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Mike Wiles
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Reged: 02/04/09

Loc: Goodyear, AZ
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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Reged: 06/06/10

Loc: San Antonio, TX
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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Reged: 04/06/08

Loc: NJ
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
Photon Catcher
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Reged: 07/20/07

Loc: California
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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Reged: 12/09/09

Loc: Merced CA
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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Reged: 10/12/07

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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Reged: 12/09/09

Loc: Merced CA
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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Reged: 10/12/07

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
scholastic sledgehammer


Reged: 05/09/10

Loc: Croatia
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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Reged: 03/04/07

Loc: Bulgaria
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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Reged: 10/12/07

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
sage
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Reged: 01/19/13

Loc: LP Land
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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Reged: 12/09/09

Loc: Merced CA
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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Reged: 06/06/10

Loc: San Antonio, TX
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
sage
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Reged: 01/19/13

Loc: LP Land
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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Reged: 10/12/07

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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Reged: 05/09/10

Loc: Croatia
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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freestar8n
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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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freestar8n
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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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neptun2
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Reged: 03/04/07

Loc: Bulgaria
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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