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Does increasing aperture improve threshold contrast: Nope

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#1 GlennLeDrew

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Posted 24 June 2018 - 03:45 PM

A current thread on eyepiece contrast brought up this matter, which I feel deserves its own treatment. Not that it hasn't been discussed here over the years! ;)

 

It's something of a durable myth, among some folk, that the very fact of increased aperture by itself results in the visual discernment of subtler contrast. Some years back I went through several pages of discourse with another CNer who steadfastly asserted this. The dearth of interaction from other forumites suggested a general lack of certitude on their part. One on my missions is to beat back against such misperceptions.

 

In the context of this discussion, threshold contrast could defined as the limit at which a source of given surface brightness just becomes perceived as differing from its surrounds. The source could be darker (as in a dark nebula seen against unresolved starlight or bright nebulosity), but more commonly we tend to think in terms of brighter-than-sky objects such as glowing nebulae. And here we will restrict to the situation of dim, extended DSOs, where threshold contrast is especially crucial, although it's certainly important in the realm of the bright, such as planetary surface observation.

 

Other things being equal (basic instrument desgn, optical quality, transmission efficiency, control of unwanted light and exit pupil), threshold contrast for visual detection is invariant with aperture. If a 2" scope just permits to perceive [object + sky] surface brightness of, say, 0.07m brighter than sky, a 20" scope will do no better.

 

The preceding naturally assumes that the object in each case is easily larger than the minimum size for detection. And therein lies the key to this myth I'm knocking down.

 

There has arisen a veritable conflation of contrast transfer--as a *general* concept--with the modulation transfer function (MTF), as frequently used as a measure of optical performance. But the MTF principally concerns the small scale regime hardly much larger than the Fresnel pattern of diffraction. In other words, it has relevance where the subject is well within the bright photopic range and the exit pupil is small enough--typically less than about 2mm--to permit the discrimination of the effects concerned. Or to put it another way, the MTF for the most part concerns resolving power, at least when the optics are halfway reasonable.

 

Even if we are dealing exclusively in the realm of the small and bright, the MTF doesn't tell the full story. Besides diffraction, aberrations and small-scale scatter, contrast is afflicted at intermediate and large scale by such causes as optical reflections and non-optical surface scatter/reflections; this we term veiling glare. An actual measured MTF over all relevant scales would represent the full reality, but a calculated MTF (which we typically see) based only on and restricted to that range indicated by a wavefront measurement and consideration of diffracting obstructors misses the full picture.

 

In short, contrast transfer concerns more than the realm the tiny range the usual MTF chart covers.

 

Indeed, for dim, low contrast DSO observation, the common MTF chart is essentially irrelevant. For a difficult nebula against a fairly dark sky, the human visual resolving power *on the retina* is not the 1-2 arcminutes for daylight conditions, but instead is a truly awful 1/2 degree, or 1 degree, or as bad as 5-6 degrees at the limits of faintness/contrast. If f we were to choose a characteristic number, we could say that for faint fuzzy DSOs our resolving power is 100 times poorer than for lunar/planetary observing.

 

To see this most viscerally, install your solar filter and look at the Moon. The result will be a sunlit lunar surface dimmed to that of a moderately bright planetary nebula (not bright enough to discern color, but neither a difficult detection.) A quarter or gibbous Moon is best, for you then have still the intrinsically *very* high contrast terminator and shadows to examine. Note how terribly poor your ability to resolve familiar features now!

 

Now, back to the telescope's inability to improve upon threshold contrast detection when made larger. To this end, an easy to perform experiment might drive home the lesson better than can any amount of verbiage. This test is based on the following premise:

 

Getting physically nearer to a subject does not improve threshold contrast.

 

If one doubts this, the test would resolve the matter if a suitably large target comprising a low-contrast pattern is devised.

 

For a first order, simple appreciation, the target could be a poster. The poorer its contrast the better. Set it up at some goodly distance where the smaller features are not fully resolveable. Measure this distance.

