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  #41  
Old 24-10-2013, 08:40 AM
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Originally Posted by multiweb View Post
I can relate to this. I always take my OSC with me when I drive out to dark skies once in a blue moon. At home I'm stuck doing NB with a mono. I would never contemplate a filter wheel and doing color on the field. Not enough time for this.
Not only that, but if you've got a hyperstar, like you have Marc, and you want colour piccies, the OSC is the only way to do it. You could take your mono along too and shoot lum or Ha shots, if you wanted to.

But like I said, lugging along filter wheels and all the other extraneous stuff that can go south on you if it decides to just have a night off doesn't make for a pleasant imaging experience or a good night of astronomy.
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Old 24-10-2013, 09:07 AM
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Here's an interesting point...we've been talking about the advantages of various camera types and scope combination, integration times for optimal photos etc etc, but what would you think the optimal scope/camera combination would be for each area of astropics? You could do a Ken Crawford, Rob Gendler etc and have scopes in the 24-32" range with 16803 chipped cameras and that would be great. But they're not the best for nebs and other large targets since you need a wide field to take in most, like the NA neb or Rosette. Not only that, going large isn't always the best, since you have to contend with other problems that become magnified due to the larger aperture, such as scintillation, air currents etc. If you're looking for a system that will give you maximum details in your piccies for aperture and bang for buck, where do you think such a scope would sit? It most likely means you'll need two scopes, one for general shots and one for wide field, but there should be a "sweet spot" for each type. Where do you think that lies?
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  #43  
Old 24-10-2013, 09:28 AM
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I haven't used that many camera/scopes combinations but I have used OSC and mono on short to very short FL. My OSC with large pixels will fall apart with 100/200mm lenses. Even with an image scale of 2-3 seconds of arc per pixel. The debayering process just hacks the data to pieces. The only reasonable channel is the green channel because it has two pixels instead of one. The mono shines in the same situation.

But I was pleasantly surprised when I used the same OSC on my 8" F/4. The image scale was closer to 1-1.2 seconds of arc per pixel, better sampling and the RGB channels were balanced. I couldn't really tell visually of any significant loss of details. Of course the mono in NB was sharper but not using straight LUM due to seeing conditions which then become an important factor.
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  #44  
Old 24-10-2013, 06:23 PM
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If you're looking for a system that will give you maximum details in your piccies for aperture and bang for buck, where do you think such a scope would sit? It most likely means you'll need two scopes, one for general shots and one for wide field, but there should be a "sweet spot" for each type. Where do you think that lies?[/QUOTE]


I agree Carl. One scope won't fit all. A widefield high quality scope around 4 inches is a proven formula like FSQ106ED, I like my TEC110 fluorite for this. There are others. Pentax 125 was one. Televue NP101 and 127i.

Then refractor sweet spot is around 6 inches. My TEC180 is also close to the sweet spot in that its the largest refractor (perhaps a 200mm is still fine) that is still portable. Above that and you probably start needing a PME to handle or an AP1200. So 6 to 8 inches of APO.

Then reflectors. There are lots of great images from the 8 to 12 inch range. 12 inch compound of any type is starting to get large. A 12 inch RCOS is too large to handle easily although my closed tube 12 inch RCOS was still "portable".

Above 12 inches and I think you are looking at a permanent installation unless you are really strong and have a large cargo space in your vehicle and a large powerful mount to handle it all.

So 4 inch refractor for widefield, 6-8 inch APO for more detailed clear round star no diffraction spikes work and 8-12 inch compound scope for bright enough aperture to capture many galaxies and dim objects with fine detail and still be portable.

Then add to that the question of F ratio and flexibility of the imaging system.

For example Marcus Davies used a very nice Tak TOA150 APO. Tak makes an extender to take it up to F12+ for some magnification on small galaxies and a reducer to bring it down to F5. something. So its very flexible.

Faster scopes need more careful handling so best for more experienced scope handlers who don't mind the careful adjustments, fussy collimation, flexure issues. So perhaps not in the sweet spot.

