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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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Wmacky
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Reged: 11/24/07

Loc: Florida
Re: How much difference do more subs make? new [Re: Footbag]
      #6267598 - 12/22/13 09:22 PM

I hate to ask, but a google search comes up with nothing.

Whats BOC?


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pfile
Post Laureate


Reged: 06/14/09

Re: How much difference do more subs make? new [Re: Wmacky]
      #6267609 - 12/22/13 09:29 PM

the 1/3 rule is for back of camera ("BOC") histogram. or the histogram that BYE shows you. those histograms represent a default stretch (the same stretch that the camera would apply if you let it create a jpeg for you)

the 1/3 rule is sometimes also given as "20%" or "well-detached".

note that if you are at a very dark sky site it may be impossible to push the histogram that far to the right without really long sub exposures.

rob

edit: dave you will have to take test shots; the histogram is not displayed until the exposure ends.


Edited by pfile (12/22/13 09:30 PM)


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Wmacky
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Reged: 11/24/07

Loc: Florida
Re: How much difference do more subs make? new [Re: pfile]
      #6267618 - 12/22/13 09:36 PM

Quote:

the 1/3 rule is for back of camera ("BOC") histogram. or the histogram that BYE shows you. those histograms represent a default stretch (the same stretch that the camera would apply if you let it create a jpeg for you)

the 1/3 rule is sometimes also given as "20%" or "well-detached".

note that if you are at a very dark sky site it may be impossible to push the histogram that far to the right without really long sub exposures.

rob

edit: dave you will have to take test shots; the histogram is not displayed until the exposure ends.





Thanks! I have one other quick question. When taking a exposure test shot. Is there a quick way to tell if you have over saturated the brighter stars, other than a visual check? The histo turn into a hard to see thin line as it approaches the right hand side.

Edited by Wmacky (12/22/13 09:37 PM)


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: How much difference do more subs make? new [Re: Cotts]
      #6268183 - 12/23/13 09:31 AM

Quote:

I am very interested in this 'minimalism' discussion. I will be travelling to Chile in March for 11 nights at Atacama Lodge. I plan to devote about 5 of the nights to astrophotography through an AT65EDQ/Canon60Da combination. (The other nights will be visual observing with the 24" dob while shooting wide-angle stills and time-lapses..)

There are waaaay more targets than I will have time for so I will have to compromise on S/N ratio by shooting some bare minimum of subs, flats, darks etc. ISO 1600 works well with my camera...

The big question is: How many subs is the minimum acceptable number? And how long should they be? Which would be better, e.g., 2x6min, 3x4min, 4x3min, 6x2min or 12x1min?

I have a 'wish list' of about 25 targets with the AT65 so, with my planned schedule, I'll do about 5 targets a night -- about 1.5 hours maximum per target. Whatever shall I do?





Hi Dave,

The brighter objects, like the Eta Carina Nebula, won't need as much exposure as the fainter objects.

Prioritize your list by which objects you really want the most, and then research how faint they are. Give more exposure to the faint objects.

Under true dark skies at altitude, you should get excellent results with shorter total integration times than at a typical observing location in North America.

You will typically need longer exposures for fainter extended objects, and shorter exposures for clusters.

Jerry


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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How much difference do more subs make? new [Re: Samir Kharusi]
      #6268216 - 12/23/13 09:48 AM

Quote:

I doubt that there is EVER a point of diminishing returns. The longer the integration time, the fainter/dimmer the wisps you can dig out at a good Signal-to-Noise-Ratio. But one should not think of x frames more, think of doubling the integration time for each step.




The equation for Stacking says otherwise. It varies with the Square root of the number of subs. It has nothing to do with total time at all.

Direct quote from the DSS site.

http://deepskystacker.free.fr/english/theory.htm

Quote:


"The more, the better but above some threshold it is less efficient.
The signal to noise ration in increasing with the square root of the number of combined frames regardless of the exposure time of each frame.
This is true with all the combining methods (average, median, kappa-sigma clipping, auto-adaptive weighted average, ...) except entropy weighted average since this one in using the entropy to weight each pixel and thus is increasing the noise that is a big entropy contributor.."