 

Pull out or borrow a binocular of 5-7X (the lower the magnification, the less the distance you will have to walk--or bike ;). Examine the the target carefully through the bino. Now walk (or bike) toward the target, stopping at that distance whose ratio equals the inverse of the magnification. For example, if the initial viewing distance with bino is 100 feet, and the bino magnification is 5X, you will stop 20 feet short of the target. Now examine it with eyes alone.

 

If necessary, repeat (and get a bit of exercise ;). Do you find any notable differences in the two views?

 

A fuller test using a battery of binos would be more instructive. For instance, if one had 2X, 4X and 8X binos, comparisons could be made at respective eye-alone distances of 50', 25' and 12.5' (for the same 100' starting distance as above.)

 

And better yet, under conditions of low light, and using binos having at least near to 7mm exit pupils.

 

You see, employing a telescope is just like moving nearer to an object. This is exactly true if the scope's exit pupil is at least as large as the observer's iris.

 

For our earlier experiment, suppose you place beside the target a friend who looks back at you all the while you conduct your observations. Furthermore suppose your 5X bino has an exit pupil equaling your iris diameter; let's say this is 3mm, making for a 5X15 bino.

 

If your friend has a fixed-maginification telescope which can resolve your iris, he will see it subtend some apparent angle in the eyepiece. At the initial 100' starting distance, let's say he sees your 3mm iris subtending an apparent 1 degree in his eyepiece. Your bino's 15mm objectives will then subtend an apparent 5 degrees.

 

Now you move the 5X closer to the target, stopping at 20'. Your friend willl now see your iris subtending 5 degrees, which is the same angular size as your iris when 100' distant. This is why your naked eye view from 5X nearer to the target is exactly the same as the 5X magnified view from 5X farther away. In both cases the entrance pupil of the system viewing the target, as seen from the target itself, is identical.

 

Finally, as one point of evidence that the contrast threshold does not vary with viewing distance (and by extension, with changes in aperture), consider that as you look at the wall in your room right now and move nearer to and farther from it, its surface brightness does not vary in the slightest. Certainly NOT as a function of the inverse square law! (Which is a *huge* variance.) And so if the basic surface brightness is not varying, it must follow that any local differences in surface differences must not vary either. In other words, contrast does not vary. And if contrast is not varying, neither can threshold contrast.


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#2 TOMDEY

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Posted 24 June 2018 - 04:32 PM

Hi, Glenn! That's interesting, especially your summary paragraph. Noted, boundary condition throughout the discussion that viewing is ideally rich-field throughout.

 

But... but... although it true that the Object wall with structured variations in luminance is an invariant Object, (So far, so good) BUT: The angular frequency spectrum of specific, ostensibly identifiable features a, b, c, d, ... in the Image (on your retina) shifts scale in proportion to your range. Twice as far doubles the spatial frequency(ies). Even if object-space is ideally-fractal (lateral angular spatial freq spectrum invariant with scale/range) nevertheless, Specifically-Identifiable features a, b, c, d will eventually drop to undetectability as you recede. If feature B is a half-degree at five feet, it will NOT be detectable at ten, per your own criterion! Grab these Vixen 2.1x binos and look at it from ten feet and... voilà! feature B is discernable again.

 

And THAT resurrects (as correct) the contention that more aperture renders "smaller" Object-Space Targets of Interest discernable, because of the coupling of the necessarily-assumed  Rich-Field boundary condition to maximum allowed magnification, which is proportional to Aperture!  QED  Tom

 

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#3 GlennLeDrew

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Posted 24 June 2018 - 06:34 PM

Tom,

What I was getting at is this. Say we have a completely featureless blob of an object *well large enough* for a small aperture to permit detecting, but which is only glimpsed with some uncertainty due purely to low contrast. Some folk think that bringing a larger aperture to bear will now permit to see this blob more definitely because the larger aperture *lowers* the contrast detection threshold. Or, if the object were to be just barely below the detection threshold for the smaller instrument (in spite of being well large enough otherwise), these folk think the bigger scope will now bring it into visibility.

 

This would be equivalent to the following thinking (and using purely imaginary numbers for illustration.)

 

Consider a step scale of grey tones whereby the difference between adjacent patches were to be, say, 4% in brightness. A small scope permits to discriminate brightness differences of 8%. The view would be effectively of pairs of patches being seen as one, with half the total number being counted. But, this thinking would suggest, a sufficiently large aperture, due to the attendant lowering of threshold contrast, would permit to discriminate between all adjacent patches, resulting in all--not just half--being counted.

