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Surface Brightness and Performance

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#1 NGC7088

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Posted 31 January 2013 - 10:47 PM

I don't know if this has been discussed before but I've been reading this:
http://www.rocketmim...Brightness.html
and I've had some thoughts about it.
It is very scientific and strikes very true with experience but also against it in many ways.
I've examined many objects for hours many a night at magnifications 400x+ and 500x + with large optics. The limit of 200 x by the atmosphere for maximum magnification is only true for small optics in my experience and
then only for very bad nights. But for the sake of discussion I'll accept it just "may" be as high as you'll ever have to go. There is nothing to complaint about an astronomical object at 200x. However:
This immediately takes Mr. Culp's article to another level. Since it clear cuts the minimum and Maximum magnifications for a telescope. And so if you're looking to get the most economical high performance telescope, the choices are way cut down by this article.
How is that?
If the maximum magnification (limited by the atmosphere) is indeed 200x and That magnification as defined by Mr. Culp is when it is equal to the diameter of the objective, then clearly one should aim for a telescope with an objective with 200mm diameter. You're then allowed to magnify a little
further to (1+2/3)x that diameter.You never get to the .5mm exit pupil of legend (equal to 2x the diameter in mm or 50x diameter in inches) but you're forevermore restricted to 0.67 minimum exit pupil. You know what? not a bad
idea, not at all.
What it means is that the smallest high performance telescope is 200mm wide.
That is almost 8 inches for a refractor objective. So based on price alone a 200mm apo is only for the rich and famous. A reflector of similar "clear" area would be 8.8", call it 9". There's not many of those around but it is no wonder why 8" newts are so popular, they are very close to the mark. The best telescope you'll ever need is 8 to 10" diameter.(or is it?, read further)
So one of my first criticisms of this article is that it is biased towards reflectors...
Can we stretch this cheaper?
Let's see. We could assume that the 2/3 "extra" magnification is equal to200x. That reduces the clear aperture to 133.33mm. It's not exactly a grab
and go refractor, but such sizes (130mm) do sell. 5 and 1/4 (5.25) inches would the be the "ideal" telescope. No not cheap for refractors at all, but "doable" for many.
Coincidentally an equivalent "clear" area for a reflector is almost exactly 6". No wonder they are so popular too.

Another implicit consequence of that article is that once you choose your aperture, you have basically chosen your magnifications regardless of focal ratio and then you choose your eyepieces to suit.
For 6 inch reflector (clear area similar to 5.25 refractor) the magnifications are: (regardless of focal ratio)

21x, (Aperture/7) 100% Bright, Max field of view, minimum Mag, 7mm exit pupil, eyepiece equal to 7xfocal ratio

31x, (Aperture/5) 50% Bright, 70% field of view, 5mm exit pupil, eyepiece equal to 5x focal ratio. Best compromise for horse head nebula (with H beta).

75x, (Aperture/2) 8% Bright, Optimum Mag., eyepiece equal to 2x focal ratio.

150x, (Aperture) 2% Bright, Max Mag., 1mm exit pupil, eyepiece equal to focal ratio. No increase of detail after this.

225x, 1% Bright, Extra Magnification. .67mm exit pupil, no additional detail but you are stretching the image and see detail larger.

If an f/4.5 the eyepieces are:(work out other f ratios by yourself)
32
22
9
4.5*
3*
* the 4.5 is a 9mm with 2x Barlow and the 3mm a 6mm with 2x Barlow, why?
because that way you get a little better eye relief.
So a 6 in reflector might be all you'll ever need.

So if you manage to get your "ideal" 133.33mm refractor (5.25 in) regardless
of the focal ratio you are already restricted to magnifications:
19,27,67,133,200 and depending of your focal ratio you should choose youreyepieces. For example:
f/7:
56
40
16
8
5

Is there bias against folded optics?
I think so, let's see an extreme:
The Celestron nextsar 6SE could be all you'll ever need but:
The magnifications are exactly the same as above, yet the eyepieces to get
there are:
70
50
20
10
7
Do they exists?
Yes the 70 and 50 exists from Russell Optics but...only at 2" diameter. Close but no cigar. ( The 6Se is only 1.25" focuser) Unless there is a way to mod the magnificent 6SE (others please chip in here). Yes there is a bias against folded optics.
Is the bias by Mr. Culp. Not really but by the atmosphere and the physics.

