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dawziecat
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Re: 5 or 30 minute exposures for narrowband? new [Re: vpcirc]
      #5649862 - 01/29/13 08:45 AM

Quote:


Also, narrowband signals are inherently weak, requiring long exposures of 20 40 minutes.





Yes, but at what focal ratio?

You most certainly do not require such exposure durations with f/2.8 optics. Lots of iconic objects in astronomy are not small in angular diameter and not especially faint in Ha either.

These can be imaged very satisfactorily in NB with fast camera lenses and almost all of us do image these objects when starting out in this hobby.

Sure, it is ideal that one can do long subs and great if your gear is up to it. But, for a great many of us, our gear limits what we can reasonably accomplish.

My biggest thrill in this hobby to date was when I first tried an Ha filter with a DSLR and a 300mm f/2.8 camera lens. No one should deny themselves this out of fear their gear is not up to 20-40 minute exposures. Stack up a dozen or two exposures of a few minutes each (or even less) and be prepared for a kick!

Seems to me there is a conflict of expectations here. What you need to image a small SNR in NB and what you need to do The Rosette, The NA Nebula complex, the Horsehead, and the like are pretty different in regards to FL, associated focal ratio and attendant sub exposure length.


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vpcirc
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Re: 5 or 30 minute exposures for narrowband? new [Re: dawziecat]
      #5649887 - 01/29/13 09:07 AM

That's a great example, he said to try and image the horsehead at short exposures and see what happens compared to longer exposures. A very bright DSO like the Rosette isn't going to show the signal problem because so much is there.
But again the point is why stack 2 dozen images. There is zero gain from going beyond 16.


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David PavlichAdministrator
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Re: 5 or 30 minute exposures for narrowband? new [Re: cn register 5]
      #5649891 - 01/29/13 09:10 AM

Remember to play nice and leave the personal comments out of the discussion. It adds nothing to the conversation.

David


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Inverted
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Reged: 01/19/13

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Re: 5 or 30 minute exposures for narrowband? new [Re: dawziecat]
      #5649898 - 01/29/13 09:16 AM

I'm no expert on CCDs or ccd noise models. However, just to state the obvious, it seems clear that if read noise is the dominant source of noise, and read noise varies considerably between cameras, then it would seem that the same snr, over a fixed integration time will vary considerably by camera. For example, if you have a camera with 3e- vs one with 15e- , take 1 hour of integration, with 6x10 minute exposures, with the 15e- camera, then the same read noise contribution, with the 3e- camera should be archived with 30x2 minute exposures. However, through sampling, all random noise should tail further towards the asymptote with the 30x2 minute exposures. Also, if you either your shots, fixed pattern noise should as well. So, your total snr I think may actually be better, as long as the total read noise contribution is roughly the same or better. also, depending on how low the read noise is and how close it is to the point where read noise is no longer the dominant noise is, it should also start to become more beneficial to "sample out" other sources of noise, by taking more exposures. I have no idea what that level is though, or how close modern cameras are getting to it.

Anyways though, still in practice, you may well be better off taking longer exposures wot the low e- cameras, but as mentioned, I'm sure there a lot of factors to consider, even without considering camera noise models.


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Inverted
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Re: 5 or 30 minute exposures for narrowband? new [Re: vpcirc]
      #5649910 - 01/29/13 09:30 AM

Quote:

There is zero gain from going beyond 16.




No doubt gains decrease as you go, but anyone who understand statistics and the central limit theory, should understand that there is alway a gain until you reach the asymptote. Again, i'm no expert on ccds, but do know enough to know there will be a gain... Whether the gain improves snr noticeabley depends on all sorts of factors, but I would be interested to hear how he came up with 16. I suspect that is a rough example, based on a certain setup, sky conditions, target brightness etc.. That may be true for that one instance,, but clearly is not generalizeable. I know I can certainly see a difference between a statck of 16x5 min exposures and a stack of 50... Whether or not the stack of 50 would be better than a single 250 minute exposure, I don't know "generally" but with my setup the 50x5 would be better LOL


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vpcirc
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Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5649917 - 01/29/13 09:40 AM

See Tony's research here;
http://www.astrosurf.com/davidguerit/doc/astromag.pdf

You can also email him at tonyhallas@sebastiancorp.net
He's very gracious and responds quickly.


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Alex McConahay
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Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5650001 - 01/29/13 10:29 AM

I think the number 16 is taken as a cutoff point after which there is little APPRECIABLE advantage----not that there is none at all.

The point is that to see an appreciable reduction in noise, you have to go from sixteen to thirty two exposures--so you are at the point of diminishing returns, and it just is not worth it.