 

I stress that this is independent of the matter of bringing formerly too-small objects/details into resolvability. In all instances, the test target is well larger than the minimum size for detection.

 

In that exhaustive, multi-page discussion with a CNer those many years back I mentioned in my previous post, I had brought to bear numerous ways of looking at the matter, including some in this thread. But he (and the audience, it seemed) were not convinced. Here's one concept I tried:

 

Imagine lying side by side on the sky two intrinsically precisely identical galaxies; same size, form and brightness. But one lies 2X farther than the other. One scope, at a certain exit pupil, is trained on the nearer galaxy. Another scope 2X larger in diameter and working at the same exit pupil is trained on the farther galaxy. We know (or should know ;) ) that the views will be identical. Try as I might, the other correspondent could not accept that. He maintained that the larger 2X larger scope, at the same exit pupil, would still reveal subtler contrasts for that 2X farther galaxy.

 

Perhaps I failed to state my arguments clearly enough back then. Or I misinterpreted the arguments coming back to me. ;)

 

In any event, the too-common statements of improved threshold contrast with larger aperture leads me to fear that this kind of misperception is still alive and well today. I'd be most curious to know if anyone reading this does or once did think so.


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#4 russell23

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Posted 24 June 2018 - 08:03 PM

There has arisen a veritable conflation of contrast transfer--as a *general* concept--with the modulation transfer function (MTF), as frequently used as a measure of optical performance. But the MTF principally concerns the small scale regime hardly much larger than the Fresnel pattern of diffraction. In other words, it has relevance where the subject is well within the bright photopic range and the exit pupil is small enough--typically less than about 2mm--to permit the discrimination of the effects concerned. Or to put it another way, the MTF for the most part concerns resolving power, at least when the optics are halfway reasonable.

 

Glenn,

 

There is a possibility that the reason few people responded to the previous thread you mentioned is that they have no idea what you are talking about.   For example,  I highlighted in the above terminology I am really not familiar enough with to make any sense of what you are saying.

 

I believe I understand that what you are trying to disprove is the notion that aperture difference change contrast.  But I really don't follow your proofs at all.

 

What I do understand is that the exit pupil is the important factor to visual contrast rather than the aperture.  Yet, as aperture increases you can see more detail in objects. 

 

What I'm trying to say is that you are explaining way over my my understanding of optics theory and I would suspect many (most?) others.   There is a practical understanding of the use of optics and there is optical theory that explains the physics behind what is seen.   The latter becomes relevant when people try to explain what they see in practice using incorrect optical theory.

 

So I believe what you are trying to correct is an incorrect explanation for the difference people see when they use a larger vs. a smaller aperture. 

 

I mean no insult but to get more people involved in the discussion it really is necessary to bridge the information gap between optics 101 and PhD candidate Optics 599. 

 

Dave


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#5 GlennLeDrew

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Posted 24 June 2018 - 08:58 PM

Dave,

I'm just another amateur like most folk here. My knowledge of theory is in no way what could be called advanced. I happen to have some experience in optics fabrication, but this is not necessary to tackle the concepts I raise here. Indeed, there are a good number of bright sparks who hang out in the Optics/DIY Forum who eclipse me handily on the theory front.

 

To overcome any lack of knowledge among the readers is why I often outline pretty simple and easy-to-conduct experiments.

 

It occurs to me that a likely source of confusion could well be misinterpretation of Roger Clark's exposition(s) on threshold contrast, at least in his book. He details the manner in which threshold contrast *as a function of magnified size* varies, which relates to the way in which fainter stuff requires to be more magnified in order to be detected. And of course a larger aperture affords higher magnification, with no or lesser image diminution (dependent on the exit pupil). The less attentive reader could easily interpret this to mean that by itself a larger aperture confers a lower contrast threshold, not realizing the crucial matter of magnified size.