But If all this is really true, how come Stephen James O'meara saw all
he saw with only a 4"????
Please discuss.

#2 GlennLeDrew

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Posted 01 February 2013 - 01:53 AM

It can be all too easy to over analyze all this.

The 209X 'limit' applies for objects having sufficient brightness for our visual system to work at or near full resolution. As object brightness decreases, our resolving power also decreases, and so a higher magnification can be employed without our being aware of the impact if seeing.

There is no 'bias' against certain optical systems. This is because there are other reasons for a variety of configurations than just allowing a practicable set of eyepieces to operate at the full range of exit pupils.

#3 NGC7088

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Posted 01 February 2013 - 07:08 AM

Yet if I owned a telescope store I could memorize a little summary of Mr. Culp's article, rid the floor of all Cats and Maks and use it cleverly to sell only 6" to 10" Newtonians, with the occasional 130mm refractor.

You say there are other reasons for a variety of configurations and I agree. For example is it possible these brightness limitations are practical for photographic uses rather than for visual astronomy?

#4 Tony Flanders

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Posted 01 February 2013 - 07:14 AM

That article is all very clearly written, but it uses a lot of words and math to go around in circles, it makes generalizations that aren't true, and it misses some very important points.

Let's start with the statement "It turns out that an exit pupil of about 2-3mm (2.4mm to be precise) is the optimum point for maximizing the resolving power of the eye. This is about where theory would put it, and it has been confirmed by observational studies."

Say what? What theory would put the maximum resolving power at 2.4 mm? Any such theory would have to include a model of the eye's imperfections. If the eye's optics were perfect, maximum resolution would be at maximum aperture.

But in fact nobody's eyes are perfect and everybody's eyes are imperfect in different ways. That 2.4 mm figure might be a valid median -- sounds about right to me -- but I betcha that the normal range is anywhere from 1 to 5 mm.

The article also assumes that everybody's eyes open to precisely 7 mm, when in fact values anywhere from 4 to 8 mm are common.

Regardless, it is an empirical fact that when seeing is not limiting the views, almost all planetary observers and absolutely all tight double-star observers prefer to work at exit pupils well below 1 mm. Right off the bat, that indicates that for all the apparent logic of the analysis, something is fundamentally wrong with it.

Even more important, the article ignores the fact that resolution is drastically worse when using night vision -- the kind that matters most to deep-sky observers. And contrary to his assertions, people who observe faint fuzzies generally employ exit pupils much smaller than 7 mm. Not to achieve better resolution, but to see fainter objects, for reasons first explained by Roger Clark.

I'm also pretty baffled by your extrapolation from his arguments. Forget all of that analysis and look at the facts:

Bigger apertures always show fainter objects -- and finer details in small objects -- than smaller apertures.

Seeing permitting, an ideal eyepiece collection lets you get to exit pupil of 0.5 mm or smaller. However, this places no constraints on the telescope, since Barlows can be used to achieve arbitrarily high magnifications.

Although in theory there are cases where a 7-mm exit pupil will allow you to see faint objects invisible at higher magnifications, such objects are in fact vanishingly rare. In practice, a 4-mm exit pupil is ample for the faintest objects and the most aggressive filters.

Nonetheless, all other things being equal, focal ratios slower than f/8 are undesirable, because they make it hard to achieve adequately low magnifications. Nonetheless, since most observing is done at exit pupils of 2.5 mm or less, this turns out not to be a huge limitation in real life. Moreover, many slow scopes can be fitted with focal reducers -- a standard commodity for SCTs.