Edited by mpgxsvcd (12/23/13 10:26 AM)


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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How much difference do more subs make? new [Re: Tonk]
      #6268218 - 12/23/13 09:49 AM

Quote:

Yes - I don't count number fo subframes but the total integration times.

Put it this way, the Gendler's, Croman's and Hallas's of this world do 20 to 30 hours integration. A few DSLR folks here have run up to 20 hours with stunning results.

Personally I find things look very good after 10 hours worth.




Your wasting your time then. Check the equations for stacking. They have nothing to do with total time. Just shear number of subs. However, at some point even adding subs won't help enough.

I believe that point is somewhere around 60 subs for High ISO subs(ISO 6400) and lower(Perhaps 32 subs or less) for lower ISO subs for my camera. That is just my observation though. The camera makes a huge difference in whether that observation is accurate or not.

Edited by mpgxsvcd (12/23/13 10:26 AM)


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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How much difference do more subs make? new [Re: Cotts]
      #6268288 - 12/23/13 10:24 AM

Quote:

I am very interested in this 'minimalism' discussion. I will be travelling to Chile in March for 11 nights at Atacama Lodge. I plan to devote about 5 of the nights to astrophotography through an AT65EDQ/Canon60Da combination. (The other nights will be visual observing with the 24" dob while shooting wide-angle stills and time-lapses..)

There are waaaay more targets than I will have time for so I will have to compromise on S/N ratio by shooting some bare minimum of subs, flats, darks etc. ISO 1600 works well with my camera...

The big question is: How many subs is the minimum acceptable number? And how long should they be? Which would be better, e.g., 2x6min, 3x4min, 4x3min, 6x2min or 12x1min?

I have a 'wish list' of about 25 targets with the AT65 so, with my planned schedule, I'll do about 5 targets a night -- about 1.5 hours maximum per target. Whatever shall I do?

Thanks

Dave




I have been interested in this solely because of the Messier Marathon. I want to not only capture all ~110 objects. I want to stack them as well.

From my dark site I can get at least 1 minute ISO 6400 subs at F4.0 before blowing out the sky. That is actually a lot of exposure. If I can take at least 60 of those images with good tracking I know I can get a fantastic image once I figure out my collimation issue.

However, the fact is that at most I am only going to be able to spend about 4-5 minutes on each object. 4 subs is not great but it does help.

For the brighter objects like M42, M31, M45 and the clusters I can definitely knock the time down to 30, 15 or even 8 seconds per sub to retain the brightest elements of those objects. That might give me a few extra minutes on the darkest objects.

My hope is that next spring I can stack at least 4 subs for all of the Messier objects plus the Horse Head in a single night without guiding. To me that seems like an admirable task. I only got through stacking 55 objects last year before my mount failed do to a collision with the tripod leg. It was well below freezing and I was not dressed appropriately so I had to give up then.

I have the warm clothes now and I am closer on figuring out the collimation so I think it will work this year. I don't expect perfect images. However, I do think this will give me nice results of an awful lot of objects for a single night.


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Footbag
Postmaster
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Reged: 04/13/09

Loc: Scranton, PA
Re: How much difference do more subs make? new [Re: mpgxsvcd]
      #6268329 - 12/23/13 10:47 AM

You do need enough subs that average and median stacking methods begin to work properly. I've found this to be tough with narrowband where you have fewer longer subs.

Once you cross that threahold, assuming properly exposed subs; you can treat a sub as an interval of time.

Keep in mind that we are doing this for aethetic photographs. If 5 hours of subs didn't reduce the noise to visually acceptable levels, then quadrupling the exposure is really the only option.

Is it diminishing returns, or diminishing motivation? At some point, its a lot of work for a gain you may not be able to see.


