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Peter in Reno
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Re: 5 or 30 minute exposures for narrowband? new [Re: freestar8n]
      #5652304 - 01/30/13 01:15 PM

Mike,

Can you upload a good unprocessed sample sub in FIT file format that you were having issues? Maybe include master bias, dark and flat if possible. I am having difficulty understanding your issues. I cannot imagine a brighter site would result better than your dark site. I have processed other people's images taken at a dark site and they were so easy to process because there were so much data and so little noise, light pollution or sky glow.

Like Frank said, you are still much better at a dark site than where I live.

Peter


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Inverted
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Reged: 01/19/13

Loc: LP Land
Re: 5 or 30 minute exposures for narrowband? new [Re: freestar8n]
      #5652323 - 01/30/13 01:26 PM

Quote:

Read noise usually shows a pattern across the ccd - and a master bias captures that pattern so it can be subtracted. But there will remain a random, Poisson component that cannot be subtracted. The pattern noise is the "fixed pattern noise" in the bias - and it can be subtracted away. The Poisson noise is the "read noise" - and it cannot. If you have a good master bias, the pattern noise is removed well, but the read noise remains. And you get one hit of read noise every time you expose an image - regardless of the exposure time.

So - if the read noise is visible in the sub-exposures, as happens at a dark site, you want to go longer so that sky glow, which is a fundamental limit at your sites, overcomes it.

If the site is so dark that you can't really do that - no problem - it still means your image will have low noise and will turn out well.

Frank




I see, so basically the sky-glo is being used to add a pedestal to the data? Also, I realize bias subtracts the pattern part of read noise, what I meant was to what extent is there a noticeable remaining after subtracting bias? I'm having trouble comprehending the relative scales. For example, if we take 16 bias frams, we'd still have 25% of the pattern noise. Is that enough to notice, if the sky glo pedestal is low enough? What if we take 64, now we're down to 12.5%, or heck, I could set the camera to just take lots of bias frames while at work, at 10,000 now I'd be down to 0.01%. (Also, I am interested if read pattern noise changes much over time - i.e. how long can I use the same bias frames, but maybe that's another thread..) If we can sufficiently eliminate pattern noise, then I think the pedestal shouldn't be perceived to be beneficial even; usually random noise just isn't that easy to see... I guess as you say though, if there is no sky noise, against a perfectly black background, maybe it is still more noticeable...


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Fogboundturtle
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Reged: 05/20/09

Loc: Burnaby, BC
Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5652376 - 01/30/13 01:47 PM

I am having hard understanding the concept. Skyglow light spectrum would have been excluded from the Narrowband filter in the first place so can it have any influence of the final result ?

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vpcirc
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Reged: 12/09/09

Loc: Merced CA
Re: 5 or 30 minute exposures for narrowband? new [Re: Peter in Reno]
      #5652398 - 01/30/13 01:58 PM

Peter it's not that a brighter site is "better" It's that it requires longer sub exposures in my case. The quality of the data is better. Peter I'll send you a private message with a link

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Mike Wiles
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Reged: 02/04/09

Loc: Goodyear, AZ
Re: 5 or 30 minute exposures for narrowband? new [Re: Fogboundturtle]
      #5652401 - 01/30/13 02:01 PM

Quote:

Skyglow light spectrum would have been excluded from the Narrowband filter in the first place so can it have any influence of the final result ?




Its influence in the final result is the lack of skyglow. Because the narrowband filter does such a good job of blocking the sky glow, it becomes nearly impossible to overcome the read noise with skyglow in a narrowband image.

Mike


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Inverted
sage
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Reged: 01/19/13

Loc: LP Land
Re: 5 or 30 minute exposures for narrowband? new [Re: Fogboundturtle]
      #5652421 - 01/30/13 02:08 PM

Quote:

I am having hard understanding the concept. Skyglow light spectrum would have been excluded from the Narrowband filter in the first place so can it have any influence of the final result ?




I know looking at the background ADU in some images taken with a HA filter recently, and plugging in the numbers to the Starizona CCD calculator, I was still getting a recommended exposure time of about 10 minutes. This was taken from a very light polluted site with a full moon though.

Looking around I did find this

from the sbig site here:
http://sbig.impulse.net/dss7/dss7.htm

The dark site is much darker, but if not zero, presumably, given time, some skyglow will get through as it appears pretty broadband.

I don't know to what extent. An image can't have a lower SNR by removing noise though...


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Fogboundturtle
professor emeritus


Reged: 05/20/09

Loc: Burnaby, BC
Re: 5 or 30 minute exposures for narrowband? new [Re: Mike Wiles]
      #5652425 - 01/30/13 02:09 PM

Quote:

Quote:

Skyglow light spectrum would have been excluded from the Narrowband filter in the first place so can it have any influence of the final result ?




Its influence in the final result is the lack of skyglow. Because the narrowband filter does such a good job of blocking the sky glow, it becomes nearly impossible to overcome the read noise with skyglow in a narrowband image.

Mike




are you suggesting it would be preferable to capture with 7mn instead of 3nm filter because it would than be easier to offset the read noise?


