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Question about F/ratio limitations for dim targets?

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#1 ImaLibra

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Posted 14 November 2017 - 06:37 PM

Hi all,

 

Here is a question I thought might be valuable to address, especially for newcomers like myself:

 

I'll try to explain my question as best I can: Let's say I am limited to very small aperture and then I use a powermate to really get a close-up view of a small object, say, only several arcseconds in size. Let's also say it is very dim, maybe Magnitude 16. Is there a certain point at which a small aperture and too much FL, or too much F/ratio, prevent me from EVER seeing all the details of a dim object, because even if I sat on the object for a loooong time with the shutter open, is it true that any detail that would show up is drowned out by the continually building dark current as a shutter is open? I hope that makes sense.

 

Basically, having too small of an aperture, and too great of a FL or F/ratio on a dim object, that no matter how long you sit on it you will never get it, because continuously building electronic current and noise from the equipment hide anything that would be gained.

 

If that is true, does anyone know of perhaps a rough estimate of a limiting magnitude for a particular F/ratio or QE? 

 

I hope to hear back from you guys, it's something that's been on my mind, especially when it comes to wanting to resolve all of the details of these faint DSOs, but also having limitations on the size of the payload we can manage to capture those photons. 

 

Thank you!



#2 Alex McConahay

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Posted 14 November 2017 - 06:53 PM

>>>>>>If that is true, does anyone know of perhaps a rough estimate of a limiting magnitude for a particular F/ratio or QE?

 

Since you are asking about "details" then you are asking about resolution. 

I don't know about F ratio, or QE, but resolution is determined by the Airy DIsk, which is dependent on aperture. Google up "Airy Disk," which is the restricting concept when discussing resolution. You get a formula that says, among other things, that the larger the aperture of a lens, the smaller the airy disk can be (the better the resolution).  

 

Alex


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#3 havasman

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Posted 14 November 2017 - 07:15 PM

There are 2 very important variables that should be considered - experience and conditions. Conditions is simple: dark is required & seeing and transparency are enablers.

 

Experience is incredible. Look up observing posts by Sasa and Astrojensen and Sarkikos and a few others to begin to realize just what talented, experienced observers can see in their 63mm (!!) scopes. It consistently just completely boggles my mind to hear what they've seen and see their sketches. Those guys, to me, are amazing. I have no where near that skill so I have to use aperture.

 

So the answer is probably yes but the value is all over the place and different for everybody. Like a lot of simple questions in the hobby, the answer is complicated.


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#4 mistateo

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Posted 14 November 2017 - 08:08 PM

As others have already stated, a dim image from a high f-ratio is only part of the problem.  Scopes also have a resolution limit that correlates with aperture size.  


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#5 bobzeq25

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Posted 14 November 2017 - 08:14 PM

My main point would be this.  Imagers very rarely use Barlows because the loss in speed is more of an issue than the theoretical gain in resolution.  The loss in speed impacts signal to noise ratio, no matter what, while, at small image scales, seeing often limits resolution more than optics.  So there's really no upside.


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#6 ImaLibra

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Posted 14 November 2017 - 08:29 PM

My main point would be this.  Imagers very rarely use Barlows because the loss in speed is more of an issue than the theoretical gain in resolution.  The loss in speed impacts signal to noise ratio, no matter what, while, at small image scales, seeing often limits resolution more than optics.  So there's really no upside.

Thanks, it makes sense. I will have to be happy with a smaller, brighter image of my target rather than shoot for a large, blurry, and less-detailed image of the target. I guess I thought I could overcome the loss of speed with more integration time, but I do think it's just not practical at a certain point. 



#7 james7ca

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Posted 15 November 2017 - 12:20 AM

Well, there is always some limit on a signal to noise ratio where you will not be able to produce a usable image. And, it really doesn’t matter much whether you are talking about a small or large aperture, or a bright or dim object, instead what matters is the final signal to noise ratio.

