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Impact of 'seeing' on DSOs

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#26 GlennLeDrew

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Posted 12 January 2013 - 10:02 PM

I used 'extreme' examples because it's easier to obtain consensus on their being little if any affected by seeing. Once that has been conceded, then at least there is the appreciation that there is a continuum over which surface brightness impacts the amount of detail possible to see.

I knew the example of a CCD image (even through a rather small instrument) would cause some to exclaim, "Unfair comparison!" which is why I took pains (unsuccessfully :grin:) to point out what a prime focus image of a planet would record. Or not record, really.

At prime focus, the image of a bright but small object such as Jupiter comes nowhere close to revealing the same detail visible through the same scope with an eyepiece working at a 1-2mm exit pupil. If the camera is sampling at, say, 3 arcseconds/pixel, it's resolving details at closer to double that sampling frequency, or near to 6 arcseconds. But the scope visually resolves to 1-2 arcseconds. This disparity carries on to ever larger apertures, until seeing eventually imposes its ultimate limit.) And so we see that for prime focus imaging, a telescope does not resolve to its real limit; the image is undersampled. That's why fir planetary imaging, the f/ratio must be increased to the f/30 range in order to exploit the available resolving power of the aperture.

Back again to the small aperture scope and its image of a dim nebula. We know that this image is undersampled. Yet you can appreciate how the detail seen with that scope through the eyepiece under the best of conditions is probably two orders of magnitude (100 times) coarser. For example, a 300mm telephoto image will reveal striations (due to intervening dust clouds aligned by magnetic fields) in the North America/Pelican field which are utterly invisible in even a meter class telescope. Yet many of these striations are of order an arcminute in spatial frequency, or the size of one of the larger planetary nebulae, or Jupiter. If we could magically pump up the surface brightness of these nebulae so as to approach that of a bright planetary nebula (from 24-24.5 MPSAS to about 15 MPSAS; a brightness increase of 5,000 times, and where color would be seen), those striations would be well seen via small-ish apertures, provided contrast is sufficient.

I suspect many backyard observers haven't yet fully internalized just how atrocious is the eye's resolving power at low illumination and low contrast. In the photopic regime the retina resolves to 2 arcminutes--as low as 1 arcminute for the eagle-eyed. But at the dim limit, the resolving power is as poor as 5 degrees or more. That's a ratio of 150, and even as much as 300 for some.

Let's suppose that at some moderate magnification of, say, 100X you are just detecting the effects of seeing on Jupiter's disk features . In other words, the 1.2 arcsecond seeing is magnified 100X so that it subtends 2 arcminutes on the retina. For a dim object at the limit of detection, it would require something like a further 150-fold increase in magnification--to 15,000X--in order for that 1.2 arcsecond seeing subtend the requisite 5 degrees for perception. And even if such a dim feature need only subtend a mere 1 degree, the magnification would still have to be 3,000X. And for worse seeing of 2.4 arcseconds, a magnification of 1,500X.

To obtain such high magnification while preserving what surface brightness there is requires larger exit pupils and hence *large* apertures. If it is accepted that at lower surface brightness and contrast an object or feature must subtend the better part of a degree or more on the retina in order to be detected at all, then it immediately becomes apparent that rather massive apertures are required to sample down to the seeing scale on the light starved and poorly resolving retina.

#27 galaxyman

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Posted 13 January 2013 - 12:49 AM

I don't think anyone is denying that certain DSO's are much more effected by transparency than seeing, but at the same time there are a number of DSO's (including galaxies) that certainly benefit from good seeing along with good transparency, as I now pointed out in two prior post.

Even compared to planets, there are far more DSO's in this category of the benefits of good seeing. In reality for most amateur scopes only three planets show any quality detail as in Jupiter, Saturn, and Mars. Even here both Jupiter and Saturn can still give quality views on nights of fair seeing, whereas tiny Mars is more of a "bugger" to see any detail. We can point to a number of DSO's that seeing isn't a big deal, though the same can be said with some planets like Venus for example.

Now we can say the same on many DSO's, that even on less than average seeing, quality views still can be had. To really push both the scopes and object for more detail, higher magnification at times needs or can be used for seeing fleeting or small detail within the DSO (as I pointed out before).