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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How much difference do more subs make? new [Re: Footbag]
      #6268396 - 12/23/13 11:33 AM

Quote:


Equations for stacking give you only part of what going on. By your reconing then we should all be shooting 1000's of 1 second exposures to get the best results ?? Makes sense that if you shoot 0 second exposures then you get no noise





I never said that. Stacking more subs will reduce the noise. However, it reduces the noise less and less as you increase the number of subs. At some point you will fail to see a difference no matter how many subs you add.

How long you image for doesn't change the way the stacking process works. If you have less noise to begin with of course you will end up with less noise in the end if you stack the same number of frames. However, the ratio of original noise to final noise will be be the exact same for either scenario if you take the exact same number of subs.

Again, Imaging time has absolutely nothing to do with how the stacking process works. It is simply a way to increase the signal to noise after the images have already been taken. There are many things that will affect the signal to noise while you are taking the image. Stacking is the only efficient way to change it after you have already taken the images.


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Tonk
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Reged: 08/19/04

Loc: Leeds, UK, 54N
Re: How much difference do more subs make? new [Re: Footbag]
      #6268410 - 12/23/13 11:42 AM

Have you read this summary

http://www.astropix.com/HTML/I_ASTROP/SIGNAL.HTM

Do so and then tell me I'm wasting my time

You have only discussed half the story - noise reduction. So far you have neglected considering collecting sufficient signal - thats a function of total integration time.

If we are not talking about very bright subjects (where lack of signal is not an issue) but the vast range of DSOs such as faint nebula and galaxies then total integration time gets the upper hand. Put it into practice and this becomes very evident. Targets such as ngc 7023 just get better with the amount time you put in capturing photons. On the otherhand bright targets such as M13 don't need this level of effort at all unless you are seeking the faint periphery stars (which will require HDR techniques to balance the core with faint edges in your final image)


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mpgxsvcd
Carpal Tunnel


Reged: 12/21/11

Loc: Raleigh, North Carolina
Re: How much difference do more subs make? new [Re: Tonk]
      #6268437 - 12/23/13 12:08 PM

Quote:

Have you read this summary

http://www.astropix.com/HTML/I_ASTROP/SIGNAL.HTM

Do so and then tell me I'm wasting my time

You have only discussed half the story - noise reduction. So far you have neglected considering collecting sufficient signal - thats a function of total integration time.

If we are not talking about very bright subjects (where lack of signal is not an issue) but the vast range of DSOs such as faint nebula and galaxies then total integration time gets the upper hand. Put it into practice and this becomes very evident. Targets such as ngc 7023 just get better with the amount time you put in capturing photons. On the otherhand bright targets such as M13 don't need this level of effort at all unless you are seeking the faint periphery stars (which will require HDR techniques to balance the core with faint edges in your final image)




All I have said is that adding subs of the same exposure duration past the point of diminishing returns will not help you. That says nothing about adding more exposure duration to each sub. However, the standard assumption is that you are using the maximum exposure for your given focal ratio and sky limits. If you aren't doing that then do so by all means. That won't affect how your subs do when stacking though.

You can also use filters to adjust the signal just like you can increase the exposure of each sub. However, that too does not affect how DSS increases the signal to noise ratio.

In my opinion 60 subs is sufficient even with ISOs as high as ISO 6400. In addition the benefit from using longer and longer exposures with lower ISO really decreases after about 4 minutes with my camera. I will post the images for that test when I get the chance. Your camera my vary from mine but I don't think it will vary much unless you are cooling it actively.

At 4 minute subs I believe that 4 hours is the most you should ever spend on an object with my particular camera. I believe that after that you are in fact just wasting your time.


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Footbag
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Reged: 04/13/09

Loc: Scranton, PA
Re: How much difference do more subs make? new [Re: mpgxsvcd]
      #6268452 - 12/23/13 12:18 PM

Quote:

Quote:

Have you read this summary

http://www.astropix.com/HTML/I_ASTROP/SIGNAL.HTM

Do so and then tell me I'm wasting my time

You have only discussed half the story - noise reduction. So far you have neglected considering collecting sufficient signal - thats a function of total integration time.