 

In any event, the very operation of our visual system suggests this misperception, and reinforces it. To wit. As we go from low to moderate magnification, the increased image scale typically results in increased detail seen in DSO fuzzies. The darkened sky gives the impression that contrast has been boosted, as though the sky has darkened more rapidly than the object. (And of course fainter stars being seen augment this impression most strongly; but for point sources contrast *has* improved.) And so in spite of the fact that with a shrinking exit pupil the sky and all extended objects dim by the same amount, we are given the *impression* that the sky dims more rapidly. And that's *without* an aperture change.

 

A larger aperture offers a larger image scale, which improved resolution out of the gate suggests improved contrast.

 

Now invoke the MTF, for which the less critical peruser might fail to appreciate the limitations. A more cursory study can lead to the misperception that improved contrast at small scale applies at large scale too. For instance, the additional diffraction induced by a secondary obstruction lowers contrast--at the scale of the diffraction pattern. But legion are those who have conflated this to mean the lower contrast results in 'greyer' skies; they have extrapolated this in their mind from the scale of the very tiny to that of the whole FoV.

 

Because it's common that Newts tend to be less well baffled that refractors, and that Newts--more so in the past--typically permit larger exit pupils, the brighter sky more likely to see with Newts could reinforce the notion of the secondary obstruction and support as causing or contributing to the 'greyer' sky.


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#6 havasman

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Posted 24 June 2018 - 08:58 PM

Yet, as aperture increases you can see more detail in objects. 

 

Yes. Due to magnification/image scale. And Glenn makes that point. But I think it would allow more readers to understand his argument more effortlessly if that point were more clearly conceded earlier in the treatise. It might allow more readers to turn off their internal argument with what they're reading if they knew it agreed better with at least part of what they already know.

 

But the subject is contrast and, as the point has been made before and often and well, contrast is maybe the most misused term of any used to describe the effects of our hobby's optics. So I appreciate Glenn's continuing effort to clarify the concept. Reading this latest attempt I hoped that the new and a bit differently slanted presentation might get through to a new set of learners. I know I am thinking of what I've learned from Glenn's earlier efforts when I use the term apparent contrast instead of misusing the term contrast.


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#7 Sketcher

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Posted 24 June 2018 - 10:23 PM

The topic question: Does increasing aperture improve threshold contrast?; asks one thing.  The first sentence in the 2nd paragraph addresses something entirely different.  The first is an absolute, with no conditions attached.  The second is an impossibility.

 

It's not possible to “increase aperture by itself”.  One may increase aperture and keep the magnification the same – but that results in operating at a larger exit-pupil.  One may increase aperture and keep the exit-pupil the same – but that results in operating at a higher magnification.  Anything else results in increasing aperture while changing both – magnification and exit-pupil.

 

There are too many variables to justify any meaningful conclusion – until one pins down all but one: aperture.  But that's not possible if one insists on sticking (strictly) with the topic question.

 

OK, I see in the fourth paragraph that the exit-pupils are being kept the same, resulting in an increased magnification with the larger aperture.  And you're attempting to get past the extra magnification by limiting the discussion to objects that are 'large enough' for the lower magnification of the smaller aperture to handle. –  A lot of restrictions/conditions are cropping up.  This is no longer a discussion concerning the effects of only increasing aperture.

 

In the 19th paragraph we see that we probably don't want the exit pupil to get too large with the larger telescope – looks like another restriction . . .

 

So getting back to the topic question:  Does increasing aperture improve threshold contrast?  It would appear to follow from statements made in the original posting that the correct answer may well be:  “It depends”.

 

In the real world, where things like exit-pupil, magnification, and object size are not being strictly controlled; increasing aperture will most definitely – sometimes – result in what appears to be improved threshold contrast; but this isn't strictly due to the increased aperture.  After all, in the real world, one cannot increase aperture without changing the exit-pupil and/or magnification.

 

So, in the real world, some people will see some objects appear as if the contrast has improved when they've 'only' increased the aperture.  And it really doesn't matter if one attributes that improvement to a change in magnification, an increased aperture, a different exit-pupil. or a monkey from the 12th dimension manipulating our reality.  What matters is that one made a change and can now see something that could not be seen before.