I guess the point is that some planetary observers want optimum views that require great seeing, but at the same time some DSO observers want both great transparency and good seeing to push the observing limits (like me).


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#28 GlennLeDrew

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Posted 13 January 2013 - 02:18 AM

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.

For some, the application of mathematics is anathema, and a relinance upon 'impressions' and 'feelings' carries more weight. In spite of the demonstrated unreliability of our senses, or more properly, the interpretation derived therefrom.

If a case study using established facts as derived from an understanding of the operation of the eye is dusputed, I would like to see a rebuttal based upon quantitative analysis, not qualitative 'feelings', in which bias is likely to be present.

#29 Astrojensen

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Posted 13 January 2013 - 04:49 AM

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.


For visual amateur observers, this is true. But perhaps we ought to think more about what instrument we are likely to be using. A 10x50 binocular will give essentially perfectly sharp views on any night, regardless of the seeing. I've yet to see seeing bad enough that it obscured a deep-sky object in my binoculars. The resolution of a 10x50 binocular on even quite bright DSOs is on the order of something like one arc minute, larger if the object is dimmer.

But if I observe with a 12" scope, I don't think the claim that *all* objects are to some degree affected should be rejected out of hand. Maybe the effect is subtle, but if the seeing is bad enough (and it can get pretty bad out in the deserts, or so I've heard), then I think that the detection or details of even large nebulae and galaxies might suffer, even at low magnifications. A dim planetary nebula, say, some ten arcminutes across, but with a relatively sharp edge, may well get fuzzed out enough that the edge becomes indistinct and a deep-sky object with a sharp edge is far easier to detect than one with a indistinct one. If the object is on the threshold of detection in stable seeing, it might well become invisible in bad seeing, even if all other factors remain identical.

I think that for each instrument, deep-sky objects will be spread across a line, with some being almost invulnerable to bad seeing, some that are in some degree affected and some that are highly affected. The smaller the instrument, the larger the number of nearly invulnerable objects, and the larger the scope, the larger the number of highly seeing dependent objects. But each instrument will have some of each category.

Perhaps this is just a summary of what everybody's been saying, but at least I've now proved that I understand it - maybe! :grin:


Clear skies!
Thomas, Denmark

#30 GlennLeDrew

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Posted 13 January 2013 - 07:53 AM

Thomas,
You have divined the essence of the matter; there is a continuum wherein object size, surface brightness, contrast, aperture and exit pupil diameter determine the degree of resolution.

I invoke mathematical characterization of these and other problems, not to 'bamboozle with statistics' or 'obfuscate with numbers', but to advance the argument with science. I find it astounding that some amateur *astronomers* should be so averse to even the simplest--and fundamental--quantifications via mathematics.

To possess even a first order understanding of the range of values occupied by such variables as object surface brightness, contrast ratio as a function of sky brightness, visual system resolving power, image surface brightness as a function of exit pupil diameter, contrast gains via filtration, and much else besides, is of great benefit in understanding the impact of same on object visibility. To be content to proceed merely by relying on 'impressions' is to remain lacking in true understanding. Science progresses via numbers, not feelings.

#31 galaxyman

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Posted 13 January 2013 - 12:56 PM

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.

For some, the application of mathematics is anathema, and a relinance upon 'impressions' and 'feelings' carries more weight. In spite of the demonstrated unreliability of our senses, or more properly, the interpretation derived therefrom.

If a case study using established facts as derived from an understanding of the operation of the eye is dusputed, I would like to see a rebuttal based upon quantitative analysis, not qualitative 'feelings', in which bias is likely to be present.


Glenn I do agree with you, though in certain particular DSO's of all types, better seeing enhances the view. Same can be said about planets, for it's mostly about what you want to get out of your personal observing expierence.


I agree there are certain DSO's that seeing has minimal effect on the view, and transparency is of course the key factor.

I do as you may have known, the extreme end of DSO observing, so I want everything I can get what the sky can give me.


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#32 blb

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Posted 13 January 2013 - 06:11 PM

My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.

For some, the application of mathematics is anathema, and a relinance upon 'impressions' and 'feelings' carries more weight. In spite of the demonstrated unreliability of our senses, or more properly, the interpretation derived therefrom.