Alex


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Inverted
sage
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Reged: 01/19/13

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Re: 5 or 30 minute exposures for narrowband? new [Re: Alex McConahay]
      #5650048 - 01/29/13 10:58 AM

Thanks for the link Mike. I definitely wish I knew more about this stuff. I know as far as sampling goes, random noise should end up as 1/sqrt(n) where n is the number of exposures.

So at 4 exposures, you'd have half the noise of 1 exposure. To get to half the noise of 4, you'd need 16. But, then to start really seeing an improvement, it does drift off, to cut that remaining noise in half, you'd then need to go to 64, and to cut that in half, you'd need to go to 256 etc...

It looks like the example is for dark noise, but the same applies to any unbiased normally distributed random noise, so, read noise etc... in theory anyways...

It looks like he uses "8" as a start value, I'm not sure what units, but the scale will be larger with a larger starting noise. I think in practice, there are lots of factors, such as sky fog, dynamic range of computer monitors, pixel size, tracking, seeing etc... that will limit real-world appreciation of gains after some point. I know though, you can see a gain after 16, as I've added integration to images, taken previous night for example, and watched the images improve. It does definitely take more and more to see an improvement as you go, and if the image is good enough to begin with, the improvement certainly may not be perceivable.


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Tandum
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Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5650144 - 01/29/13 11:54 AM

My 2 cents worth. Noise is something you deal with after you have signal. I'm sure an f3 lens gives you lots of signal quickly with little noise but we don't all use f3 lenses.

Capture what you can and if it's noisey get more, simple as that. The longer the exposure the deeper you will see. My exposures are limited by polar alignment mainly. Longer than 15 minutes and I see rotation.

Here's an example with noise. Weather stopped getting more data but it still looks ok.


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Inverted
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Re: 5 or 30 minute exposures for narrowband? [Re: Tandum]
      #5650221 - 01/29/13 12:36 PM

Quote:

Noise is something you deal with after you have signal.




Exactly. One thing to point out though is that, the accepted theory is that signal increases with time linearly. So, if your using the same equipment, the signal will be the same if you have 1 60 minute exposure, or sum 60 1 minute exposures. The remaining difference between 60 1 minute exposures and 1 6 min, is noise. Different sources of noise, can have different properties and can be best eliminated with different methods. If you reduce one source of noise to negligible levels, but still don't have a nearly perfect representation of the signal, then there must be another source of noise that hasn't been reduced sufficiently.

Of course, I honestly don't know much about all the sources of noise with this regard and how they all fit together. And since they say this hobby is all signal to noise, and we can see that we can write signal out of the equation... all I really know is I don't really know much of anything


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freestar8n
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Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5650257 - 01/29/13 12:53 PM

Quote:

Anyways though, still in practice, you may well be better off taking longer exposures wot the low e- cameras, but as mentioned, I'm sure there a lot of factors to consider, even without considering camera noise models.




Hi-

I think your note sums up key points - and mentions the key word, "asymptote". The basic noise model used in this context - by everyone basically - is that shot noise in the signal and in the sky glow depends only on total integration time, while the read noise is independent of integration time and only depends on the number of exposures. There are other noise terms, but this works well for typical imaging sessions involving a small number of exposures - and it completely ignores other factors such as guiding and field rotation.

A more practical view would be to consider how much sky glow signal you are getting - and that is heavily dependent on f/ratio. At f/15 with narrowband, you will need much longer sub-exposures than f/2 hyperstar in order to make read noise unimportant.

What is "unimportant"? Well - as you make the sub-exposures longer (and fix the total integration time) there will be an improvement as you reduce the impact of read noise - but at some point you won't notice an improvement anymore - and instead your image goes bad due to other factors such as guiding and field rotation.

For anyone interested in this stuff there are many web sites that describe it reasonably well - and refer to the same basic noise model. Some web pages get misinterpreted and make people think you *need* to go longer at a dark site, for example, when in fact they are just saying you *can* go longer and still get benefit - because the sky glow is small.

I think the example given earlier by Dawziecat is great and should make several points. You can do very well with short exposures if you have a fast system - even with 3nm filters. His example stacks a very large number of subs - and I think the point of diminishing returns would be met much earlier - but the key point is that you would need to know the read noise and sky glow to determine how much of a win there is in using very long sub-exposures.

If there are any web pages or people who disagree with this my assessment here - I'm happy to address them.

Frank


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vpcirc
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Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5650286 - 01/29/13 01:09 PM

Quote:

Thanks for the link Mike. I definitely wish I knew more about this stuff. I know as far as sampling goes, random noise should end up as 1/sqrt(n) where n is the number of exposures.

So at 4 exposures, you'd have half the noise of 1 exposure. To get to half the noise of 4, you'd need 16. But, then to start really seeing an improvement, it does drift off, to cut that remaining noise in half, you'd then need to go to 64, and to cut that in half, you'd need to go to 256 etc...