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#8 Jon Isaacs

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Posted 24 June 2018 - 10:25 PM

The preceding naturally assumes that the object in each case is easily larger than the minimum size for detection. And therein lies the key to this myth I'm knocking down.
This is exactly true if the scope's exit pupil is at least as large as the observer's iris.

 

These are two important assumptions.  The fact that they do not necessarily apply are the reaso that when one is viewing the same appropriately small object, that a larger telescope will show finer details and lower contrasts will be seen.  The larger telescope can provide a larger and brighter image.  Contrast threshold is a function of brightness since this is really a S/N ratio.  

 

Jon


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#9 xiando

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Posted 24 June 2018 - 10:45 PM

In terms of reflectors, a larger aperture scope provides a higher likelihood of obtaining a scope with lower CO. Even one step up from my 41% CO 6" newt to an 8" drops that value to near 30%. You're suggesting that's not a good thing for contrast? Is the transfer equation bull?


Edited by xiando, 24 June 2018 - 10:45 PM.


#10 Redbetter

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Posted 24 June 2018 - 11:53 PM

Must have been before I was here since I am not sure what the argument is.  I take it as a given that contrast of an object against a background is independent of aperture assuming that the relative image scale and brightness are the same.  I also assume that the black background cut off point will be the same in terms of illumination (e.g. exit pupil in a given sky condition) regardless of aperture. 

 

The part that is hardest to quantify is the loss of resolution as image brightness is reduced.  For bright, high contrast objects like planets and double stars this seems to be around 3 or 4 arc minutes for me from what I can tell.  For averted vision, threshold magnitude, dim objects it might be over a degree from what I can detect separation in and the power it takes to do it if the seeing allows.  For even larger lower surface brightness objects (therefore lower contrast, all the way to the detection limit) effective resolution is indeed far worse.    

 

Larger aperture is primarily for finer detail and smaller/dimmer objects than could otherwise be seen in small aperture.  Reflectors/SCT's/Maks all have some non-trivial loss of brightness and contrast.   Larger instruments make up for this by providing higher resolution at equivalent or larger (brighter) exit pupils.  Well corrected and decently figured smaller refractors can provide the maximum surface brightness when paired with the right eyepiece...and sometimes the right filter.    It depends on the scale that is required for the object.  Smaller apertures have the advantage of providing more context in the same field of view. 


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#11 GlennLeDrew

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Posted 25 June 2018 - 01:32 AM

Perhaps my epistle was too long. ;)

 

It all boils down to what I think is a reasonably clear and definite statement in the 1st sentence of paragraph 4. I stated that:

 

Other things being equal (basic telescope design, optical quality, control of unwanted light and exit pupil), threshold contrast for visual detection is invariant with aperture.

 

In the next paragraph I specified that the object in all cases was fully larger than the minimum size for detection. This was to obviate the factor of the detection threshold causing the object to disappear at lower magnification.

 

I should have clearly constructed the object as being an utterly featureless circle of uniform surface brightness across its entire face. This would keep us from immediately jumping straight to the matter of details revealing themselves with increasing aperture/magnification. And I should have stressed the constancy of the exit pupil, so that there is no difference in scene surface brightness (which obviously impacts signal to noise and hence threshold contrast.)

 

This would seem to be why folks have a hard time grasping the most fundamental tenet of my argument. They straightaway recall their own experiences, necessarily involving a myriad of variables. I'm trying to isolate to the most basic aspect, which omits resolvability and scene brightness as variables.

 

If we consider that all instruments are of equal design and quality and operate at the one and same exit pupil, and that the object is big, featureless and of uniform surface brightness, the fact of there being no difference in threshold contrast should become clear. I hope. ;)

 

If you recall, I sketched out the situation of a scope 2X bigger than another examining a galaxy half the size of one examined by the smaller scope. Because both scopes are operating at the same exit pupil, each provides a view of its target galaxy identical to the other. Same apparent size, same resolved detail, same apparent integrated brightness and same surface brightness. If you, dear reader, understand completely why this is so, you understand my thesis here.

 

If there is uncertainty or doubt, consider this.