I always consider F5 to be a sweet spot for F ratio. You can still get a rigid system that is fast at a nice dark site with a good camera that is still easy to handle and super fussy to flexure and miscollimation etc etc.

So if I can get F5 in my scopes or close to it I am happy and how well F5 works in the images.

For example my TEC110 is F5.6 -it could be a tad faster but then a Proline camera is very heavy so no thanks. My AstroPhysics 140 has a reducer that makes it around F5.6 as well and that is very nice for that scope.

My TEC180 though at F5.25 was a bit ho-hum. But an adapter to use the AP reducer on the TEC could be spectacular as Yuri does not make reducers for TEC scopes. He doesn't believe in them.

Greg.
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  #45  
Old 24-10-2013, 11:42 PM
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Interesting discussion, one that seems to pop up again now and then.

The other discussion might be diminishing returns as well as your cameras maximum dynamic range limits (ADU limits)that determines which method suits and 'how many hours is long enough'...

I might have my understanding wrong(learning continues), but I couldn't see how multiple short exposures amounted to the same results as a single long exposure method, it just didn't make sense to me unless the light gathering conditions could be met with both instances where the same sum of signal brightness is gathered.

But if you're averaging you can only have so much signal to weed out of the noise in the end?

I see it that The count total of short exposures you'd want to take could work two ways, one would be using a light bucket with a low end camera, your ADU might blow out so you take say a 5min exposure, then you take multiples, as many as you want but your only improving the SNR of limited range of signal over the noise.

Then you use a high end camera with a huge well depth, as long as the tracking mechanics of your equipment allows you to with good accuracy, you could image for an hour. Here your SNR also is improved and you're seeing an increase of the signal brightness over time, and this can also be improved more by improving the SNR again using multiple images and averaging them.

But from both scenarios depending on how long you expose for with the cameras well depth and the telescopes ability to gather photons slow or fast, every instance has a point of disminishing return that is limited by your camera and telescope.

So whether it's 20hours of short 5min exposure stacks, or 20hours of 60min stacks, they can never be equaled.
And considering that, what you decide on in the end has a diminishing return value of how much signal you capture, it's just the method you decide on to get there and how dark a site you have that can capture light fast than noisy light polluted sites.

Edit: I see you guys have touched on this already at this first posts
My bad?

Last edited by leinad; 24-10-2013 at 11:57 PM.
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  #46  
Old 26-10-2013, 12:37 AM
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I agree with Ray's original point that total integration times do not directly correlate with the same SnR across different systems and locations. A 12" scope has over twice the aperture (area) than my 8"... so all else being equal, 20 hours for someone else may be similar to 45 hours for me.

However, if you look at the subset of people who actively aim to go really deeeep, produce images that elicits "wow" reactions, gets APODs, and so on, things start to look very similar: mag 21-22 dark skies, 100 - 130 mm refractors for nebulae, 12" reflectors with appropriately matched pixel sizes for galaxies, Astrodon/Astronomik/Baader/etc filters, 11000 / 16803 / 694 chips are all pretty common.

In this case, comparing 1 vs 10 vs 50 vs 100 hours does give a rough idea of what to expect.
Following up on Dave's idea, I did some modelling of a few typical hi res galaxy imaging systems with the attached results. The starting point was a Mag23 galaxy against a Mag21 sky (per arc sec2) and 0.8 atmosphere transparency. The output is time taken to reach 20dB SNR, the ISO standard for "adequate" image quality - better quality will require longer. The optimum sub is based on the Starizona 5% read noise criterion - it is the shortest sub where shot noise will still overwhelm read noise and you could use longer if you wish. Of course, this only applies to visible broadband and the optimum sub length for any system in narrowband is "as long as possible".

there is lots of variability even in this smallish group - one system needs 6.5 hours , another needs 1.5 hours to get the same SNR. Sub length varies from 3 minutes to 40 minutes - and this is for the same environment and task. Actual times could be a bit different due to initial assumptions, but nothing like enough to account for such a large spread in imaging time. Seems that, even within this subset of imagers, there is no such thing as a universal best imaging time or an optimum sub length - it strongly depends on the system design. The most significant factor is the pixel scale (imaging time will depend on 1/pixelscale^2), but QE is also very important and read noise is a major factor in determining the sub length. regards Ray
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Last edited by Shiraz; 26-10-2013 at 08:24 AM.
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  #47  
Old 27-10-2013, 02:43 PM
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This evidence-based discussion is like a breath of fresh air. My perception of medicine and pathology for the last 30 odd years is that (often younger) specialists are now almost embarassed to express an opinion without being able to reference evidence in peer-reviewed literature. Very different to many years ago when senior specialists and their empirical knowledge were unquestioned.

