on first image that would not be visible on second image no matter how many 5 min subs I take?

Edited by orion69 (02/09/13 09:26 AM)


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neptun2
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670354 - 02/09/13 09:28 AM

Well at least i understand it following way. It is reasonable to go as long as possible if you consider the following:

1. Not to overexpose to a point where you start to loose detail.

2. Not to make the background too bright due to a light pollution.

3. Not to start experience mount induced guiding problems.


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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670382 - 02/09/13 09:41 AM

Hi-

Thanks for a specific question that is also in the context of this thread and how realistic imaging situations relate to the noise model.

The key issue is how much sky noise is in the sub vs. read noise, as others have mentioned above concerning the OP's images. Since you don't fully specify the f/ratio, it could be that the 5m exposure - even though it is narrow band - will have sky noise that dominates read noise - in which case there will be much less win with 30m subs.

But if we take the more interesting case, which I think you are asking about, in which the 30m exposures are way sky noise limited, while the 5m exposures are read noise limited - then there will be a large reduction in noise and a huge benefit.

But your question has an additional layer to it - which, if I am reading it right, is asking - if some faint detail is not visible in the sub at all - is it possible for it to appear after 8, 16, 200 stacked subs? The answer from the noise model, and from professional astronomers is - absolutely yes. The accumulation is a statistical process and even if you don't "see" it in the sub - even faint signal is present in that noise, and the accumulation of many subs will let the SNR of that signal go as the square root of the number of exposures. That's why the Hubble Ultra Deep Field involved stacking 288 frames for some wavelengths. They used the same model we are discussing here.

So - the idea that stacking "runs out of gas" at 16 is not supported by the model and is not supported by professional work - nor is it supported by many of the nice images in CN that show many subs over many nights stacked into a high snr result.

Frank


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Inverted
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670385 - 02/09/13 09:43 AM

Quote:

Frank (and others of course), I have question that has been already addressed in this and other posts but to make things more clear (at least for me): suppose I shoot somewhat dim object and I'm using NB filters. After 10x30 min subs I've got my new stacked picture. Now I'm shooting same object, with same gear, same sky quality (completely same conditions as before) only now I'm shooting 5 min subs x times.

Question: is there some detail on first image that would not be visible on second image no matter how many 5 min subs I take?




That's an interesting question, i keep thinking about. The models seem to assume that photons arrive instantaneously, in large quantities. So, no. However, in practice, while photon emissions rates are "astronomically" huge, what other factors influence how many hit your sensor, from the faintest regions of a nebula, I have no idea. I seem to recal that when I did this stuff more often, I could usually see the detail, in shorter subs was there,but the problem was I just couldn't stretch the image sufficiently without flooding it with noise, to get it into the final image. From my recent trials, that seems to be the case too. If there are photons that just aren't hitting our sensors over a five minute period, then we are getting to some remarkably faint detail. Clearly, whatever the reason may be, some objects do just do better with longer subs, at least if you really want to bump the faintest regions up above the noise floor.


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vpcirc
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670396 - 02/09/13 09:48 AM

Quote:

Frank (and others of course), I have question that has been already addressed in this and other posts but to make things more clear (at least for me): suppose I shoot somewhat dim object and I'm using NB filters. After 10x30 min subs I've got my new stacked picture. Now I'm shooting same object, with same gear, same sky quality (completely same conditions as before) only now I'm shooting 5 min subs x times.

Question: is there some detail on first image that would not be visible on second image no matter how many 5 min subs I take?




Your shorter subs will show far less detail resolution and contrast no matter how many you take. That is the point of Jimmy's experiment. Even with far less total imaging time the amount of faint data he was able to obtain is clearly visible. Secondly, with narrowband, you are never going to reach the sky limit from what Don Goldman has told me. There are bright objects that will get blown out by longer subs such as M 42, but if you stick to short subs, you'll miss this wonderful detail that exists elsewhere. Adam Block has a great tutorial on how to combine short and long subs in CCDStack using missing value to combine. Tony Hallas demonstrates another great procedure in his newest addition to Photoshop processing in Vol 5 using the reveal all layer mask in CS5 and CS6. Jimmy used the Hallas method in his M42.


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JWalk
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5670399 - 02/09/13 09:49 AM

The skies I get to shoot in are very good and VERY dark. They are usually 1.5 seeing or better. In no way did I give the 20min subs more attention than the 5min subs. I gave the 5min subs more time and effort because it needed extra processing to get it to the level that I wanted it at. The 20min stack just exploded with signal after 3 levels stretches.