 

When the Hubble Space telescope does one of its deep fields it will expose for dozens and dozens of hours. But does that mean that the Hubble is a “small” aperture or a “slow” system? Certainly not in amateur terms, but perhaps yes in terms of the largest professional observatories.

 

I think the only absolute limit to how long you can expose on a single object would be determined by the saturation point of the sensor, when the noise and signal combine to completely overload the recording capabilities of the photo site (the so-called full-well depth). Up until that point you will continue to build signal, but you may never reach a point where you have enough signal to noise to produce a useful image.

 

Then there is the technique of image stacking where in theory you could stack an infinite number of images into an infinite accumulator (128 bit, 256 bit, etc.). But practically speaking you’d never do that and in fact most users will probably tire of that process after a few thousand subs.

 

In terms of stacking, I’m not certain whether anyone has demonstrated any limit to how far you can go. And note when it comes to that limit there is really no reason why a small aperature should be any different than a large aperture. Although a small aperture (or high f-number system) will probably take longer to reach a given signal to noise ratio.

 

So, in the final analysis can you use a barlow to record greater detail in a “small” object? The answer is most likely yes, but you’ll need good seeing, good guiding, and good optics/focus and you will certainly need a longer total exposure time. In any case, as was previously mentioned, there is a limit on resolving power that is based upon aperture and once you reach that limit you won’t gain any more fine detail with greater focal length (unless you also increase your aperture).

 

Lastly, remember that an object’s magnitude is usually given in terms of its total integrated brightness, not in terms of its surface brightness. Thus, a small planetary nebula or galaxy that has a low total integrated magnitude (say the OP’s sample of magnitude 16) may actually be no more difficult to record than a larger object that has a higher integrated magnitude (within limits, since small remains small and very few objects have uniform surface brightnesses). In fact, the surface brightness on some small planetary nebulae can be so high as to make it easy to overexpose in just a few seconds, even though its magnitude can be rather unimpressive. This is one reason why it may be perfectly fine to use a barlow on a small, high-surface brightness object like a planetary nebulae.


Edited by james7ca, 15 November 2017 - 01:05 AM.


#8 Jared

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Posted 15 November 2017 - 01:19 AM

Hi all,

 

Here is a question I thought might be valuable to address, especially for newcomers like myself:

 

I'll try to explain my question as best I can: Let's say I am limited to very small aperture and then I use a powermate to really get a close-up view of a small object, say, only several arcseconds in size. Let's also say it is very dim, maybe Magnitude 16. Is there a certain point at which a small aperture and too much FL, or too much F/ratio, prevent me from EVER seeing all the details of a dim object, because even if I sat on the object for a loooong time with the shutter open, is it true that any detail that would show up is drowned out by the continually building dark current as a shutter is open? I hope that makes sense.

 

Basically, having too small of an aperture, and too great of a FL or F/ratio on a dim object, that no matter how long you sit on it you will never get it, because continuously building electronic current and noise from the equipment hide anything that would be gained.

 

If that is true, does anyone know of perhaps a rough estimate of a limiting magnitude for a particular F/ratio or QE? 

 

I hope to hear back from you guys, it's something that's been on my mind, especially when it comes to wanting to resolve all of the details of these faint DSOs, but also having limitations on the size of the payload we can manage to capture those photons. 

 

Thank you!

So you have thrown in a couple complications here where you probably only need one.  Specifically, what I really think you want to know is how dim an object you could image regardless of size, and what causes the limit.  This is the problem of imaging low surface brightness objects.  It’s not a question of how much total light the object is putting out or how big or small the object is, it’s more a question of the brightness per unit area or magnitudes per arc second.  

 

While there is no theoretical limit to how dim an object you could image if you had perfect technique and were willing to devote many, many hundreds or thousands of hours to collecting data, there are certainly some very practical challenges.  For most amateurs, for example, just getting an image of the brightest areas of the integrated flux nebula or “galactic cirrus” is an achievement.  That”s something like 24.5 magnitudes per square arc second in surface brightness.