Opinions from experienced astrophotographers will always be important, but evidence and sold mathematics are great to see, especially as the number of options in range of amateurs continues to expand. And the statement that there's never one perfect 'scope for everyone has never been more true.

I'm just embarassed I've never looked at a decent spreadsheet for exposure times, although I have read up a bit. Must get on with it!

Last edited by RobF; 28-10-2013 at 07:52 PM.
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Old 28-10-2013, 08:51 AM
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Thanks for that, Ray; very interesting. I think it shows that an appropriate image scale is critical for decent sensitivity (as we all know from painful experience or learning from others).

Ignoring the clear case of poorly matched scope/camera (KAF-8300M on a 12-inch RC...), there's not *that* much difference in total integration time amongst the other contenders - a constant factor, rather than orders of magnitude.

Object selection, such as Orion Nebula versus a mag 10 galaxy with low surface brightness, will make more of a difference. However, I'm guessing that some of those scope combinations will have a much deeper limiting magnitude for the same integration/sub time.
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  #49  
Old 28-10-2013, 08:10 PM
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Originally Posted by leinad View Post

So whether it's 20hours of short 5min exposure stacks, or 20hours of 60min stacks, they can never be equaled.
My understanding of why summed short subs and one long exposure can be the same is that the primary broadband noise is due to shot noise - that comes as part of the signal-plus-background and you get the same number of photons (target, background and noise) in a given period however you choose to collect them. This argument falls over if you have a camera with high read noise - then the read noise can add to the overall noise and make the SNR worse for multiple short subs. However, it is often still possible to use relatively short subs and still keep the total read noise contribution below the level where it makes any real difference.

Quote:
Originally Posted by RobF View Post
This evidence-based discussion is like a breath of fresh air. My perception of medicine and pathology for the last 30 odd years is that (often younger) specialists are now almost embarassed to express an opinion without being able to reference evidence in peer-reviewed literature. Very different to many years ago when senior specialists and their empirical knowledge were unquestioned.

Opinions from experienced astrophotographers will always be important, but evidence and sold mathematics are great to see, especially as the number of options in range of amateurs continues to expand. And the statement that there's never one perfect 'scope for everyone has never been more true.

I'm just embarassed I've never looked at a decent spreadsheet for exposure times, although I have read up a bit. Must get on with it!
Thanks Rob. I was surprised to find that there was no readily available design methodology for putting together an imaging system. Suck it and see will eventually give a result, but you can waste a lot of time and money getting there in this hobby. A decent model is a lot less costly and can provide insight that is just not available from empirical considerations. There is a place for the wisdom of the elders, but I am personally too old to wait around for understanding to come via that route.

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Originally Posted by naskies View Post
Thanks for that, Ray; very interesting. I think it shows that an appropriate image scale is critical for decent sensitivity (as we all know from painful experience or learning from others).

Ignoring the clear case of poorly matched scope/camera (KAF-8300M on a 12-inch RC...), there's not *that* much difference in total integration time amongst the other contenders - a constant factor, rather than orders of magnitude.

Object selection, such as Orion Nebula versus a mag 10 galaxy with low surface brightness, will make more of a difference. However, I'm guessing that some of those scope combinations will have a much deeper limiting magnitude for the same integration/sub time.
Yes you are right Dave (crikey that sounds like HAL from 2001....) - there is not such a large difference between the reasonably well sampled systems - image scale sure is critical.

I am not sure where the concept of limiting magnitude fits in, since it seems to generally be applied to unresolved objects and all of the modelling to date has been on extended objects - will have to do a bit of reading.