My thoughts are go as long as your sky allows. You want to grab the dimmest stuff you can. I applaud the guys who shoot from the backyard and have to deal with light pollution and bad seeing. I enjoy seeing great work from not so great skies. As for narrowband, I shoot HA with my canon at home and an FSQ and with the moon out at about 50% I can usually shoot away from it and go 15 min subs. That is about it before the DSLR gets blown out

Orion is shot in HDR style. I did exposures for the core to bring that out. That is a no brainer that we all know. 10min exposures worked perfect for
Me on that area of sky.

Edited by JWalk (02/09/13 09:51 AM)


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Inverted
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: vpcirc]
      #5670415 - 02/09/13 10:01 AM

Quote:


Your shorter subs will show far less detail resolution and contrast no matter how many you take. That is the point of Jimmy's experiment.




In practice, that may be true. as photons are emitted at a high frequency and presumably arrive at a high frequency, they should register even in short exposures though... So, if all the other noise is random, then we should absolutely be able to stack and get the same result. So, it would seem the limitation is likely non-random, error, or noise, or unwanted signal or whatever people want to call it. In that case, it may just be limitations of calibration for example. I don't know, I do know professionals, such as the Hubble operators, do stack pics and get really faint detail, so, it is possible....


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freestar8n
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: JWalk]
      #5670423 - 02/09/13 10:06 AM

Quote:

Orion is shot in HDR style. I did exposures for the core to bring that out. That is a no brainer that we all know. 10min exposures worked perfect for Me on that area of sky.




Yes - you want to expose long, but avoid saturation. The time to saturate will depend on f/ratio, camera sensitivity, and well depth. Therefore - someone else with hyperstar might be better off on the same object not going beyond 5m, and should not be given the impression his images are suffering by not being 20m.

You are fortunate in having one of the darkest spots available, along with an OSC camera that blocks a lot of sky signal. That is very different from many suburban imagers doing monochrome LRGB at around f/4, who would be quickly sky limited in short exposures by light pollution. For them, there would be little gain in increasing subexposure time because their main noise source is sky background. All they can do is stack - and they should not be given the impression that stacking beyond 16 has no benefit.

So - there is a model that works well, and the benefit of longer exposures depends on factors that are different for each imaging setup. In your case there may be a win going 5m to 20m, but for many others there would be little gain in SNR and it may make things worse due to other factors.

Frank


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orion69
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Inverted]
      #5670435 - 02/09/13 10:11 AM

Reason I asked the question is that I never tried shorter then 30 min subs with NB filters. The thing is, when my mount and camera arrived I tried 10 min subs with clear filter just to test setup and since that went well I jumped strait to 30 min subs NB.
It would be useful if I can get same detail with more shorter subs but I see that there are different opinions in that question...


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670453 - 02/09/13 10:25 AM

Quote:

It would be useful if I can get same detail with more shorter subs but I see that there are different opinions in that question...




Well anyone who is doing perfectly fine at 30m and has no downside from saturation or lost frames has no reason at all to go shorter - as long as he exposes enough frames for a good combine.

The penalty of going to 15m exposures depends on the ratio of sky noise to read noise in the subexposure. Your SNR will go down with 15m exposures, but it will be slight if you are sky limited - but big if you are read noise limited - at 30m.

If you find the numbers for your images at 30m - read noise and sky background noise (sqrt of sky background signal in electrons) that will tell you a lot.

Either way - the model, and professional astronomers, would say the SNR goes as the sqrt of the number of subexposures. So you can achieve the same SNR with 15 as you can with 30 but it will take some additional amount of time. But this does require you to have very good master flat/dark/bias so the calibration is excellent and your stacking does behave according to the model.

Frank


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Inverted]
      #5670473 - 02/09/13 10:38 AM

Quote:

The models seem to assume that photons arrive instantaneously, in large quantities.




No - the photoelectrons arrive randomly, slow or fast, with some mean arrival rate. The resulting counts in each exposure then obey the Poisson distribution. You could have only one signal electron every 10 frames - the other 9 being zero counts at a pixel - and the signal would still be "there" - even though it doesn't even have a single electron to show for it in most subs.

The signal will nonetheless slowly accumulate with each added frame in a linear fashion - while the noise goes as a square root. Eventually the signal will win over the noise and you will start to "see" it.

That's the theory, that's the model, and that's why the Hubble ultra deep field summed 288 frames.

Frank


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670477 - 02/09/13 10:41 AM

Well thanks for the explanation. The calculation with the square root definitely makes sense. I have seen that with my own pictures - when you stack for example 5 frames the SNR improvement against the single frame is quite high - the snr is more than 2 times higher. To go further you need good amount of frames. For example to get 4 times better SNR than the single frame you will need 16 frames.

