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Any tips and tricks for dark nebula and dust clouds?

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#1 DrGomer

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Posted 08 January 2020 - 08:30 PM

When it comes to galaxies vs nebulas, there definitely are different gear imaging considerations to maximize results. How about with dark nebulas and dusty bits?  Is it still just a game of matching optics to field of view and no substitute for more integration time, good calibrations and dark skies?  Or is there something that one would keep in mind that is a bit different vs bright nebulas and galaxies?  



#2 Jerry Lodriguss

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Posted 09 January 2020 - 12:54 AM

Optimize your noise sampling for the faint stuff by using the ISO or gain with the lowest read noise. This is where you will need the extra bit depth when you stretch these faint details that are just above the sky background.

 

And there is no substitute for more integration time and dark skies... or faster optics. :-)

 

Jerry


Edited by Jerry Lodriguss, 09 January 2020 - 12:57 AM.

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#3 Maxtrixbass

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Posted 09 January 2020 - 12:22 PM

I find the dark nebula/dust clouds to be the extreme sport of astrophotography. Like someone described its like imaging at night through a window and trying to get the dust on the glass.

 

Jerry's comments are right on. Its not really different than imaging faint galaxies, just more extreme in what you are trying to capture.


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#4 jerahian

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Posted 09 January 2020 - 12:48 PM

Optimize your noise sampling for the faint stuff by using the ISO or gain with the lowest read noise. This is where you will need the extra bit depth when you stretch these faint details that are just above the sky background.

 

And there is no substitute for more integration time and dark skies... or faster optics. :-)

Jerry, wouldn't using a gain with the lowest read noise normally reduce your FW and DR, thus constraining the range one would need for the faintest of details?  For example, looking at the ASI1600MM Pro, FW of 20K @ gain 0 vs. 500 @ gain 300.

 

Forgive me, I'm not doubting your knowledge or experience here, just trying to understand.


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#5 AhBok

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Posted 09 January 2020 - 01:43 PM

Yes, this is confusing to me as well. Is bit depth the same as full well capacity. Jerry is far more knowledgeable than me, so I am sure there is something missing in my understanding.

#6 Jon Rista

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Posted 09 January 2020 - 03:52 PM

When it comes to galaxies vs nebulas, there definitely are different gear imaging considerations to maximize results. How about with dark nebulas and dusty bits?  Is it still just a game of matching optics to field of view and no substitute for more integration time, good calibrations and dark skies?  Or is there something that one would keep in mind that is a bit different vs bright nebulas and galaxies?  

The best tip I can give you is ditch the LP when imaging dark nebula. The difference between the middle of a red zone, and a green zone, is about 16x. That 16x difference in sky background brightness is HUGE to imaging dark nebula. It is WELL worth the time and effort to drive out to a dark site (and it does not have to be gray/black zone...a deep yellow or green zone will do, if you can point away from LP bubbles) and image dark nebula there. You will still need a decent amount of integration...7-10 hours with a cooled, low noise camera will produce a pretty clean result, but if you can manage multiple trips, 20 hours will give you excellent results. 


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#7 Jon Rista

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Posted 09 January 2020 - 05:43 PM

Yes, this is confusing to me as well. Is bit depth the same as full well capacity. Jerry is far more knowledgeable than me, so I am sure there is something missing in my understanding.

No, bit depth is not the same as FWC. FWC is a physical trait. Bit depth is a data trait. The FWC may be, say, 40000e-. Gain may be 0.65e-/ADU and the ADC 16-bit. That 40ke- would then convert to a maximum of 61538 ADU...which is less than the full numeric range offered by 2^16. Similarly, the gain could be 0.5e-/ADU, same ADC. Technically speaking, such a gain would need 80000 ADU, but a 16-bit ADC is still limited to 2^16, so your maximum numeric value would be 65535. In fact, we also have to account for the offset, which diminishes the real-world range... So if your offset is say 1000 ADU, then your maximum ADU would be 64535. Some cameras actually limit you to 64000, to keep you from exposing into non-linear parts of the dynamic range. 

 

So FWC and bit depth are not the same thing. FWC is a more true physical limit, but even then...depending on the exact gain used, the physical well capacity may in fact result in a converted ADU count that is less, or technically more, than the numeric range allowed by the ADC (the bit depth). 