Regards ray

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  #50  
Old 29-10-2013, 11:58 AM
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With my new system I took a leap into the 'unknown' only because it had not been done.

My background is in Physics and optics and over forty years in scientific research.

I did calculate all aspects of every part of my new system. My solution was a unique solution because it was correct. For me!

This is not a recipe for anyone else's system.

What is the strength of us amateurs, is that no matter where you look at the sky the chances are that the pro's are not looking at your bit of sky!

Evidence based decision making rules!

Bert

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Old 29-10-2013, 10:25 PM
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With my new system I took a leap into the 'unknown' only because it had not been done.

My background is in Physics and optics and over forty years in scientific research.

I did calculate all aspects of every part of my new system. My solution was a unique solution because it was correct. For me!

This is not a recipe for anyone else's system.

What is the strength of us amateurs, is that no matter where you look at the sky the chances are that the pro's are not looking at your bit of sky!

Evidence based decision making rules!


Bert
thanks Bert. Sounds like we have a very similar background, even down to the how long bit.
When you designed your quite unique system, what tools did you use? - seemed to me that there was nothing much out there.
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Old 30-10-2013, 02:37 PM
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[QUOTE=Shiraz;1028469]My understanding of why summed short subs and one long exposure can be the same is that the primary broadband noise is due to shot noise - that comes as part of the signal-plus-background and you get the same number of photons (target, background and noise) in a given period however you choose to collect them. This argument falls over if you have a camera with high read noise - then the read noise can add to the overall noise and make the SNR worse for multiple short subs. However, it is often still possible to use relatively short subs and still keep the total read noise contribution below the level where it makes any real difference.


Something is not right here. The top imagers are going for longer exposures and getting better results. If your tracking can handle it I have noticed for example you start getting dimmer, fainter details when using 15 minute subs versus 10 minutes.

Don Goldman R Jay Gabany both mentioned they are now using 30 minute subs for their imaging.

John Gleason of Ha fame uses 40 minute subs on his FSQ and Ha imaging.

The usual barrier is tracking. Can your setup handle 30 minute subs and still get tight round stars or do they become elongated or even just bloated from the bouncing around from corrections and periodic error?

I plan on going longer (I currently image at 10 or 15 minutes for LRGB and 20 minutes for narrowband) as it seems to me the really faint parts have to get above the noise floor of the system. It has the best chance of being imaged if you go longer if your skies permit it (otherwise you will get an excellent bright image of the light pollution!). I intend tweaking PEC and Polar Alignment and getting large T-point models to make long exposure with tight round stars possible every time.

Greg.
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Old 30-10-2013, 03:51 PM
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All this discussion on SNR.....
Actually how DO you guys actually measure the SNR in the images you take????
(I have to use some processing software to determine the SNR in our spectra..not easy.)
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Old 30-10-2013, 05:32 PM
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[QUOTE=gregbradley;1029014]
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Originally Posted by Shiraz View Post
My understanding of why summed short subs and one long exposure can be the same is that the primary broadband noise is due to shot noise - that comes as part of the signal-plus-background and you get the same number of photons (target, background and noise) in a given period however you choose to collect them. This argument falls over if you have a camera with high read noise - then the read noise can add to the overall noise and make the SNR worse for multiple short subs. However, it is often still possible to use relatively short subs and still keep the total read noise contribution below the level where it makes any real difference.


Something is not right here. The top imagers are going for longer exposures and getting better results. If your tracking can handle it I have noticed for example you start getting dimmer, fainter details when using 15 minute subs versus 10 minutes.

Don Goldman R Jay Gabany both mentioned they are now using 30 minute subs for their imaging.

John Gleason of Ha fame uses 40 minute subs on his FSQ and Ha imaging.

The usual barrier is tracking. Can your setup handle 30 minute subs and still get tight round stars or do they become elongated or even just bloated from the bouncing around from corrections and periodic error?

I plan on going longer (I currently image at 10 or 15 minutes for LRGB and 20 minutes for narrowband) as it seems to me the really faint parts have to get above the noise floor of the system. It has the best chance of being imaged if you go longer if your skies permit it (otherwise you will get an excellent bright image of the light pollution!). I intend tweaking PEC and Polar Alignment and getting large T-point models to make long exposure with tight round stars possible every time.