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shams42
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Loc: Kingsport, TN
Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5652426 - 01/30/13 02:09 PM

Quote:

Also, I realize bias subtracts the pattern part of read noise, what I meant was to what extent is there a noticeable remaining after subtracting bias?




You have to distinguish between noise and unwanted signal. The noise is random and cannot be subtracted out. Some aspects of unwanted signal, including any fixed pattern that occurs on readout as well as dark current, can be removed. Indeed, calibration with a master dark frame removes both of those aspects of unwanted signal. It is desirable to use an adequate number of exposures for the master dark so that both of these aspects of unwanted signal are well-sampled from a statistical point of view.

The noise components remain. Noise can't be subtracted.

As Frank said, the a master bias frame has two functions. First, to represent any unwanted signal resulting from reading the CCD (often called "pattern noise"). This is only really important if you aren't using dark subtraction. It's really only useful for removing any readout pattern from the flats. Second is to represent the pedestal added to each pixel's value. Knowing this pedestal value is crucial for calibrating the master flat(s). Remember the lights will be divided by a normalized flat, and if the pedestal is not removed the division will not be correct.


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Alph
Carpal Tunnel


Reged: 11/23/06

Loc: Melmac
Re: 5 or 30 minute exposures for narrowband? new [Re: Fogboundturtle]
      #5652441 - 01/30/13 02:20 PM

Quote:

I am having hard understanding the concept. Skyglow light spectrum would have been excluded from the Narrowband filter in the first place so can it have any influence of the final result ?



If the background noise is not much higher than the readout noise, then stacking is not a very effective way to increase the S/N.


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vpcirc
Post Laureate


Reged: 12/09/09

Loc: Merced CA
Re: 5 or 30 minute exposures for narrowband? new [Re: Fogboundturtle]
      #5652459 - 01/30/13 02:29 PM

Quote:

Quote:

Quote:

Skyglow light spectrum would have been excluded from the Narrowband filter in the first place so can it have any influence of the final result ?




Its influence in the final result is the lack of skyglow. Because the narrowband filter does such a good job of blocking the sky glow, it becomes nearly impossible to overcome the read noise with skyglow in a narrowband image.

Mike




are you suggesting it would be preferable to capture with 7mn instead of 3nm filter because it would than be easier to offset the read noise?




Going to a a 3nm compared to say a 7 will give you greater contrast from what Don told me. Sky glow is signal and from what I understand can actually hide noise because the SNR changes with "that signal" It's also a limiting factor and prevents some of the signal you do want. It's not that the noise isn't still there, it's that the "signal from sky glow hides some of it so it's not as noticeable. In my case at a dark site, I needed more signal to help overcome the read noise. At least that's the way I understood it. I've only been doing narrowband for a year so I'm still learning a lot. I don't profess to understand any of this, I'm only sharing what I was told.


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mikeschuster
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Reged: 08/25/11

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Re: 5 or 30 minute exposures for narrowband? new [Re: Alph]
      #5652460 - 01/30/13 02:30 PM

Frank,
Treating dark noise as sky due to its time dependence is a nice idea. But on the other hand it is a defect that would not be present in a "perfect" camera, whereas sky remains unchanged. So maybe I will continue to treat dark noise as a defect to be overcome. And LOL about floodlights. So true!
Thanks,
Mike


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Inverted
sage
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Reged: 01/19/13

Loc: LP Land
Re: 5 or 30 minute exposures for narrowband? new [Re: shams42]
      #5652468 - 01/30/13 02:35 PM

Quote:

You have to distinguish between noise and unwanted signal.




The definition of noise is unwanted signal. In statistics there is random noise and bias ("pattern noise"). Sorry, this is semantic, but if we're not speaking the same language we're going to misunderstand each other...


Quote:


The noise components remain. Noise can't be subtracted.





Right, random noise remains and this is can be"averaged out" due to properties of the Central Limit Theorem (or if you like, the less rigorously defined "law of Large Numbers").


Quote:


As Frank said, the a master bias frame has two functions. First, to represent any unwanted signal resulting from reading the CCD (often called "pattern noise"). This is only really important if you aren't using dark subtraction. It's really only useful for removing any readout pattern from the flats. Second is to represent the pedestal added to each pixel's value. Knowing this pedestal value is crucial for calibrating the master flat(s). Remember the lights will be divided by a normalized flat, and if the pedestal is not removed the division will not be correct.




That's interesting, the flat does capture bias as well, as does the darks. However, as we saw earlier, remaining noise follows the rule 1/sqrt(n), and I can take bias frames a lot faster than darks. There are diminishing returns, but clearly the n=16 rule doesn't make much sense, as 25% of the noise is still remaining, but it's true the returns diminish substantially after that, to cut that in half I need to go to 64 for example. However, bias frames being easy to take, I could take 10000 and drop the remaining noise down to 0.01%. of the original noise. And really, even if I don't take that many I can still drop the noise down pretty substantially from n=16. At n=400, I'd be at 0.05% which is a heck of a lot less proportionally than 25% at n=16. Taking 400 darks or flats seems like a royal PITA, but taking 400 bias frames is no big deal at all. So, I'm not sure I agree that "there is no benefit to bias frames if taking darks. Unless I'm missing something, totally possible... The big question seems to be under "what" circumstances, does "what" level of noise become perceivable?