It looks like the example is for dark noise, but the same applies to any unbiased normally distributed random noise, so, read noise etc... in theory anyways...

It looks like he uses "8" as a start value, I'm not sure what units, but the scale will be larger with a larger starting noise. I think in practice, there are lots of factors, such as sky fog, dynamic range of computer monitors, pixel size, tracking, seeing etc... that will limit real-world appreciation of gains after some point. I know though, you can see a gain after 16, as I've added integration to images, taken previous night for example, and watched the images improve. It does definitely take more and more to see an improvement as you go, and if the image is good enough to begin with, the improvement certainly may not be perceivable.




Since most stacking programs "average" the images you might see an improvement in image quality as the new images may be of higher quality than the previous. I think the result would be even better though if you again measured all images and deleted all but the best 16. I do not profess to have any understanding of the science behind this but I can assure you the folks agreeing with Tony's findings are some of the top imagers in the country. I did ask Tony if that theory applied to light frames in person at PATS this year. The answer was yes.


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vpcirc
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Re: 5 or 30 minute exposures for narrowband? new [Re: freestar8n]
      #5650306 - 01/29/13 01:18 PM





Some web pages get misinterpreted and make people think you *need* to go longer at a dark site, for example, when in fact they are just saying you *can* go longer and still get benefit - because the sky glow is small.

Frank




I was told directly by John Smith at AIC that indeed you do need to go longer at a dark site, especially with narrowband. John is an expert in this field. For those who do not know John, you can find his many useful articles and tips here; http://www.hiddenloft.com/


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freestar8n
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Re: 5 or 30 minute exposures for narrowband? new [Re: vpcirc]
      #5650333 - 01/29/13 01:33 PM

Quote:

I was told directly by John Smith at AIC that indeed you do need to go longer at a dark site, especially with narrowband.




Yes - I think you misunderstood what he was telling you. I would be very surprised if he, or anyone familiar with the underlying noise model, would say you need to use longer subs at a dark site to achieve the same quality in the same time as at a bright site. The message he was probably trying to convey is - you *can* and you *should* go longer - and your results would be even better at a dark site.

The benefit you get from longer subs depends on read noise and sky glow. Short subs at a dark site will end up with a much better image than the same subs at a bright site. But going longer at a bright site won't help much and isn't really worth it - whereas at a dark site it is worth it.

It all comes down to knowing the noise terms - which in this case is read noise and sky glow - for your camera, equipment, and location.

Frank


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vpcirc
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Re: 5 or 30 minute exposures for narrowband? new [Re: freestar8n]
      #5650342 - 01/29/13 01:41 PM

I don't think I "misunderstood him" as I was surprised to hear him say that as I thought a dark site meant I could go shorter. He was helping me evaluate my images. I will find out, but it had to do with sky glow "hiding noise", and since sky glow is "signal" I had very little to hide any in noise in the background and therefore needed longer images to overcome that noise. I was shooting 15 Min LRGB and 30 min NB.

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vpcirc
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Re: 5 or 30 minute exposures for narrowband? new [Re: vpcirc]
      #5650359 - 01/29/13 01:46 PM

For a great understanding of the benefit of longer images and why they are much better than multiple short images, John has an easy to understand writeup here

http://www.hiddenloft.com/notes/SNR.txt


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freestar8n
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Re: 5 or 30 minute exposures for narrowband? new [Re: vpcirc]
      #5650378 - 01/29/13 01:52 PM

From what you are describing, it sounds like he assessed the noise in your subs and deduced that the sky glow was small compared to your read noise - and you would benefit from longer exposures. That sounds perfectly fine and I would probably agree with it - assuming it really is read noise dominated. If you took the same setup to a place with big sky glow, or used a faster system or wider filter, you would be doubly impacted in a bad way. Your subs would be much noisier - due to sky glow - and you would not get any benefit from longer subs. All you can hope for is to take many more subs. But - as you say - beyond some number of subs, the stacking just doesn't gain much - so you are just stuck with what you get.

So dark sites are great: your subs look great, and you can make them look even better by going longer. At a bright site - you can't use that trick.

Many people are imaging from brighter sites and/or using fast systems - and they shouldn't feel a need to expose long sub-exposures if it doesn't really help - and in fact can make the end result worse due to other factors. So I just say people should be aware of the noise terms in their imaging so they can make an educated guess of how long they should go - and it all depends.

Frank


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Mike Wiles
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Re: 5 or 30 minute exposures for narrowband? new [Re: freestar8n]
      #5650613 - 01/29/13 03:41 PM

This thread has given me more to think about all by itself than the last 100 threads I've read put together. Excellent discussion. As I'm likely about to demonstrate, I'm anything but an expert on any of this topic.