Greg.
thanks Greg - you have proved my point. I have no desire to second guess DG or RJG, but put their systems into the system model with very dark sky (21.6). The suggestion for Don's system is for 27min subs and for RJay, 21 minute subs. Since I made a number of unknowable assumptions, I think that this is pretty good validation of the model. All along, the suggestion for NB has been "as long as possible", so JG's use of 40 minutes is also some sort of validation.

The point of the original post was that just because someone like DG uses 30 minute subs, there is no valid for anyone else to immediately change over to 30 minute subs (unless they have a similar system) - sub length should be individually chosen for the characteristics of the scope, camera and sky. For example, Mike S's system under the same conditions does not need subs longer than 3 minutes, even though Mike uses longer ones - he would be throwing away a lot of dynamic range if he used 30 minute subs. Your CDK with the 16803 needs subs of at least 15 minutes under very dark sky. There is no single sub length that is required if you want quality images and there may even be some very good reasons for using shorter subs than recommended by the maths to overcome seeing variability over a scale of a few to tens of minutes.


Quote:
Originally Posted by Merlin66 View Post
All this discussion on SNR.....
Actually how DO you guys actually measure the SNR in the images you take????
(I have to use some processing software to determine the SNR in our spectra..not easy.)
Trying to measure SNR for a single complex image is even less easy - it is not possible to separate out structure and noise. In assessing equipment, have used a flat, removed bias and then selected by eye a region where there is no obvious gradient. then used Nebulosity to get the stats for a 21x21 region in the selected part of the image.

ISO has standards/measurement techniques for image quality/SNR. http://en.wikipedia.org/wiki/Signal-...ratio_(imaging) might be worth a read.

Last edited by Shiraz; 30-10-2013 at 05:53 PM.
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Old 30-10-2013, 05:45 PM
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Ray,
WIKI repeats the theoretical methodology....
Do programs like Nebulosity/ Maxim etc actually give a SNR for a nominated area??
I'm just interested as AP's seems to take this into acount in their analysis but I haven't seen the data.....
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Old 30-10-2013, 05:56 PM
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Ray,
WIKI repeats the theoretical methodology....
Do programs like Nebulosity/ Maxim etc actually give a SNR for a nominated area??
I'm just interested as AP's seems to take this into acount in their analysis but I haven't seen the data.....
Neb gives mean and SD, so yes it provides SNR for a flat image. The only object I know of that can be used to get an estimate of SNR in a real image is Helix - parts of that are very flat. Or you could use a bit of featureless sky background if you can find one. Otherwise, it is only reasonable to measure system SNR with controlled images.

Last edited by Shiraz; 30-10-2013 at 06:10 PM.
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Old 30-10-2013, 06:11 PM
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Ray,
""it is only reasonable to measure system SNR with controlled images""
Is this what AP's actually do???
I'm tending to think SNR is very subjective and not rigorously applied....
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Old 30-10-2013, 06:15 PM
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Originally Posted by Merlin66 View Post
Ray,
""it is only reasonable to measure system SNR with controlled images""
Is this what AP's actually do???
I'm tending to think SNR is very subjective and not rigorously applied....
I have done so when validating imaging system models, can't speak for others.

SNR is of no use in assessing real images and is never used that way as far as I know - it is used for doing comparative assesment of equipment and imaging technique options - where it can be applied rigorously.

Last edited by Shiraz; 30-10-2013 at 06:46 PM.
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Old 30-10-2013, 10:02 PM
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thanks Bert. Sounds like we have a very similar background, even down to the how long bit.
When you designed your quite unique system, what tools did you use? - seemed to me that there was nothing much out there.

There is only one criteria, maximise the signal to noise without sacrificing too much spatial information! For the last thirty years of my working life I collected x-ray diffraction data from protein crystals with very large unit cells. Most of the experimental innovative effort in our lab was to maximise the signal to noise of our data. Our home laboratory system had a better signal to noise than many/all synchrotron based systems. The protein beam line at Melbourne's synchrotron uses many of our concepts and is one of the best on the planet.