#8 DrGomer

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Posted 09 January 2020 - 05:48 PM

Thanks Jon. I live in a green zone fortunately with largely unobstructed views if I move to one of 3 locations around the house... and my LP bubbles are manageable if I avoid targets in the vicinity of the cities. . spilling over from the other thread, I think I'm going to try Sharpcap's brain and really maximize, then image over multiple days, moving to 2-3 targets through the night to only image when objects are at least 45deg above horizon. Then do this till I have the 20 or so hours of data on a target.  Stacking 600-1000 images in the end is going to be fun tongue2.gif  Sounds like it is time to get some good day and report back in a few weeks.  

 

(I know that I have some really nice dark sites in reasonable driving distances, but there is something really nice about imaging from home in a nice warm house when it is below freezing at night!)

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#9 calypsob

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Posted 09 January 2020 - 06:16 PM

Thanks Jon. I live in a green zone fortunately with largely unobstructed views if I move to one of 3 locations around the house... and my LP bubbles are manageable if I avoid targets in the vicinity of the cities. . spilling over from the other thread, I think I'm going to try Sharpcap's brain and really maximize, then image over multiple days, moving to 2-3 targets through the night to only image when objects are at least 45deg above horizon. Then do this till I have the 20 or so hours of data on a target.  Stacking 600-1000 images in the end is going to be fun tongue2.gif  Sounds like it is time to get some good day and report back in a few weeks.  

 

(I know that I have some really nice dark sites in reasonable driving distances, but there is something really nice about imaging from home in a nice warm house when it is below freezing at night!)

Do you have an sqm-l? If you live in that area the you should be able to easily build a massive SNR. Fast glass helps out a ton for faint dark fuzzies.



#10 Jerry Lodriguss

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Posted 09 January 2020 - 06:40 PM

Jerry, wouldn't using a gain with the lowest read noise normally reduce your FW and DR, thus constraining the range one would need for the faintest of details?  For example, looking at the ASI1600MM Pro, FW of 20K @ gain 0 vs. 500 @ gain 300.

 

Forgive me, I'm not doubting your knowledge or experience here, just trying to understand.

No need to apologize, we are all here just trying to understand.

 

First, full well is not defined by bit depth. Any amount of dynamic range can fit in any bit depth. It might not be optimum, it might not be pretty, but you can do it.

 

Dynamic range is defined as the full well divided by the noise. So, if you have a camera with a full well of 20,000e, and a read noise of 1.2e @30db gain, you get 20,000/1.2 = 16,666, or about 14 stops of dynamic range.

 

That dynamic range is then sampled by the bit depth. In our example you would sample, or measure, the dynamic range into individual steps of tonal detail in about 14 bits or 16,384 individual steps. 

 

The noise determines the resolution of useful steps of tonal detail. If you have 1.2 electrons of noise floating around in your data, you can't distinguish a signal less than that, it's lost in the noise (single exposures, not stacking).

 

When you use the full well at lowest gain, you get the maximum dynamic range, but, HALF of your bits are being used by the brightest stop of your exposure data.  So, roughly 8,192 steps of your total dynamic range is being used by just the brightest stop of detail.  The second brightest uses 4096. ... and so on, until you get down to the least significant bit, which is the noise (or half that if you try to Nyquist sample the noise). So your really faint detail is going to be down there in the shadows where you don't have hardly any bits of tonal separation. Then you will try to increase the contrast down there to make the faint stuff more visible, and that can make a mess.

 

If you want to increase the contrast of the really faint stuff, like IFN, using the highest ISO/Gain gives you more bits on the faint end for stretching later.

 

So, ideally, you sample at about 2x the noise (which is way down the Nyquist rabbit hole that I am not going to go here), and devote as many bits as you can to the shadow end (which means the lowest read noise at a high gain/ISO usually). 

 

Now, you have to understand, this advice is based on best recording faint stuff, and not maximizing, or even caring much, about dynamic range.  This is for the hard core who want the really faint stuff.

 

You will throw away a ton of the top end of dynamic range and your full-well will dwindle to seemingly nothing, but, hey, you want the faint stuff, right?  you can always drop in bright colors later with an exposure for the bright stuff maximized for DR.

 

Jerry


Edited by Jerry Lodriguss, 09 January 2020 - 07:06 PM.