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670485 - 02/09/13 10:45 AM

Frank could you provide us a link to your images. I bet you do fantastic work. I'd really like to see how you apply your knowledge.

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: neptun2]
      #5670496 - 02/09/13 10:51 AM

Yes - I think when people talk about hitting a brick wall - it's simply describing the slowness of sqrt(N) as N gets big.

With one sub, you add another and get a big gain. To get the same gain again, you add 2. To get the same again, you add 4. Once you reach 16, you add 4 and - not much change. So you add 4 again and - not much change. But if you keep going from 16 to 32, you will get a net big change. Same with 64 to 128.

But as the image improves, something else happens in that you reach a limit of SNR where the image is very clean and you just don't get any improvement in appearance. But that should be when the image shows no obvious speckly pattern anymore.

But if you really do hit a brick wall, and the image is just as noisy at 16 as it is at 128, then I would look into a problem in the calibration and combining. If you see noise in the bright areas - that points to a problem with the flats. If in the dark areas, a problem with darks or bias. Anyway - it points to a problem.

But I think for most people - it is just less rewarding to have to add a bunch of exposures to get an extra bump in quality. So many people just switch to a different target and move on. But it doesn't mean the SNR isn't still improving according to the model. The model says the relative change with each added frame should decrease - and so it does.

Frank


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: vpcirc]
      #5670637 - 02/09/13 12:23 PM

Quote:

Your shorter subs will show far less detail resolution and contrast no matter how many you take. That is the point of Jimmy's experiment. Even with far less total imaging time the amount of faint data he was able to obtain is clearly visible. Secondly, with narrowband, you are never going to reach the sky limit from what Don Goldman has told me. There are bright objects that will get blown out by longer subs such as M 42, but if you stick to short subs, you'll miss this wonderful detail that exists elsewhere. Adam Block has a great tutorial on how to combine short and long subs in CCDStack using missing value to combine. Tony Hallas demonstrates another great procedure in his newest addition to Photoshop processing in Vol 5 using the reveal all layer mask in CS5 and CS6. Jimmy used the Hallas method in his M42.




Unfortunately you are not able to benefit from the discussion as you do not seem to understand the simple concept of statistical sum. If you did appreciate this, you would know why there can be really no sharp cutoffs on number of frames and SNR. I very much doubt the experts you refer to say this, and if they do they are wrong.
But one is free to shut themselves off from basic math and science, and instead base their understanding on discrete bits of information from authority they worship.


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Re: A Comparison of Exposure Lengths... XMas Tree *DELETED* new [Re: korborh]
      #5670668 - 02/09/13 12:40 PM

Post deleted by vpcirc

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670710 - 02/09/13 01:06 PM

Without falling into the unfortunate polarization that has fractured this and the original thread, I am curious about the choice made by the STSCI imaging team for the ultra deep field Hubble images.
The Hubble is a native f/24 system . . . VERY slow! Who of us here would even consider an f/24 OTA for doing AP?
The individual subs comprising the UDF used a typical exposure time of 1200 seconds according to this Wikipedia article.

Now, I am totally unsure if the Hubble UDF images were actually taken at f/24. If not, just what was the EFR for the exposures?

1200 seconds at f/24 is mighty short! f/24 is about 4.5 f stops slower than a more typical imaging OTA used by folks here . . . say f/5.6 just for discussion.

A 1200 sec exposure at f/24 is approximately equal to a mere 120 seconds at f5.6. Or a 1 minute exposure at f/4.0

Seems to me the STSCI folks are using mighty short subs considering the slow FR of the instrument. Of course they are using total integration times approaching two weeks!!!
More than 800 20-minute subs! No "16 sub rule" here!
What wuzzes we are in comparison!!

Why? Perhaps . . .
1/ They are limited by the observing window of opportunity, i.e., the target is not available long enough per orbit to allow longer subs. Or . . .

2/ I am completely wrong and they are NOT imaging at f/24 at all, but at a much faster FR.

I am unsure why this thread has become so contentious. The consensus seems correct to me, i.e., go as long as your gear and sky allow but, do not deny the obvious either. At some point, short and long sub results will converge so as to make the the difference in the final result imperceptible. The point at which this happens will be a factor of equipment, sky condition and the image scale at which the final image is viewed. Also exposures must not be so long that saturation and/or blooming become overly obtrusive.

The exposure limit determined by "practical" matters will, for many of us, be less than the ideal as defined by the mathematics. For some of us, it will be WAY less!