There are objects such as the Pleiades that are simply too large to view in big telescopes. That's why small refractors (and their extreme, hand-holdable binoculars that operate at 10x or lower) fill an important niche.

Any telescope is better than no telescope. And given sufficient skill, any observer will get great results with any decent telescope.

#5 Jon Isaacs

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Posted 01 February 2013 - 07:34 AM

.You never get to the .5mm exit pupil of legend (equal to 2x the diameter in mm or 50x diameter in inches) but you're forevermore restricted to 0.67 minimum exit pupil. You know what? not a bad
idea, not at all.



There are a lot of assumptions made. I think Glenn and I are both familiar with the concepts here, Glenn more so than I. Glenn points to the fact that resolution of the eye is not fixed but rather is a function of contrast and brightness. I am not sure where the "studies" came from that suggested the "optimum" resolution was at 2.4mm exit pupil, I tend to imagine they were not done at night.

The suggestion that the eye's resolution is maximized at a 1mm exit pupil because one begin to see the Airy disk is a questionable assumption. As any double star observer knows, the Airy disk is not a circular disk of uniform brightness but rather the brightness decreases with the radius. When one is working between the Dawes limit and the Rayleigh criteria, the disks are overlapping and the reason the split can be made is that the eye can actually see the minima in the brightness between the overlapping disks, it's only about a 5% drop. For me to see that thin dark line requires magnifications much higher the 0.67mm exit pupil suggested, usually closer to 0.3mm.

Sedgwick discusses the maximum magnifications, I don't know if he is the original source of the 25x-50x rules of the thumb but he also says that higher magnifications may be used.

The assumption of the seeing limiting the maximum magnification to 150x to 200x combined with the "optimal exit pupil of 2.4mm" suggests something close to 20 inches is still a gain. Around here, seeing is often much better than that.

This webpage is an attempt to quantify the reasons for the choosing eyepieces and particular magnifications and I think for that it does a reasonable job. But at some point, one has to recognize that these limits and suggestions are only guidelines based on a crude model of the human eye, the actual response of the human eye is far more complicated.

Bottom line: Don't be afraid to experiment, let your eye tell you what magnification/exit pupil provides the optimal view. It's not a crude approximation, it's the real thing.

Jon Isaacs

#6 Madratter

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Posted 01 February 2013 - 09:47 AM

If an article, no matter how fancy the math, is contradicted by clear experience, clearly the math is wrong, or the assumptions behind the math are wrong. There are all kinds of brilliant ideas in physics that simply don't meet experimental tests.

Since I routinely find magnifications above 200x useful, the article is wrong. End of story.

There is no need to over analyze this. I've used my 150mm f/8 refractor with a 7mm eyepiece and a 2x barlow and found that useful. That happens to be 343x.

#7 BillFerris

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Posted 01 February 2013 - 09:53 AM

Bigger apertures always show fainter objects -- and finer details in small objects -- than smaller apertures.

Seeing permitting, an ideal eyepiece collection lets you get to exit pupil of 0.5 mm or smaller. However, this places no constraints on the telescope, since Barlows can be used to achieve arbitrarily high magnifications.

Although in theory there are cases where a 7-mm exit pupil will allow you to see faint objects invisible at higher magnifications, such objects are in fact vanishingly rare. In practice, a 4-mm exit pupil is ample for the faintest objects and the most aggressive filters.

Nonetheless, all other things being equal, focal ratios slower than f/8 are undesirable, because they make it hard to achieve adequately low magnifications. Nonetheless, since most observing is done at exit pupils of 2.5 mm or less, this turns out not to be a huge limitation in real life. Moreover, many slow scopes can be fitted with focal reducers -- a standard commodity for SCTs.

There are objects such as the Pleiades that are simply too large to view in big telescopes. That's why small refractors (and their extreme, hand-holdable binoculars that operate at 10x or lower) fill an important niche.

Any telescope is better than no telescope. And given sufficient skill, any observer will get great results with any decent telescope.