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#11 DrGomer

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Posted 09 January 2020 - 06:45 PM

Wes, I don't have a meter, just what the dark skies websites show. The 2015 Atlas shows 21.57 mag./arc sec2 at zenith which is pretty good on a new moon. Even with a full moon, I can see a ton of stars, which really makes me appreciate it cause when I visit family in Southern California, I can barely see the brightest stars on a new moon. I'm amazed at what people have done with narrow band imaging in bortle 8/9.

 

I do gather light fairly well. Not as good as mono. Scope is reasonably fast.  SV80A which is F/7 with a 0.8 FF, so F5.6.  not going to get substantially faster unless I move to the RASA8 (which  is a wish list item :p)



#12 DrGomer

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Posted 09 January 2020 - 06:48 PM

Jerry, for what you describe, it sounds like you'd use that very low DN setup and merge it with high DN data since the all the high gain data probably has wildy blown out stars. Sound about right?



#13 Jerry Lodriguss

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Posted 09 January 2020 - 08:19 PM

Jerry, for what you describe, it sounds like you'd use that very low DN setup and merge it with high DN data since the all the high gain data probably has wildy blown out stars. Sound about right?

Yep. Or use a ton more total integration.

 

Jerry


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#14 jerahian

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Posted 09 January 2020 - 11:08 PM

No need to apologize, we are all here just trying to understand.

 

First, full well is not defined by bit depth. Any amount of dynamic range can fit in any bit depth. It might not be optimum, it might not be pretty, but you can do it.

 

Dynamic range is defined as the full well divided by the noise. So, if you have a camera with a full well of 20,000e, and a read noise of 1.2e @30db gain, you get 20,000/1.2 = 16,666, or about 14 stops of dynamic range.

 

That dynamic range is then sampled by the bit depth. In our example you would sample, or measure, the dynamic range into individual steps of tonal detail in about 14 bits or 16,384 individual steps. 

 

The noise determines the resolution of useful steps of tonal detail. If you have 1.2 electrons of noise floating around in your data, you can't distinguish a signal less than that, it's lost in the noise (single exposures, not stacking).

 

When you use the full well at lowest gain, you get the maximum dynamic range, but, HALF of your bits are being used by the brightest stop of your exposure data.  So, roughly 8,192 steps of your total dynamic range is being used by just the brightest stop of detail.  The second brightest uses 4096. ... and so on, until you get down to the least significant bit, which is the noise (or half that if you try to Nyquist sample the noise). So your really faint detail is going to be down there in the shadows where you don't have hardly any bits of tonal separation. Then you will try to increase the contrast down there to make the faint stuff more visible, and that can make a mess.

 

If you want to increase the contrast of the really faint stuff, like IFN, using the highest ISO/Gain gives you more bits on the faint end for stretching later.

 

So, ideally, you sample at about 2x the noise (which is way down the Nyquist rabbit hole that I am not going to go here), and devote as many bits as you can to the shadow end (which means the lowest read noise at a high gain/ISO usually). 

 

Now, you have to understand, this advice is based on best recording faint stuff, and not maximizing, or even caring much, about dynamic range.  This is for the hard core who want the really faint stuff.

 

You will throw away a ton of the top end of dynamic range and your full-well will dwindle to seemingly nothing, but, hey, you want the faint stuff, right?  you can always drop in bright colors later with an exposure for the bright stuff maximized for DR.

 

Jerry

Thanks for the explanation, Jerry!  I had to read that a couple of times for it to sink it, but I get it.  If I'm not mistaken in my understanding, I think an analogy here might be to think of increasing gain almost as a zoom function, like zooming in on the left end of a histogram to see the finer data elements across the same graph space.  The gain increase effectively zooms in on the IFN details while the upper end of the DR gets pushed out/clipped.



#15 17.5Dob

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Posted 09 January 2020 - 11:20 PM

Wes, I don't have a meter, just what the dark skies websites show. The 2015 Atlas shows 21.57 mag./arc sec2 at zenith which is pretty good on a new moon. Even with a full moon, I can see a ton of stars, which really makes me appreciate it cause when I visit family in Southern California, I can barely see the brightest stars on a new moon. I'm amazed at what people have done with narrow band imaging in bortle 8/9.