If a case study using established facts as derived from an understanding of the operation of the eye is dusputed, I would like to see a rebuttal based upon quantitative analysis, not qualitative 'feelings', in which bias is likely to be present.


Glenn I do agree with you, though in certain particular DSO's of all types, better seeing enhances the view. Same can be said about planets, for it's mostly about what you want to get out of your personal observing expierence.


I agree there are certain DSO's that seeing has minimal effect on the view, and transparency is of course the key factor.

I do as you may have known, the extreme end of DSO observing, so I want everything I can get what the sky can give me.


Karl
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My point exactly Karl. We must have similar experences. Seeing does not effect all DSO's the same. Poor seing hurts a number of the objects we view and is hardly noticeable in others. This has more to do with the type of object being viewed than the seeing. Still seeing effects what we see, it is just that some objects do not show bad seeing like other objects but it still effects what we see. So giving the impression that it does not is wrong in my book.

#33 blb

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Posted 13 January 2013 - 06:20 PM

I used 'extreme' examples because it's easier to obtain consensus on their being little if any affected by seeing. Once that has been conceded, then at least there is the appreciation that there is a continuum over which surface brightness impacts the amount of detail possible to see.


My only point is this. An across the board claim that all DSOs are affected equally by atmospheric seeing is unrealistic.


Yes, I agree completely. I guess it only sounded like you were saying that seeing didn't matter. Seeing does matter, it's effects just are not noticeable on some objects due to the nature of the object being observed.

#34 GlennLeDrew

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Posted 13 January 2013 - 07:17 PM

Here's a neat experiment Thomas Jensen performed not too long ago. He observed the Moon through a solar filter, and saw indeed just how poorly the eye resolves at 'DSO brightness levels.'

I'm sure he'll remind us of his specific results, but in the meantime I'll add these notes...

Assuming one has a Baader solar film filter of density of 5.0 (dims light to 1/100,000), the dimming amounts to 12.5 magnitudes. (Interestingly, the ratio 1:100,000 is very nearly equal to the ratio of the solar disk's area to the area of the 180 degree hemisphere of the sky.) This brings the -10.58 MPSAS Sun down to a rather more comfortable +1.92 MPSAS, but still not much less bright than sunlit snow.

Through the solar filter, the first quarter Moon's +5 MPSAS surface brightness would be brought down to +17.5 MPSAS, which is not far brighter than the color detection threshold of 18-19 MPSAS. The Moon would now have a surface brightness of reasonably bright planetary nebula which might show a greenish hue.

Observing the familiar lunar terminator, where one has the benefit of very high contrast afforded by shadows, estimate how much poorer is the eye's resolving power.

And this is nowhere near the lower limit of brightness the eye can work with, it being about +27 MPSAS, or near 10,000 times dimmer. To appreciate what further dimming does, you could install a variable polarizing filter, which at maximum dimming should get you perhaps another 4 magnitudes fainter, or about 21.5 MPSAS (a decent night sky brightness level.)

But even at the fairly bright +17.5 MPSAS for a quarter Moon, can one resolve even awful seeing? I wonder. I hope someone who has a large-ish solar filter (I have none, myself) and matching scope will try this experiment. The bigger the better, for then one can obtain sufficient magnification without having to go to a small exit pupil, thus retaining image brightness (in order to make the test as much a best case scenario as possible.) Naturally, the full magnification regime is to be explored nonetheless, as one might do for such a bright DSO having a surface brightness as high as 17.5 MPSAS. (The brightness range of nebulae and galaxies ranges from 14 MPSAS down to the contrast limit of about 26 or thereabouts.)


If you have a different filter whose density is known, you can calculate its dimming in magnitudes very easily. If the density is given as the logarithm of the attenuation, merely multiply this by 2.5.

For example, the Baader film having a density of 5 means it attenuates by 10^5, or a factor of 100,000. The logarithm of the attenuation is 5, which when multiplied by 2.5 yields a dimming of 12.5 magnitudes.

A filter of density 6 attenuates by 10^6, or 1,000,000. 6 * 2.5 = 15 magnitudes of attenuation.