The above is great stuff. Tony's sage and solid advice is a natural product of an understanding of theory informed by substantial personal experience and knowledge of how amateur astronomers use telescopes. His last point sums it up, nicely.

Bill in Flag

#8 FirstSight

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Posted 01 February 2013 - 11:09 AM

Seeing permitting, an ideal eyepiece collection lets you get to exit pupil of 0.5 mm or smaller. However, this places no constraints on the telescope, since Barlows can be used to achieve arbitrarily high magnifications.


Let's not forget another *very* important practical limitation of the human eye which all too many observers are unfortunately subject to: somewhere in the sub-1mm exit pupil range, internal eye imperfections (floaters, etc) start to become sufficiently visible to not merely be an annoyance, but to interfere with the eye's ability to see detail. There are two (often distinct) thresholds where: a) internal eye imperfections start to become visible; b) these imperfections become prohibitively interfering. These thresholds vary from person to person (and the threshold of prohibitive interference also depends somewhat on the nature of the object being observed), but nevertheless a quite substantial portion of observers will often experience problematic difficulty by around 0.5mm exit pupil size. The size scale of visible "floaters" is often interferingly similar to the size scale of e.g. fine planetary features the observer is trying to perceive by using higher magnifications when they are accompanied by problematically small exit pupils. Of course this may not matter as much if the object is a diffuse extended one, e.g. a galaxy. Nevertheless, despite the principle that relative contrast of extended objects against the background is constant across different magnifications, at some point the absolute amount of illumination available from an extended object at a small-enough exit pupil falls below the individual eye's threshold of perception, and the presence of internal eye imperfections doesn't exactly help with maximally extending this threshold.

#9 NGC7088

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Posted 01 February 2013 - 08:49 PM

Wow! I knew there was something wrong (just from my experience, everyone and their sisters use folded optics at my local club and we have no complaints, personally I built my own 15" dob from Kriege and Berry's book plus own a cheap Ioptron 90mm refractor and a large collection of binoculars, and all of those have led me to a rather different preferences than Culp's article would suggest.)
I think the first most important point to a bias (or "something wrong") was hit right on the head by Mr. LeDrew.
Restricting yourself towards a practicable set of eyepieces to operate at a full range of exit pupils is not all there is to it.
I'm happy that other great respected personae of these forum went deeper and found problems with other assumptions ( my own use of the information was quite logical if simulating an attempt to use the article to save money Mr. Flanders, no need to be baffled about me trying to provoke reactions) thank you. Particularly pointing out by more than one of you about the 2.4 eye resolution problem thing.
I think one thing remains: collaboration to improve the information. At the bottom of his article Mr. Culp invites:
"Your questions and comments regarding this page are welcome. You can e-mail Randy Culp for inquiries, suggestions, new ideas or just to chat. "
Would one of you Big Personae here let him know how to improve the article, please?

#10 NGC7088

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Posted 01 February 2013 - 08:58 PM

P.S.
I was daydreaming "I could call them "Culp's Grade High Performance Telescopes". Brand them "COOLPRITS". Include a 9" f/5 DOB...etc." oh well...

#11 Starman1

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Posted 02 February 2013 - 12:52 AM

All this makes me go hmmm. :question:

What is the practical highest magnification that can be used?
Well, as has been pointed out, it's going to be where the internal imperfections of the eye interfere with the view and where seeing doesn't blur the image, which ever comes first.

And if that level happens to be above the seeing conditions of the night (i.e. the magnification is so high there is never a moment of clarity in seeing during the normal variation that is always occurring), then a lower magnification may show you a sharper image and more small details even despite the reduction in size.

And if the magnification is above the point where the Airy disc becomes a visible size, higher magnifications will expand the detail but won't resolve any smaller details. Note that if a detail gets large enough to cross the threshold of visibility, you may find exceptions in such a statement.