Bert

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Old 30-10-2013, 10:24 PM
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Further to my earlier post, here are some posts made by Scott Rosen in a Yahoo group I'm part of....

Quote:
If you have a moment, read through the entire message between Roger Clark and me. While I was asking my questions of him, I made a point of explaining my best understanding of the theories involved, and I was quite happy to find that Roger confirmed that my general understanding was correct. As such, I think its a good guide to REALLY comprehending what we're trying to accomplish when capturing our photons.

As to the 100 x 1 minute versus the 10 x 10 minute - if we assume that 1 minute is long enough to bring the faint parts of the image above the read noise, then I don't particularly agree with the conclusion. Yes, the SNR in each of the 10 minute frames will be 3.16x higher than a corresponding 1 minute frame. However, there will only be 1/10th as many frames. Ergo, you'll correct for the lower SNR by stacking 10 times as many low SNR 1 minute frames resulting in a 3.16x restoration in SNR.

Now, real world, 1 minute subs are often not long enough to bring a separation between the left axis of your histogram and the data mountain (even at a high ISO). This may be the case where you have a reasonable amount of light pollution, but this wouldn't be so in most dark skies. So, there may very well be some difference between 100 x 1 minute and 10 x 10 minute.

I shoot from a pretty dark sky (SQM typically around 21.4). I usually shoot my RGB images at ISO 1600 and 3 to 5 minute subs. This gives me a little bit of separation between the left axis of the histogram and the data mountain - right around 15%. I choose these relatively short subs (particularly for a dark sky) because they give me the histogram I want. Any higher, and I just risk more clipping of my highlights. Additionally, if I have relatively short 5 minute subs, each of them is 1/2 as likely as a 10 minute sub to be ruined by an airplane flying through the FOV, or some guiding issue, etc. This way, when the airplane strikes, I've only wasted 5 minutes of my dark time and not 10 minutes. Moreover, I have 5 minutes more of photons to improve my SNR.

I've probably posted this image as a demonstration more times than it deserves, but here is an image of M101 where I captured 7 hours of 45 second subs (I hadn't retrofitted my mount for autoguiding yet):

http://www.astronomersdoitinthedark....hp?c=157&p=409

Now, I can assure you that my histogram for this image was pegged against the left axis. This is NOT optimal. But, the image still has surprisingly good SNR in the faint parts of the galaxy. My 9 hour image taken a year later with 3 minute subs doesn't look that much better:

http://www.astronomersdoitinthedark....hp?c=113&p=438

Keep in mind, too, that the latter image had the benefit of being reprocessed using the DSLR-LLRGB workflow, whereas the earlier image did not. This probably accounts for more of the improvement in chromatic noise than the longer subs would.

Now, I don't recommend 45 second subs (unless that is your unguided limit), but I think this helps to show that we may be giving too much credit to long subs. Long subexposures DO look better individually than their short sub equivalents, but their aggregate stacks don't look that much different.
and....

Quote:
If you want to skip the back and forth and find the crux of the matter, look at the end of the message for the last paragraph or two where it says:
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> --- BEGIN QUOTATION ---
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> >
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> > But, at the shadow end of an image, am I correct that the rule
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> > of thumb to have the back of the camera histogram showing
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> > a separation of the data "mountain" from the left axis of
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> > 15% to 50% along the axis is based upon the principle of
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> > ensuring that you've separated the faintest parts of the image
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> > from the read noise of the camera?
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>
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> Certainly separation, but once separated, it matters little if at 15%
>
> versus 50% as the sensor is linear.
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> --- END QUOTATION ---
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> The important point here is that there's no great value in taking longer exposures once you're able to separate the faintest parts of the image from the read noise (i.e., once you have some separation of your histogram from the left axis). In fact, one of the down sides to longer exposures once you achieve this minimum separation is that you're going to be at risk of clipping more of your highlights (typically the cores of the bright stars).
but....

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Now, if you were trying to capture narrowband data (i.e., Ha), you'll probably need the longer sub length and higher ISO.
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