Incidentally, do not confuse this with the units of filtration as employed by photographers. These filters are rated either by the fraction of light transmitted or the number of f/stops of attenuation.

#35 galaxyman

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Posted 14 January 2013 - 12:57 AM

Glenn I don't think anyone is disagreeing with you about some of the DSO's you've mentioned. Just not sure if you are agreeing that at the same time many DSO's benefit on nights of the combo of good transparency and good seeing :question:

I believe I have pointed this out already with some examples.

Here in Pennsylvania we have heavily varied seeing conditions. Even on nights of poor to fair seeing, there are times while observing an object that seeing will get very good for moments at a time while the object is in the eyepiece, showing a bit more detail within that object at those moments. This also happens when I'm viewing an Abell galaxy cluster when seeing the faintest members my scope (and eye) can see. Bad seeing smears the fleeting light, so of course better seeing at a particular dark site will enhance the view of a galaxy cluster by reeling in more members.

I wonder how many observers out there with large scopes realize what can be seen in many planetary nebula at very high power as I pointed out with Ngc 2392 and Ngc 1501? The same with some inner detail of some bright galaxies using very high power that good seeing helps in both cases.

I will reiterate that I will take the darkest skies over good seeing, but having both is optimum for my type of observing.




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#36 GlennLeDrew

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Posted 14 January 2013 - 09:04 AM

Karl,
I did acknowledge earlier here that those DSOs comprised of stars, are of small size and have higher surface brightness will be affected by poor seeing. My emphasis has been directed toward the lower surface brightness regime, as a way of demonstrating that *not all* DSOs are impacted. I never did say *none* are impacted.

I wish all deep-sky observers could view the Moon through a solar filter so that they could obtain a most immediate appreciation of the eye's atrocious resolving power at DSO levels of surface brightness. It's so bad that it would probably qualify as functional blindness. Imagine in your daily life wearing 'Coke bottle bottom' glasses which threw everything out of focus so that any point source or sharp edge were to be spread out to a degree--or the five(!) degrees the dimmest stuff is seen at. What a blurry view!

#37 blb

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Posted 14 January 2013 - 10:08 AM

Imagine in your daily life wearing 'Coke bottle bottom' glasses which threw everything out of focus so that any point source or sharp edge were to be spread out to a degree--or the five(!) degrees the dimmest stuff is seen at. What a blurry view!

We all do, It is the air above us that makes it hard to see. That's why we need nights of good seeing.

#38 galaxyman

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Posted 14 January 2013 - 10:41 AM

Agree with the low surface brightness objects which by far suffer under poor transparency, though the true test are the individual objects (galaxies and planetary nebula) themselves, and many do have a high surface brightness. Paying attention as I mentioned with moments of good seeing does bring out small details in a number of galaxies that under poor seeing does not. This is why I do say to have patience when viewing a particular object, and play with different magnifications. We would hear the same from other serious DSO observers like Barb Wilson and Alvin Huey of Faint Fuzzy fame.

Roger Clark's book Visual Astronomy of the Deep Sky talks about using more magnification on DSO's. Under a night sky with the great observers (and great scopes) of the CAS, anyone could see this demonstration of the amount of detail a number of objects can produce, using some wide ranges of magnifications including over 1000x. When using this kind of magnification, better seeing does come into play.

The original post question of if DSO's are effected by seeing. I think the answer and a better way to conclude this is that better seeing is helpful to those who are very serious about DSO observing.


Karl
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#39 russell23

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Posted 19 January 2013 - 09:28 AM

The last time I had my scope out I had nice dark skies, but the upper air was so turbulent that I couldn't get sharp pinpoint focused stars at magnifications as low as 53x. That was the worst seeing I've seen and it made enjoying any kind of starfields, star clusters, or deep sky objects with stars that are an integral part of the aesthetics completely unenjoyable.

Dave

#40 Dennis_S253

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Posted 20 January 2013 - 08:59 PM

I think they should break the reflector section down to three parts.
1. Up to 8".
2. 10" to 16".
3. Bigger than 16".
I get tired reading post about people with there 16", 22" or 30".
like I could see that in my 6". Even if the seeing was good or bad.

#41 Dennis_S253

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Posted 20 January 2013 - 09:15 PM

oops, the DSO section also. :getem:






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