I had a case recently where I found that to be true:
Jupiter at 140X (11X/inch in my 12.5") was magnificently detailed and it was obvious there was no atmospheric scintillation. So I went for broke and tried a barlow on my 228X eyepiece, yielding 456X (36.5X/inch). Seeing was so good, the small amount of scintillation in the image was negligibly damaging the quality of the image, and I could begin to see some albedo markings on Ganymede which, at this power, displayed a noticeable disc that was larger than the other Galilean satellites.
So I inserted my 6mm into the barlow, yielding 608X (48.6X/inch), still below the theoretical maximum magnification of the scope. The image was still fairly sharp--maybe a little blurrier--but still sharp. This was a good night.

Unfortunately, I have too many floaters in my eye to use this magnification. My image of Ganymede was obscured by an annoying floater that was bound and determined to fall right where I was looking.
So I started moving down in magnification until I could still see the details I wanted to see but without any floaters interfering. Since then, I've experimented on the Moon (we've had some superb seeing recently), and I've come to the conclusion that my experimentally verified highest practical magnification is 304X (24.3X/inch). I would note that I am not a double star viewer particularly, but this magnification would expand the Dawes limit of my scope (0.36") to 1.8', a little below the resolution of the eye, so I am aware that keeping the magnification this low does reduce the maximum abilities of the scope somewhat.

The point? As has been pointed out, the subject of maximum magnification has multiple answers. If I had no floaters in my eyes, I could have used the 608X with aplomb. But I can't.

Someone wiser than me told me once that the magnifications used in a scope were, "Low enough to avoid the flotsam in the eye and high enough to avoid astigmatism". For me, that's roughly exit pupils between 4.5mm and 1mm (the magic 1X/mm of scope). What is it for you? Probably different.

If we're talking a group, it's probably 7mm to 0.5mm or less. But people differ a lot in that parameter of viewing. Does any formula calibrate for all observers? No. That's why experience is important. Any formula that says you shouldn't do what you know you can do is obviously wrong.

#12 demiles

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Posted 02 February 2013 - 07:18 AM

I believe this site does try to explain some basic princles and does a pretty good job of putting it in easy to read and understand format. I'm not saying its perfect, and may not apply to every situation all the time. In my 31 observing sessions last year there were only 5 that allowed magnifications to go consistently above 200x

#13 NGC7088

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Posted 02 February 2013 - 08:46 AM

I think that something missing is to bring the light gathering power of the aperture into play. A 1mm exit pupil from a 6 inch aperture and a 1mm from a 15 inch will both be 2% as bright as they could be. But the 15 inch will pack 6 times more light in that 1mm. With more light to play with, you can magnify more before it degrades. The quality of that light depends on the atmospheric conditions so that does have a role. I've never taken account of how many observing sessions allowed more or less than 200x or any other magnification. Looking back my "feeling" is that I rarely have been constrained by such limits. Maybe I'm lucky with a steadier than average atmosphere in my location.
My eyepiece configuration includes 244x and 380x regularly. I'm pretty sure most of the time I get to use those 2 without problems.
In the end the answer seems to be that maximum limiting magnification depends on the person and the location's average seeing conditions. But I would advice, always keep an eyepiece configuration for 60x aperture in the bag for that special night when it comes.

#14 NGC7088

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Posted 02 February 2013 - 09:16 AM

One more thing. I have floaters too. I've mastered a trick to sort of move the worst of them out of the way for a moment. Close your eyes lightly and move your pupils way up, or way down or way to one side or the other, depending on what works for a particular floater, and hold it there a few seconds. When you release, the floater does not com back to exactly where it was before but "lags behind" that position. Maybe enough to let you see. If not then repeat. Eventually you should find enough opening between the floater and what you're trying to see.

#15 kansas skies

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Posted 02 February 2013 - 09:53 AM

Starman1 said:

Someone wiser than me told me once that the magnifications used in a scope were, "Low enough to avoid the flotsam in the eye and high enough to avoid astigmatism".


As someone who suffers from both, I think this statement pretty much says it all...

Bill

#16 GlennLeDrew

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Posted 02 February 2013 - 11:57 AM

The statement that a larger aperture at an exit pupil of 1mm "packs more light" into that pupil than dies a smaller aperture at the same exit pupil is not correct.