If we are not talking about very bright subjects (where lack of signal is not an issue) but the vast range of DSOs such as faint nebula and galaxies then total integration time gets the upper hand. Put it into practice and this becomes very evident. Targets such as ngc 7023 just get better with the amount time you put in capturing photons. On the otherhand bright targets such as M13 don't need this level of effort at all unless you are seeking the faint periphery stars (which will require HDR techniques to balance the core with faint edges in your final image)




In my opinion 60 subs is sufficient even with ISOs as high as ISO 6400. In addition the benefit from using longer and longer exposures with lower ISO really decreases after about 4 minutes with my camera. I will post the images for that test when I get the chance. Your camera my vary from mine but I don't think it will vary much unless you are cooling it actively.

At 4 minute subs I believe that 4 hours is the most you should ever spend on an object with my particular camera. I believe that after that you are in fact just wasting your time.




This ignores the object brightness. That is a mistake because as objects get dimmer, more exposure is required to improve the SNR.

Tonk's example of the iris nebula is a good one. I have probably 15 hrs on that object and very little to show for it.


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tazer
sage


Reged: 12/22/11

Loc: Central North Carolina
Re: How much difference do more subs make? new [Re: Footbag]
      #6269674 - 12/23/13 11:16 PM

I had the same question myself and created an animated GIF of a high ISO stacking comparison at one point. Each sub-exposure was for 30s @ ISO 3200:

http://www.astrobin.com/full/32323/0/

It's a 17MB GIF so I decided not to embed the image.


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whwang
sage


Reged: 03/20/13

Re: How much difference do more subs make? new [Re: tazer]
      #6270104 - 12/24/13 08:20 AM

There will be no "point of diminishing returns" if you calibrate (bias, dark, flat) your images perfectly. You can essentially get as many subs as you like, and continuously go deeper.

The only thing is, the image quality (S/N) will go with square root of the number of subs. How much you can improve 1 sub by taking another 1, is the same as how much you can improve 60 subs by taking another 60. If I have 1 sub in hand, usually I would not hesitate to add one more. If I have 60 subs in hand, on the other hand, I would think very hard whether or not to take another 60 more. (I think this applies to most of us.) That's a mental effect, however, not a scientific one.


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tazer
sage


Reged: 12/22/11

Loc: Central North Carolina
Re: How much difference do more subs make? new [Re: whwang]
      #6270218 - 12/24/13 09:33 AM

I think "point of diminishing return" should actually be "point of diminishing return on investment of time" where 'return' is whatever we're trying to glean from a stack. Most often this is an aesthetic return. Is it worth spending 5 hours in the field to double your SNR 3 times? For me it is. Is it worth spending an additional 80 hours in the field to double it two more times? For me it isn't, as the net gains won't be that apparent in my stacked image (based on my scope/imaging camera/computer monitor/etc.)

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Steve OK
scholastic sledgehammer


Reged: 09/22/07

Loc: OKC, OK
Re: How much difference do more subs make? new [Re: tazer]
      #6270749 - 12/24/13 02:11 PM

Well said. Stacking the first 100 subs gives a 10x increase in S/N, while the next 100 subs gives an additional 1.4x increase in S/N. As Whwang points out, all additional subs lead to greater S/N, but they do so at a decreasing rate.

Steve


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rkayakr
sage
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Reged: 10/27/10

Loc: Northeast Ohio
Re: How much difference do more subs make? new [Re: Steve OK]
      #6270766 - 12/24/13 02:21 PM

Dave
You can shoot the darks and flats at another time and devote your time in Chile to lights.
Enjoy
Bob


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: How much difference do more subs make? new [Re: rkayakr]
      #6272124 - 12/25/13 12:29 PM

Here is my take on the subject of diminishing returns. I agree with Samir and Wei-Hao.

I had just posted this to another similar question and thought it would be appropriate here also.

Some people argue that you reach a point of diminishing returns, but I don't think there is ever a point where more subs hurt.