In terms of getting a shot noise limited narrowband image....I have everything working against me. I have a slow-ish system (f/7.5) with narrow bandpass filters (3nm) and I image from dark to really dark skies (mag 5.4 at home, 6.5+ in the field). With a measured read noise of 9 electrons my camera isn't overly noisy either. I've done the calculations using test exposures for my own setup and from my magnitude 5.4 backyard at home I'd have to shoot upwards of 5 hour subexposures in order to bury the read noise in the shot noise. From the dark sites that I go to where limiting magnitude is up around 7.5 - there's not enough dark hours in a single day to take a shot noise limited subexposure. So there is some credence to the claim that it's extremely difficult to capture shot noise limited subexposures using narrowband filters. I wouldn't say it's impossible - but extremely difficult and almost certainly impractical in real world conditions. With a 7nm filter on an f/2.8 system in Manhattan...it's possibly do-able. It seems unlikely to me that you're going to get a shot noise limited narrowband image without going 20 or 30 minutes on a subexposure regardless of the equipment configuration.

Having said all that, the S/N ratio of an image is 0 if it's not attempted. As a result, a stack of 300 ten second subexposures will always have a higher S/N than not trying at all. You work with the equipment and environment that you have. To my experience - which is limited compared to many here - narrowband exposures should be as long as you dare to take them based on your equipment, conditions and comfort level. You should also take a significant number of subexposures to gain the maximum benefit from modern statistical data rejection algorithms. I think the point of diminishing returns is somewhere up around 30 subs....but I would agree that 16 is the minimum.

With it being impractical at best to take a shot-noise limited narrowband exposure it seems to me that a decent alternative is to:

  • Create a master bias frame taken from an enormous number of bias subs. This will give a really clean master calibration frame to do as much as possible for reducing read noise.
  • Shoot a lot of subexposures on the light frames. Statistical data rejection works more in your favor - and you're going to need it for random read noise from frame to frame.
  • Dither between subexposures. Any pattern noise and random noise left after the above two steps will disappear for the most part.


Mike


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Inverted
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Re: 5 or 30 minute exposures for narrowband? new [Re: vpcirc]
      #5650627 - 01/29/13 03:48 PM

Quote:


Since most stacking programs "average" the images you might see an improvement in image quality as the new images may be of higher quality than the previous.




I used the term "sum" because most people seem to think of signal as something that changes and intensifies. Really, the signal is more conceptual though and is basically fixed. I guess you could look at it as summing, but relative to the noise. The important part is the SNR. However, mathematically, if we had infinite range, it works out the same whether we sum or average.

To illustrate, if I take 3 images, and the signal is 5 in all and the noise is 1,2,1 respectively, then the sum is 15/4 and the average is 5/1.3, but either way, the ratio (SNR) is 3.75. However, our dynamic range isn't infinite, so, for data storage and processing etc... it becomes easier to average. They key though, is statistically speaking, the signal is really a conceptual idea rather than a physical entity.


Edit: by the way, the above is a simplified example, it is intended to illustrate summation versus averaging, not how noise is actually distributed.

Edited by Inverted (01/29/13 04:15 PM)


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freestar8n
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Re: 5 or 30 minute exposures for narrowband? new [Re: Mike Wiles]
      #5650726 - 01/29/13 04:50 PM

Quote:

I have everything working against me. I have a slow-ish system (f/7.5) with narrow bandpass filters (3nm) and I image from dark to really dark skies (mag 5.4 at home, 6.5+ in the field). With a measured read noise of 9 electrons my camera isn't overly noisy either. I've done the calculations using test exposures for my own setup and from my magnitude 5.4 backyard at home I'd have to shoot upwards of 5 hour subexposures in order to bury the read noise in the shot noise.




Hi-

I think the main confusing thing about these sub-exposure calculators is it makes people feel *bad* when the *have to* expose a long time. If the problem is that you have a very high read noise camera - that is indeed a bad thing. But if you are "stuck with" narrowband filters and a dark site - those are both *great* things. As long as your read noise isn't really bad, your images will be *much better* than at a bright site where you are "blessed" with short sub-exposures.

So if you are in this situation - where you have a decent camera but slow optics and a dark sky - you will still be doing much better than if you did not have a dark sky. The only downside is that you know that if your camera had lower read noise, or if you had greater aperture, a given total exposure would be slightly better. But that same total exposure at a bright site would be much worse - so be thankful.

I think for most people the general guideline would be to go as long as you can as long as you end up with more than perhaps 5 sub-exposures. If you are using a guidescope on an sct you will probably be limited by flexure and have to keep it short. If you have field rotation - same thing. If your stars saturate - same thing.

But - yes - go as long as you can if the calculations suggest you should go longer - but you shouldn't feel bad about not using the optimal sub-exposure time if it isn't practical.

Frank


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