Before laying out why, it's important to differentiate the targets into two classes; those which are unresolved point sources, such as stars, and those which are resolved as extended, such as planets and nebulae.

As long as a point source remains unresolved, then its brightness does scale exactly as the area of the aperture.

A resolved, extended source has a surface brightness which scales only as the area of the edit pupil.

At some point, when the exit pupil gets down to some diameter (about 1mm, but varying among observers), a star will be 'resolved' as the Airy disk. It will then behave like an extended source, with further increases in magnification dimming the image as the disk swells and increases in area on the retina.

So we see that in in a limited case, where the exit pupil is above some threshold, and when restricting to point sources only, it can be said that at given exit pupil a larger aperture "packs more light" into said pupil.

But in all other cases, where resolved, extended objects--and just as importantly, the sky glow itself!--are concerned, such a statement is utterly wrong. This is because the optical principle known as etendue preserves surface brightness, the latter of which depends on the exit pupil, or the eye's pupil, whichever is the smaller.

At the same exit pupil, a tiny finderscope and a light bucket deliver images (of resolved objects) having the same surface brightness (and identical contrast, it must be said.) The big scope merely allows to see smaller objects and finer detail.

#17 BillFerris

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Posted 02 February 2013 - 02:04 PM

I don't know if this has been discussed before but I've been reading this:
http://www.rocketmim...Brightness.html


It's an interesting site and a good catalyst for discussion. There is an error in his second formula near the top of the page. When calculating the minimum magnification for a telescope, he defines the term, D_ep, as the diameter of the (telescope) exit pupil when, in fact, it should be the diameter of the eye pupil. There are many exit pupil diameters at which a telescope will not be operating at the minimum magnification for its aperture. But there is only one exit pupil--one matching the diameter of the observer's eye pupil--at which the telescope will be operating at its lowest magnification.

I would also suggest he take different approach to understanding the difference in an extended object's apparent surface brightness. He sets up the discussion by talking about the surface brightness an object has when observed with the naked eye. He fails to mention that this is the surface brightness of the object--period--and that object surface brightness to the eye remains unchanged at all times and under all observing conditions.

If you know the visual magnitude and dimensions of an extended object, you can calculate its surface brightness. This object surface brightness is what the eye sees...or doesn't, if the foreground sky is too bright.

Also, there is a fairly straightforward formula for calculating the surface brightness reduction resulting from observing an extended object with an aperture: 5*log(Mag/Ap*3.387), where Mag is the magnification, Ap is the aperture in inches and 3.387 is a conversion factor based on an assumed eye pupil size of 7.5 mm and a telescope transmission factor of 100%. If you plug in random apertures and select magnifications producing a 7.5mm exit pupil, you'll see that surface brightness reduction is essentially 0. Of course, no telescope has a perfect 100% transmission factor. Accounting for this, it becomes obvious that the naked eye view always presents an extended object as having a higher surface brightness than any telescope is capable of producing.

On the subject of maximum magnification, the author is applying Dawes limit, which defines the angular resolution limit for an aperture as 4.56/D. In this formula, D is the aperture of the telescope in inches. The author is also assuming that an object having an apparent size of 2 degrees will be resolved as extended by the dark adapted eye. Setting aside the discussion of how Dawes arrived at this formula and whether or not it can be broadly applied to deep-sky observing, let's focus on the second assumption: An object having an apparent size of 2 degrees will be resolved as extended by the dark adapted eye.

Generally speaking, this is correct. Most deep sky objects will be resolvable as extended if you apply a magnification presenting them as 2 degrees (or larger) in apparent size. As you go after objects of extremely low surface brightness, you may first need to apply more aperture and will, eventually, encounter objects needing to appear larger than 2 degrees in apparent size to be seen as extended. And of course, inherently bright objects can appear much smaller in size and still be seen as extended. But generally speaking, a faint extended object appearing 2 degrees in size will--provided a large enough aperture is being used--be seen as extended by the dark adapted eye.