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Alph
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Reged: 11/23/06

Loc: Melmac
Re: 5 or 30 minute exposures for narrowband? new [Re: mikeschuster]
      #5652488 - 01/30/13 02:45 PM

Quote:

Treating dark noise as sky due to its time dependence is a nice idea



It is not just an idea. You have to subtract skyglow (and dark current) from the captured signal in order to calculate the S/N. Mike Newberry in the above-mentioned article did fold skyglow and dark current into the background noise.


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Inverted
sage
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Reged: 01/19/13

Loc: LP Land
Re: 5 or 30 minute exposures for narrowband? new [Re: Alph]
      #5652505 - 01/30/13 02:52 PM

Quote:


If the background noise is not much higher than the readout noise, then stacking is not a very effective way to increase the S/N.




I feel like we need a table of random noise and fixed noise Another for example would be "seeing". Is atmospheric distortion random or pattern noise? I guess patter noise, as that would seem to explain adaptive optics. It seems there are possibly lots of other sources of random noise though, although, unclear that they significantly contribute, but that may be relative to the pedestal. And as mentioned, dithering, sigma reject stacking, dropping bad subs can still help eliminate sources of pattern noise too.


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mikeschuster
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Reged: 08/25/11

Loc: SF Bay area
Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5652518 - 01/30/13 03:09 PM

Alph, I am doing the same thing. I just choose to think of dark current as a camera defect rather than a sky source.
Mike


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shams42
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Reged: 01/05/09

Loc: Kingsport, TN
Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5652537 - 01/30/13 03:23 PM

Quote:

Quote:

You have to distinguish between noise and unwanted signal.




The definition of noise is unwanted signal. In statistics there is random noise and bias ("pattern noise"). Sorry, this is semantic, but if we're not speaking the same language we're going to misunderstand each other...




When I say "signal", I mean that aspect that is fixed, non-stochastic, always the same each time it is measured.

When I say "noise", I mean that aspect that is random, the variable and unpredictable aspect, different each time it is measured.

The wanted signal comes from stars and DSO. Unwanted signal comes from the CCD and camera electronics. Among other sources.

Each signal brings with it shot noise. Shot noise from the object, shot noise from the background, etc. Read noise not a "tag-along" shot noise but errors that occur when reading the CCD.

Are you using the terms differently?

I hear lots of people saying that they "use darks to remove the thermal noise" or that "bias frames remove read noise." No -- bias and darks can remove certain aspects of unwanted signal but not noise. Distinguishing between true noise and unwanted signal is clarifying IMO.

Edited by shams42 (01/30/13 03:43 PM)


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pfile
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Reged: 06/14/09

Re: 5 or 30 minute exposures for narrowband? new [Re: shams42]
      #5652608 - 01/30/13 03:54 PM

everyone should own a copy of HAIP. reading the first few chapters of that book would answer all the above questions.

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Inverted
sage
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Reged: 01/19/13

Loc: LP Land
Re: 5 or 30 minute exposures for narrowband? new [Re: shams42]
      #5652626 - 01/30/13 04:00 PM

Quote:


Are you using the terms differently?





Signal
http://www.statistics.com/index.php?page=glossary&term_id=844

Noise
http://www.statistics.com/index.php?page=glossary&term_id=805

Bias
http://www.statistics.com/index.php?page=glossary&term_id=717


Really though, I think the "signal" definition given above, is bit vague, without considering the broader context.
In practice, a more useful way of defining, in the context of AP, I think, would be to say "it is the asymptotic mean rate of photons hitting the collector, given an infinite number of unbiased samples, collected over the integration time." The estimator of this will deviate due to noise and bias as defined above.

Usually we define noise more as anything unwanted, but random and bias as anything unwanted and not random.


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Peter in Reno
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Re: 5 or 30 minute exposures for narrowband? new [Re: Inverted]
      #5652648 - 01/30/13 04:10 PM

Hi Mike,

Thanks for uploading the images. I think your NB images look similar to my NB images and they appear to be normal to me. If you took enough subs, do proper calibration of each sub, and stack them, the end result will look much better.

I think it's tricky to evaluate a single NB sub because the signal is very weak. When stretching a single NB sub heavily to reveal DSO, it appears to look bad because more low ADU bad pixels show up. Calibration and stacking will take care of this.

My bottom line is I think your subs look normal to me.

Peter


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Inverted
sage
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Reged: 01/19/13

Loc: LP Land
Re: 5 or 30 minute exposures for narrowband? [Re: pfile]
      #5652708 - 01/30/13 04:33 PM

Quote:

everyone should own a copy of HAIP. reading the first few chapters of that book would answer all the above questions.




Thanks! I've been out of the hobby for a while though and am not familiar with this. What does it stand for?


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