 

Imagine you have a large sheet of white paper, with one half painted the subtlest grey that is *barely* discernible as darker than white. You hang it on a wall. Look at it from the far side of the room, then from fairly close up. You'll find no difference in the ability to discern the brightness difference; threshold contrast has not varied.

 

When looking from close up (and assuming your irises retain the same diameter), this is precisely equivalent to using an instrument of magnification equaling the distance ratio for the farther view and utilized at that farther distance. And this is therefore equivalent to increasing the size of your eyes (not just the iris diameter) had you stayed at the farther distance. And this in turn proves that threshold contrast is invariant with aperture.

 

Probably too complicated? Try this on for size...

 

Imagine a weird phenomenon whereby the eastern half of the sky is, say, 0.1 magnitude brighter than the western half. A razor sharp and dead straight discontinuity runs from the north point on the horizon, through the zenith, and on to the south point on the horizon. Even high magnification shows a razor sharp discontinuity.

 

Your pupils are 7.1mm in diameter. You have on hand three binos, all delivering 7.1mm exit pupils: A 3.5X25, a 7X50 and a 28X200. (Let's suppose the binos transmit 100%.) Between the naked eye view and the views delivered by the three binos, you will find no difference in the ability to discriminate the sky surface brightness difference. Threshold contrast has not differed.

 

This is what I'm getting at; the most fundamental aspect, stripped of all other variables. Including that most pernicious of all, the human visual system. And this is what I mean when I say "aperture alone": All other variables are fixed, such as exit pupil, transmission efficiency, control of unwanted light, and keeping to an idealized target that obviates consideration of resolvable detail.


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#12 GlennLeDrew

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Posted 25 June 2018 - 01:53 AM

Part of my motivation here is to make clear that there is no 'magic' attending a larger aperture. When relative newbies who use smaller scopes ask what can be seen with big 'uns, I get the distinct impression that some might be picturing some instrument-intrinsic boosting of surface brightness that brings into view lower surface brightness stuff that smaller apertures cannot (irrespective of size.) This impression I get is augmented by the supposition that formerly colorless extended objects can be made to exhibit color by virtue of a larger aperture, which would imply a boost to surface brightness.

 

In case of a lack of awareness of the crucial role played by apparent subtended angle, as well as how the exit pupil affects things, I feel it's wise to cover foundational concepts as here. By examining an aspect of instrument operation reduced to the minimum of free variables, a clearer understanding of the fuller picture obtains. Just one misperception can afflict overall understanding, at worst leading to persistent myths.



#13 Jon Isaacs

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Posted 25 June 2018 - 10:20 AM

Part of my motivation here is to make clear that there is no 'magic' attending a larger aperture. When relative newbies who use smaller scopes ask what can be seen with big 'uns, I get the distinct impression that some might be picturing some instrument-intrinsic boosting of surface brightness that brings into view lower surface brightness stuff that smaller apertures cannot (irrespective of size.) This impression I get is augmented by the supposition that formerly colorless extended objects can be made to exhibit color by virtue of a larger aperture, which would imply a boost to surface brightness.

 

In case of a lack of awareness of the crucial role played by apparent subtended angle, as well as how the exit pupil affects things, I feel it's wise to cover foundational concepts as here. By examining an aspect of instrument operation reduced to the minimum of free variables, a clearer understanding of the fuller picture obtains. Just one misperception can afflict overall understanding, at worst leading to persistent myths.

Glenn:

 

The members of this particular forum are not relative newbies, I think all of us understand the concept of surface brightness and angular size.  In some sense, you are preaching to the choir.

 

In your carefully worded statements where object size is a function of aperture and exit pupils are identical so as to isolate the single factor of contrast threshold, clearly the contrast threshold as a single variable is a single variable...  

 

But the real world is a different situation, it is a multivariable problem and like most problems, it is the way the problem is defined that determines the answer.  If one is looking at the same object in both telescopes, it is a much different situation, image brightness and image size are functions of aperture. 

 

 To say there is no "magic" in the views a large aperture scope can provide, that's denying reality.. In the strictest sense, it all can be explained, why a large scope shows objects and details that are beyond the reach of a smaller aperture, why the eye likes a brighter image, why there is more contrast in a brighter image, why the eye likes a larger image, this is all science so it is not magic.