But does this reality translate to the author's claim that the application of additional magnification is without benefit. Clearly, that is the implication of establishing a 1mm exit pupil as a maximum magnification. His use of that term suggests a limit beyond which no benefit or value will be found. And he's mistaken.

The analogy I would use is that of aperture. One can use theory and math to establish a formula for modeling the smallest aperture needed to resolve an extended object of a certain brightness and size under a night sky of a given surface brightness. Does that mean there is no benefit to the application of larger aperture? Will you not enjoy a better, more detailed view by exceeding the minimum aperture? Of course you will. That's a big advantage of using a larger scope. Objects are more visually impressive and detailed than they appear in smaller apertures.

Rather than using Dawes limit to define a maximum magnification, he should use it to define a minimum magnification. A 1mm exit pupil (a magnification equal to the telescope's aperture in millimeters) is the minimum magnification at which you will enjoy the full resolving power of the telescope. If conditions allow, you can use and benefit from higher magnifications. Some objects or details which were at the threshold of your eye's resolution limit will be more obvious to the eye. This isn't true for all targets but will be for some. And in those rare cases when you're hunting down an extremely low surface brightness extended object, you may actually need the additional magnification simply to present that object with enough image scale to be resolved. Then, there are bright objects which are seen as extended at much lower magnifications but which reveal their structure and detail more easily when higher magnification is applied.

Once you apply a magnification allowing resolution of the smallest angular detail, you're not done. In many cases, you're just getting started.

Finally, I would point out that the author does not address the subject of contrast. Contrast is the key to deep sky observing. The dark adapted eye is not able to discern color and has poor spatial resolution capabilities. But at night, we're very sensitive faint light sources and, as a result, we visually interpret a scene based on the contrast--relative brightnesses--of objects occupying that scene. Under low light conditions, the human visual system is primarily a contrast detector. No discussion of deep sky observing that fails to address contrast will yield a complete understanding of how or why we're able to observe such distant, faint objects.

That said, the site is interesting and offers a lot of good information. The author has clearly invested significant time and energy in creating the site and I hope he continues the work. I would especially encourage him to challenge his own conclusions when out under the stars with his telescope. If he does, he'll deepen his understanding of how astronomical observing works and revise the content of his site to reflect real world experience.

Bill in Flag

#18 Dave Mitsky

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Posted 02 February 2013 - 02:42 PM

But in all other cases, where resolved, extended objects--and just as importantly, the sky glow itself!--are concerned, such a statement is utterly wrong. This is because the optical principle known as etendue preserves surface brightness, the latter of which depends on the exit pupil, or the eye's pupil, whichever is the smaller.


Drew,

Thanks for bringing the concept of etendue (étendue) to our attention. I'd never heard of the term, to the best of my recollection, before today.

http://eckop.com/opt...gineer/etendue/

http://www.schorsch....ry/etendue.html

Dave Mitsky

#19 NGC7088

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Posted 03 February 2013 - 06:53 AM

Mr. LeDrew, I also was unfamiliar with that concept. Grateful to know more today than yesterday. However I still don't understand it in this context. Can you or someone find simpler words to digest?
Mr. Ferris, I think you mean "2 minutes" wherever you said "2 degrees" right?

#20 NGC7088

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Posted 03 February 2013 - 09:31 AM

OK I think I got it. At the same exit pupil the larger aperture has "diluted" the available light into more magnification so the brightness is the same. Is that it?

#21 Jon Isaacs

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Posted 03 February 2013 - 09:49 AM

However I still don't understand it in this context. Can you or someone find simpler words to digest?



I hope this is what you are thinking of:

The surface brightness of an object is proportional to the exit pupil, a telescope cannot increase the surface brightness over what is seen naked eye, it can magnify it making it larger but not brighter.