Insisting on going longer than your mount can track/guide well is a sure-fire way to ruin an exposure totally. Much more so than using an equivalent total exposure duration using shorter, even MUCH shorter, subs.

Many here are pretty expert imagers but I know from my own personal experience that there are people perusing this forum that are just considering CCD imaging. They may presently be using a DLSR, and contemplating making the leap.
They could get some pretty discouraging ideas here, that you just must be able to do 20 minute or longer subs to get nice narrow band images. I once thought that. If you want "world-class, APOD contenders" that may well be so. Especially with slower OTAs.

However, for "pretty pictures" that a hobbyist can take great satisfaction in and amaze his friends with, it isn't so.


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: vpcirc]
      #5670724 - 02/09/13 01:11 PM

Quote:

From John Smith

I have found that prima-donnas, attention-seekers, know-it-alls and other personality types that vex the spirit are to be avoided. Rather than try to change or convert such individuals, an impossible mission in most cases, I will choose to disengage from these vexatious types as much as possible, whether individually or the venue in which they operate. They just aren't worth any of my remaining time.




OK, I'm outt'a here. This is just name-calling and hostility.


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670726 - 02/09/13 01:11 PM

Nicely said Terry - that's kind of the conclusion I've come to on all this stuff (both threads).

As for the Hubble...I haven't a clue, but MY SPACE TELESCOPE will definitely have a faster focal ratio.


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psandelle
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670731 - 02/09/13 01:14 PM

Oh, and anyone who wants to use my space telescope (once it's up) is free to.

Paul


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vpcirc
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: psandelle]
      #5670768 - 02/09/13 01:33 PM

I don't think Hubble has an atmosphere detracting light or light pollution to deal with!

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orion69
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5670824 - 02/09/13 02:06 PM

Frank, thanks for explanation, I'll try your advices in practice when my new astrograph (F/4.5) arrives. It's very possible that I would need to shorten my subs, at least for LRGB filters. I intent to replace my Baader NB filters with 3nm Astrodon's so hopefully I wouldn't need to shorten subs for NB.

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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: orion69]
      #5670850 - 02/09/13 02:28 PM

Quote:

I have question that has been already addressed in this and other posts but to make things more clear (at least for me): suppose I shoot somewhat dim object and I'm using NB filters. After 10x30 min subs I've got my new stacked picture. Now I'm shooting same object, with same gear, same sky quality (completely same conditions as before) only now I'm shooting 5 min subs x times.

Question: is there some detail on first image that would not be visible on second image no matter how many 5 min subs I take?





Thats a good question although it has already been answered in this thread and many times in the other thread. If the photon count from dim parts of nebulosity does not exceed significantly the readout noise then you will need to stack hundreds (maybe even thousands) of images to expose/show faint detail in nebulosity. This might be quite impractical and in practice you will need longer exposure times to increase the photon count from nebulosity well above the readout value.

Once again, if the Poisson noise of your object does not exceed sufficiently the readout noise, then stacking will not be as effective as taking longer exposures. In other words a stack of two images will not be equivalent to a single image with exposure time twice as long. This is the key thing to understand and this is why we have minimum exposure time calculators.


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670885 - 02/09/13 02:57 PM

Hi-

The details of the SNR calculation for the UDF are described here. The noise model is the same, but the question they are asking is - if I want to see a certain nebula with SNR 10 - how many exposures will I need? So the underlying model is the same, but they are asking it in terms of SNR. Note that they calculate the measured result in comparison with the model, and they come up with a freakin' error of 0.01 mag in table 5. They deal with the same issues as suburban imagers - read noise and sky glow. Obviously - the model works.

Whether imaging with the HST or with a backyard setup - there are concerns with SNR and noise rejection. The same applies on the internet - so I wouldn't let an occasional, or repeated, cosmic ray impact your contributions to this thread. It can be random noise or pattern noise - but either way it is something that obfuscates the signal - and the increase in informative contributions from you and others will increase the SNR that draws people to CN as a source of good information.

Frank


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jaddbd
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5670943 - 02/09/13 03:52 PM

Quote:

1200 seconds at f/24 is mighty short! f/24 is about 4.5 f stops slower than a more typical imaging OTA used by folks here . . . say f/5.6 just for discussion.
--edit--
Why? Perhaps . . .
1/ They are limited by the observing window of opportunity, i.e., the target is not available long enough per orbit to allow longer subs. Or . . .






In the paper Frank just referenced (see page 7), seems that they used 1200sec and sometimes 850sec subs in order to get in 2 exposures per orbit with a "micro-dither" in between...

John D


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Mike Wiles
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: jaddbd]
      #5671273 - 02/09/13 07:27 PM

Just sayin....