 

But when I point my 22 inch scope at Chi UM, I see nearby NGC3877 as a beautiful, 11th magnitude edge on spiral, 5.2' x 1.2'  Or point it at M51 and the spiral structure that looks like the photos.   Then I scan from M51 over and catch some nearby 13th-15th magnitude galaxies.. 

 

When compared to what is visible in say a 4 inch, one has to be mighty jaded to not see the magic in those views.  

 

In terms of concepts, one concept that I see rarely discussed is spacial brightness gradient, which at the same exit pupil becomes spacial contrast gradient.  This is one place where the smaller aperture can have an advantage.  Since the surface brightness at maximum exit pupil is not a function of aperture but image scale is, this means the spacial gradient of a small aperture scope can be greater than the large scope.  If one is hunting down something large like Barnard's loop, the smaller scope is superior because the spacial brightness and contrast gradients are greater.

 

There is magic in that as well.  

 

Jon

 

P.S:  "This impression I get is augmented by the supposition that formerly colorless extended objects can be made to exhibit color by virtue of a larger aperture, which would imply a boost to surface brightness."

 

We know that the surface brightness is a function of the exit pupil.  At the same exit pupil the object will be the same brightness.  At the same image scale, a large scope provides a brighter image.  Rarely does one view at the largest possible exit pupil because it is the combination of image scale and exit pupil that provide the best image.  

 

So.. one has a 60mm aperture operating at 20x, that's a 3mm exit pupil.  The guy next door has a 150mm operating at 20x, that's a 7.5 mm exit pupil.  If the guy next door has a dark adapted pupil that's at least 7.5mm in diameter, the object will be 2 magnitudes brighter.  If the 60mm is used with a 7.5 mm exit pupil, that would be 8x and the object may be too small to see.

 

At some point, one has to reconcile what is seen in the eyepiece with the science. On the same object, am I more likely to see color in a larger scope?  I have to say that as someone who regularly observes with apertures ranging from about 8mm up to 560mm, my experience is that I tend to see more color in extended objects with larger aperture scopes.  It goes back to the eye liking a brighter image, liking a larger image.  Sometimes that bright image in a small scope is just too small to detect that color.  Think M42 naked eye.. 


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#14 quazy4quasars

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Posted 25 June 2018 - 01:26 PM

  A lot of factors contribute to perception.  I like Roger Clark's Visual Astronomy of the Deep Sky for a reasoned and concise discussion of what we may consider to be the "null hypothesis" of Glenn's position.  Personally, I make no claim. 



#15 Starman1

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Posted 25 June 2018 - 01:55 PM

A big scope sees more because:

--at the same exit pupil, the magnification is higher

--at the same magnification, the exit pupil is larger

--at any magnification, the resolution is greater (post #2 is a good illustration).

And, that works for faint, extended objects.

If the faint object is 0.1 magnitude brighter than the background, it will be at a much higher magnification the background sky appears the same to the eye.

Same contrast and larger = more visible.

If a faint object is the same size, the overall image will be brighter because of a larger exit pupil (this only works to a point).

Overall image brighter = more visible.

Typically, in the field, the larger scope will be being used at both a higher power AND a larger exit pupil, so people will see faint objects easier and better than in the small scope.

 

But, this breaks down when analyzing small points (stars) in the field.

(we'll consider, for simplicity, only a star on the center axis).

Every star image will be created by the entire aperture.  As a result, a 4x larger aperture will result in a 4x brighter star image because a point is a point.

[I'm not considering the size of the Airy disc in the scope, here].

And brighter = more visible.

So the larger scope will see fainter stars because the brightness has increased.

But, did the contrast between the star and the background increase?

The "noise" in the image--the background sky brightness--will have increased just the same as the star image, so the contrast should remain the same.

 

And yet, the larger scope sees fainter objects and fainter stars.

Our language about it is inexact.  Apparently, the fainter targets are more visible because contrast with the background has somehow improved, so we inexactly say contrast has improved.

We see intellectually that, while objects have increased in brightness, so has the background, so contrast remains the same as in the smaller aperture.

 

So could it be that it is not contrast that is making the fainter stars and objects more visible, but simple brightness?