A simple example goes like this:

Consider a generic extended object, say Andromeda. Your eye sees it with it's 7mm exit pupil. You now point your 70mm telescope at Andromeda. At 10x this produces a 7mm exit pupil, the same as your eye. It also collects a hundred more times light than your eye. You have 100 times the light but the image is magnified 10 times so that light is spread out over 100 times the area. 100 times the light, 100 times the area... The result is that the image is larger but the intensity or surface brightness of the object is no greater. If you use exit pupils that are larger than the entrance pupil of the eye, the additional light falls unused...

This is a simple proof that a telescope cannot increase the surface brightness of an extended object, it can only make it larger. It can also make it dimmer. If the image were magnified 20 times instead of 10 times, that 100x the light would have been spread out over 400 times the area and thus the surface brightness would have been reduced by a factor of 4.

This is a simple proof that the surface brightness is proportional to the square of the exit pupil.

I hope this helps someone understand something...

Jon

#22 GlennLeDrew

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Posted 03 February 2013 - 12:08 PM

And Bill Ferris did indeed mean 2 degrees, and not 2 arcminutes.

He was elaborating on a very important concept regarding human vision. As light levels decrease, our resolving power decreases. Down at the level of faint fuzzies seen against a dark night sky, our resolving power has gone from the daytime limit of 1-2 arcminutes to the atrocious nighttime limit of a couple or few degrees (!)

In other words, just to be detected at all, a dim nebula must subtend on our retina a size equivalent in diameter to several full Moon diameters. Yet at the same time and in the same view we can see those very tiny points of light called stars.

Our visual system is a very marvelous and adept processor of signals. On the fly it processes both bright and dim parts of the image so as to get the most information. When parts of the image are very dim, light receptors on the retina are effectively bunched together in groups, acting like much bigger and hence more light-sensitive 'pixels'. This is a necessary tradeoff--either see nothing except visual system 'noise', or have at least a detection of *something*, even if poorly resolved.

The relationship of the relevant factors are charted out in an image (with user guide, I posted in my Gallery, linked to in my signature below. I forget the image title exactly at the moment, but you'll know it when you see it. At first, you'll probably find it all quite complicated. Save the image and copy the 'user guide' text, then print them out for continued study..,

#23 Dave Mitsky

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Posted 03 February 2013 - 12:28 PM

There are mentions of etendue, which is a measure of the flux gathering capability of an optical system, in relation to telescopes at these two sites:

http://pan-starrs.if...wide-field.html

http://www.physics.p...ures/lsst.shtml

"For many applications, the rate at which objects can be detected scales as the etendue of the telescope, which is defined as the product of its collecting area (A) times the field of view (Omega)."

"The ability of a telescope to survey large patches of the sky is given by its étendue. Étendue, the French word for extent, is defined mathematically as the product of the light collecting area and the field of view of the telescope. This quantity measures the number of photons per unit frequency per unit time a telescope will accept."

There's a mathematical treatment of the concept at http://www.physics.c...try/etendue.pdf

Dave Mitsky

#24 NGC7088

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Posted 03 February 2013 - 10:40 PM

I found the chart. It is complicated. It seems to me to be absolutely more relevant to telescope setup than the site quoted at the beginning of this thread.
An example could help. In a polluted sky (say 18) suppose I wanted to see..say something simple..high in the sky right now, say M1 (Mag 8.4, SB 11, Size 8x4') or M78 (Mag 8, SB 12 size 8x6'). The telescope is 90mm. Another choice is 381mm. How can I use the chart to select the eyepiece/filter combination to produce the best contrast (or at least enough contrast). Right now I cannot see those from here, not in the binoculars, not in the small telescope, I have seen them in the big one in the past but did not set it up today. The sky is possibly worse than 18, to the naked eye it even has a red tinge to it...

#25 GlennLeDrew

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Posted 04 February 2013 - 06:56 AM

Your surface brightness values in the two examples would seem to be in magnitudes per square arcminute. To convert to magnitudes per square arcsecond, add 8.89. Then, with your sky brightness, go back to the chart and determine the parameters for visibility.






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