 

Telescopes designed for imaging really low surface brightness objects are called “high etendue” telescopes.  They combine fast optics, sensitive cameras, and low noise.  They need not be particularly large.  For something like a hundred years now there has been a “barrier” around 30 mag/arcsec^2.  That record was set by a Schmidt camera using hyperextended film—can”t remember which Schmidt camera.  Everyone seems to hit a wall there.  Between shot noise from sky glow, noise in the object being imaged, scattered light in the telescope, read noise, and thermal noise nobody, amateur or professional, has ever been able to get past that magic number of thirty.  That”s about 100 times lower intensity than the brightest portions of IFN I mentioned above.  Didn’t matter the size telescope or the technique used.  Until recently.

 

I believe the Dragonfly telescope has finally broken that barrier, and it was using equipment readily available to amateurs (though perhaps more of it than an amateur can afford).  An array of multiple Canon 400mm lenses hooked up to multiple SBIG 8300 CCD camera’s all mounted together on a large Bisque mount has gotten down to, I believe, 32 magnitudes/arcsec^2.  Very impressive.  So far, objects at that surface brightness are the dimmest objects ever imaged.  

 

Turns out theDragonfly telescope has been able to capture a whole new class of objects through this type of imaging—galaxies that are almost entirely composed of dark matter!  It’s very cool stuff and not far out of reach for a good amateur astronomer.  You don’t need a huge telescope, just dark skies, a fast telescope (preferably a refractor, according to the designers of the Dragonfly due to lower light scatter), solid technique, and lots of data.  They get lots of data by running scopes in parallel, but the same thing could be accomplished with more patience and integration time from a single telescope.  The limiting factor for most of us is sky conditions.  You really need to get rid of the noise from sky glow, and that means a dark site.


Edited by Jared, 15 November 2017 - 01:26 AM.

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#9 Jon Rista

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Posted 15 November 2017 - 01:58 AM

 

I believe the Dragonfly telescope has finally broken that barrier, and it was using equipment readily available to amateurs (though perhaps more of it than an amateur can afford).  An array of multiple Canon 400mm lenses hooked up to multiple SBIG 8300 CCD camera’s all mounted together on a large Bisque mount has gotten down to, I believe, 32 magnitudes/arcsec^2.  Very impressive.  So far, objects at that surface brightness are the dimmest objects ever imaged.  

Aye, Dragonfly has gotten below 32mag/sq". Close to 33mag/sq" even on some of the ultra large, ultra diffuse ellipticals. The dragonfly project is all about minimizing scattering and massive throughout. I think they are up to an array of 50 400mm f/2.8 L II lenses now, each with it's own KAF-8300. Their aggregate integration times across the whole setup total in the tens of thousands of hours. O_o (Of course, they acquire it all in significantly less time, due to the multiple cameras taking exposures simultaneously). The truly crazy thing is, they only use 5 minute subs!

 

To put 32mag/sq" in perspective...it is 10,000x FAINTER than a 22mag/sq" airglow limited night sky (about the darkest skies you could possibly find anywhere on earth)...


Edited by Jon Rista, 15 November 2017 - 01:59 AM.

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#10 17.5Dob

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Posted 15 November 2017 - 02:16 AM

 

 

I believe the Dragonfly telescope has finally broken that barrier, and it was using equipment readily available to amateurs (though perhaps more of it than an amateur can afford).  An array of multiple Canon 400mm lenses hooked up to multiple SBIG 8300 CCD camera’s all mounted together on a large Bisque mount has gotten down to, I believe, 32 magnitudes/arcsec^2.  Very impressive.  So far, objects at that surface brightness are the dimmest objects ever imaged.  

Aye, Dragonfly has gotten below 32mag/sq". Close to 33mag/sq" even on some of the ultra large, ultra diffuse ellipticals. The truly crazy thing is, they only use 5 minute subs!

 

 

 

Even more impressive is finding someplace dark enough to shoot 5 min subs using a 400mm f2.8



#11 Jared

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Posted 15 November 2017 - 12:09 PM

That would be New Mexico Skies!


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