 

I do gather light fairly well. Not as good as mono. Scope is reasonably fast.  SV80A which is F/7 with a 0.8 FF, so F5.6.  not going to get substantially faster unless I move to the RASA8 (which  is a wish list item tongue2.gif)

Mag 21.57 is not "that dark" for dark nebula, so you're going to need to put in extra time...



#16 DrGomer

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Posted 09 January 2020 - 11:32 PM

Mag 21.57 is not "that dark" for dark nebula, so you're going to need to put in extra time...

I figure that anything in cant do with 2-3day of good nights dedicated to a single target  is about my limit. Going to try the gain game that Jerry mentioned earlier and see how it goes.  Thanks. 



#17 Jon Rista

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Posted 10 January 2020 - 12:06 PM

Mag 21.57 is not "that dark" for dark nebula, so you're going to need to put in extra time...

Um...it is only 0.43mag/sq" off from the darkest skies you can find on earth...which is 22mag/sq" on a good night with minimum airglow. It's DARN DARK!!! My dark site is usually 21.1-21.3mag/sq" and I can get excellent dark nebula there. The difference between 21.57 and 22 is only 1.4x.

 

For the record, the difference between the average city back yard and 21.57 is anywhere from 17-30x... So 21.57 is definitely DARK!

 

I would also bet that depending on airglow, actual measurements of such a dark site would vary, and depending on airglow...it could get brighter than 21.57 all due to natural causes. 


Edited by Jon Rista, 10 January 2020 - 12:08 PM.


#18 Madratter

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Posted 10 January 2020 - 12:15 PM

Despair not. Some targets should definitely be within doable from your location.

 

This was done from what is essentially a Bortle 4 zone (more like 5 to the N, 3 to the S).

 

get.jpg?insecure

 

Just pick your targets well. Some will be in reach, others won't.


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#19 Jon Rista

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Posted 10 January 2020 - 12:58 PM

I figure that anything in cant do with 2-3day of good nights dedicated to a single target  is about my limit. Going to try the gain game that Jerry mentioned earlier and see how it goes.  Thanks. 

Depending on how long your nights are, if it is winter and you are at a high enough latitude you could be getting up to 12-14 hours of dark. So, with 2-3 nights, if you utilize the whole nights, you could get 15-20 hours per object on SEVERAL objects. wink.gif I think 2-3 nights should be plenty of time.

 

I think you'll run into diminishing returns by then anyway. The only time I've seen people push for 50, 80, 100 hours or more is when they are either doing multi-resolution imaging, and combining deep wide field data with narrow field data on things like planetary nebula...or they are chasing ultra faint objects (24-26mag) that can't really be revealed until you've integrated 100+ hours. There have been a couple imagers in the past who have integrated over 100 hours, in one case over 130 hours, to pull out stars or other faint objects as deep as nearly 26mag/sq" (which is about the hard limit for most amateur equipment due to scattering.) 

 

For most of us mere mortals, though, 20-30 hours is usually enough to reveal all we can. wink.gif


Edited by Jon Rista, 10 January 2020 - 01:00 PM.

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#20 calypsob

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Posted 10 January 2020 - 01:32 PM

Mag 21.57 is not "that dark" for dark nebula, so you're going to need to put in extra time...

 

Anything darker than mag 21 gets you into dark nebula osc imaging territory. I have never had trouble collecting Galactic cirrus or IFN as long as skies surpass mag 21, the low horizons may have lp gradients but zenith is where you want to be working anyways.   2-4 hours at F2 lets me accomplish pretty much any goal I have. Now maybe slower glass makes a difference, im not sure as I have not really spent much time trying to acquire a big snr with scopes for the past couple of years.

Bottom line is the OP lives in an area where he could accomplish huge integrations of lrgb mono or  OSC in his own backyard, with fast or slow optics his skies are dark enough to shoot anything in the faint department.


Edited by calypsob, 10 January 2020 - 01:34 PM.


#21 Jerry Lodriguss

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Posted 10 January 2020 - 01:54 PM

Thanks for the explanation, Jerry!  I had to read that a couple of times for it to sink it, but I get it.  If I'm not mistaken in my understanding, I think an analogy here might be to think of increasing gain almost as a zoom function, like zooming in on the left end of a histogram to see the finer data elements across the same graph space.  The gain increase effectively zooms in on the IFN details while the upper end of the DR gets pushed out/clipped.

Hi Jerahian,

 

Yes, exactly.

 

Jerry


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