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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Mike Wiles]
      #5671394 - 02/09/13 08:39 PM

I Agree... "paralysis by analysis" already for me . I found the Hubble paper Frank referenced pretty neat - it offers some cool insights into what they do and it is not completely unreadable.

JD


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: dawziecat]
      #5671579 - 02/09/13 11:27 PM

Quote:

The Hubble is a native f/24 system . . . VERY slow! Who of us here would even consider an f/24 OTA for doing AP?
The individual subs comprising the UDF used a typical exposure time of 1200 seconds according to this Wikipedia article.




Let me put this in the right perspective.
The WFC3 camera has 0.13 arcsec image scale and Hubble's primary mirror has a dimater of 94.5 inches (2.4m). This results in the speed factor of 12.3 (94.5*0.13)
The Celestron 14" EdgeHD has a focal length of 3905mm. A camera with 9 micron pixels has 0.48 arcsec image scale. This results in the speed factor of 6.7 (14*0.48)
So as you can see, the HST is twice as fast as the 14" EdgeHD if you are willing to spend $10,000 on a camera.
If you are not willing to spend that much money on a camera then consider the KAF-8300 which has 5.4 micron pixel. This results in the speed factor of 4 (14*0.29) and it makes the HST configuration 3 times as fast.


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Alph]
      #5671675 - 02/10/13 12:34 AM

As described in the paper I referenced, the ACS pixels used for the HUDF are 0.05" and provide an fwhm of around 0.08". So it is indeed slower than a 14" f/11 with 9um pixels - which itself is well matched to an fwhm in the 1.5-2" range. In both cases you want to realize the resolution capabilities of the instrument - so you can capture detail in lots of tiny background galaxies.

Frank


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: freestar8n]
      #5672594 - 02/10/13 03:57 PM

I am following this thread with great interest and learning a lot.

By pure chance, I have discovered today that I had shot this same area 2 years ago with my C11 Edge + Hyperstar setup with a Canon T2i camera. At the time I shot this frame, I believed I had saturated all the stars. Today, I reprocessed the stacked file again and saw that the stars were not saturated at all. My post processing skills are still far from being on the expert level but I was glad to see color in the stars. I will share the stacked file with you guys just to add to Jimmy's comparison.



Hires image.

There is more data than I was able show here.

The stack is 9x 600 second lights, shot through a Lumicon DeepSky filter with Canon T2i at ISO400.

I guess I was experimenting with the camera and Hyperstar at that time as an explanation for using ISO 400.

Had I used ISO 800, 5 minutes would suffice I guess.

File Name CONE NEBULA_LIGHT_600s_400iso_+17c_LUMICON_DEEPSKYfilter_20110131-01h54m32s.CR2
Camera Model Canon EOS REBEL T2i
Firmware Firmware Version 1.0.8
Shooting Date/Time 31.01.2011 01:45:07
Author BILGEBAY
Owner's Name
Shooting Mode Manual Exposure
Tv( Shutter Speed ) 600
Av( Aperture Value ) 0.0
Metering Mode Evaluative Metering
ISO Speed 400
Auto ISO Speed OFF
Image Size 5184x3456
Image Quality RAW
Flash Off
FE lock OFF
White Balance Mode Daylight

And here is the FIT file for those who want to try their ...

Clear skies


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Peter in Reno
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: bilgebay]
      #5672606 - 02/10/13 04:05 PM

Hi Sedat,

Your link does not appear to work. Also, it's TIF, not FIT. It's funny that TIF is an anagram of FIT.

Peter


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bilgebayModerator
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Peter in Reno]
      #5672615 - 02/10/13 04:10 PM

Peter dropbox is still uploading. Should finish in an hour...net is slow today.

Oh, and yes...it's a tif sorry for the confusion.


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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: bilgebay]
      #5672643 - 02/10/13 04:27 PM

Peter, upload is completed.

Goodluck

Sedat


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Peter in Reno
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: bilgebay]
      #5672711 - 02/10/13 05:11 PM

Got it.

Thanks,
Peter


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Peter in Reno
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Peter in Reno]
      #5672804 - 02/10/13 06:20 PM

Sedat,

You have very good data. I could not process it any better than you did. Good data makes it easier to process. Looks like a darn good DSLR camera.

Peter


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bilgebayModerator
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Re: A Comparison of Exposure Lengths... XMas Tree new [Re: Peter in Reno]
      #5673448 - 02/11/13 03:04 AM

Peter, to my eye, my image lacks lustre.

Also, at one stage the stars had more color but I lost it during the processing


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