Personally, I don't even agree with the point of diminishing returns philosophy. More photons are always going to give you a better signal-to-noise ratio because the signal can only come from the photons, and the only way to collect more photons is by collecting more subs.

In the April 2012 issue of Astronomy Magazine, Tony Hallas argued that you reach an asymptotic boundary at about 25 frames and that shooting more frames was basically a waste of time.

But this is not true. He even mentions that the signal-to-noise ratio improves with the square root of the total number of frames, but then seems to ignore it.

For example 4 frames will give you twice the s/n of 1 frame.

Likewise, 9 frames will give you 3x.

16 frames will give you 4x.

25 frames will give you 5x.

36 frames will give you 6x.

So going from 25 to 36 frames you have needed about 50 percent more total exposure time, buy your s/ has only gone up 20 percent.

Still, it has gone up.

If you follow the logic of 25 frames as a practical limit, you would never want to attempt any deep-sky objects from light polluted areas, yet there are plenty of people doing exactly that.

Look at it like this... 25 frames is 5x better than 1 frame. But, 100 frames is 10x better than one frame, and twice as good as 25 frames.

It certainly would be nice if the improvement was linear, but you still get an improvement by shooting as many frames as you can.

Now, you won't see that much improvement with 26 frames vs 25 frames. But you certainly will with 100 vs 25. The s/n will be twice as good. Is that "worth it?" Only the individual photographer can answer what his/her own level of acceptable quality is.

And this is especially important under light-polluted skies because an individual sub exposure will be limited in duration. So you will need a "hella lotta" (scientific term) subs under those conditions.

I have a shot of M27 from my suburban driveway that is a stack of 90 one-minute exposures at f/8 at ISO 800. And 90 frames certainly was not too many.

Jerry


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tazer
sage


Reged: 12/22/11

Loc: Central North Carolina
Re: How much difference do more subs make? new [Re: Jerry Lodriguss]
      #6272664 - 12/25/13 07:50 PM

Quote:

Some people argue that you reach a point of diminishing returns, but I don't think there is ever a point where more subs hurt.




I don't think more subs ever hurt, rather the contribution of each successive sub to the resulting stack diminishes.


Quote:

More photons are always going to give you a better signal-to-noise ratio because the signal can only come from the photons, and the only way to collect more photons is by collecting more subs.




As I understand it, the primary purpose of stacking is to eliminate noise. To increase signal you take longer subs.


Quote:

Now, you won't see that much improvement with 26 frames vs 25 frames. But you certainly will with 100 vs 25. The s/n will be twice as good.




And 400 is twice as good at 100, but that's a huge step for the same increase in SNR. Ultimately you want to get to the point where you can stretch your image aggressively enough to bring out the faintest signal. Any further reduction of noise won't be detectable in the resulting image.


Quote:

Is that "worth it?" Only the individual photographer can answer what his/her own level of acceptable quality is.




But I think that's the essence of diminishing returns. You invest time to increase SNR. While your time investment remains constant, the rate of return diminishes.


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whwang
sage


Reged: 03/20/13

Re: How much difference do more subs make? new [Re: tazer]
      #6272859 - 12/25/13 10:21 PM

Quote:


Quote:

More photons are always going to give you a better signal-to-noise ratio because the signal can only come from the photons, and the only way to collect more photons is by collecting more subs.




As I understand it, the primary purpose of stacking is to eliminate noise. To increase signal you take longer subs.






We should keep in mind that the most important source of noise is signal itself, in deep-sky imaging.
(This assumes that each sub is long enough so the readout noise of the camera becomes negligible.)

Photons carry noise with them. The amount of noise they carry is the square-root of the number
of photons. The more photons we collect, the stronger noise we get. However, since the noise
is the square-root of photons, the S/N is then number of photons divided by the square-root of the
same number. In other words, S/N improves as we collect more photons. This is exactly why we
want longer total integration time, and also exactly why we want more subs. They are the same thing.

Cheers,
Wei-Hao


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