Let's take the example of a star image 1" in size at magnitude 19.0 in a background sky of brightness magnitude 20.0 per square arc second.

The star is 1 magnitude brighter than the background sky (I'll ignore the contribution of the background in the star image).

Now, increase the scope size by 2.5x.  Each has increased in brightness by a magnitude, and the difference is the same.

If contrast were the only factor, the star might not be any more visible in the larger scope.  And yet, it is more visible.

What has increased?  Brightness.  Size and contrast have not.

But the faint star has suddenly come out of the background at the exact same exit pupil, so surface brightness has not increased.

 

No amount of language can erase the fact that fainter targets, both stars and extended objects, are more visible in larger scopes.

As are colors.  I've seen it too many hundreds of times to disbelieve it.

Yet, if contrast did not improve, some other factor did to make the objects more visible.  And it isn't size, because that doesn't explain the improved visibility of faint stars.

and it isn't exit pupil because at the same exit pupil as in the small scope, fainter stars are visible.

So it must be brightness in terms of photons.

 

Does improving contrast help?  Sure, or nebula filters wouldn't work.  The nebula did not get brighter, yet it became more visible because the background darkened.

The effect is very similar to making the aperture a lot larger.  Hmm.  Could it be the larger aperture somehow improves contrast?

If so, how?  I follow the arguments that it does not, and yet.......

 

The serious question is whether the eye sees only the "contrast ratio", which does not change, or "brightness difference" which is not the same thing.

If the object has a brightness of 2 and the background a brightness of 1, the contrast ratio is 2:1.  Double the brightness in a larger aperture and the object becomes a 4 and the background a 2.

The contrast ratio is the same, 2:1, but the brightness difference has gone from 1 to 2.  Does the eye see the difference rather than the ratio?

 

I frankly don't know, but I do know that the explanation of larger size and larger exit pupil in the larger scope simply doesn't add up to make faint stars more visible in the larger scope.

Or colors more visible at the same exit pupil (indeed, colors are more visible even at SMALLER exit pupils).

Until I read a rational explanation that doesn't defy the evidence of my senses, I am going to say that this is one of those examples where I follow the arguments, but that they contradict what I see.

As Galileo supposedly muttered, "Eppur si muove".


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#16 Organic Astrochemist

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Posted 25 June 2018 - 02:58 PM

“If a 2" scope just permits to perceive [object + sky] surface brightness of, say, 0.07m brighter than sky, a 20" scope will do no better.”
This is false. The 20” scope will do better by allowing one to perceive objects that are smaller and/or dimmer.
Consider the object at the limit of detection in the 2”. If one increases the magnification to increase the apparent size, the [object + sky] surface brightness dims below the threshold and the object can’t be seen (the sky is black and can’t be dimmed). If one reduces the magnification to increase the surface brightness, the apparent size falls below the threshold and the object can’t be seen.
The narrow range of this optimum magnification will make it difficult to observe such objects in practice.
With a larger scope one will be able to use both lower and higher magnification to see the object, hence increasing the likelihood of actually observing it in practice. Furthermore, objects which are smaller and dimmer would be invisible in the 2” but might be visible in the 20”.

Of course objects that can’t fit in the field of view of the 20” would be better observed in the 2”.

#17 GlennLeDrew

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Posted 25 June 2018 - 03:31 PM

Glenn:

 

The members of this particular forum are not relative newbies, I think all of us understand the concept of surface brightness and angular size.  In some sense, you are preaching to the choir.

 

In your carefully worded statements where object size is a function of aperture and exit pupils are identical so as to isolate the single factor of contrast threshold, clearly the contrast threshold as a single variable is a single variable...  

 

But the real world is a different situation, it is a multivariable problem and like most problems, it is the way the problem is defined that determines the answer.  If one is looking at the same object in both telescopes, it is a much different situation, image brightness and image size are functions of aperture. 

 

 To say there is no "magic" in the views a large aperture scope can provide, that's denying reality.. In the strictest sense, it all can be explained, why a large scope shows objects and details that are beyond the reach of a smaller aperture, why the eye likes a brighter image, why there is more contrast in a brighter image, why the eye likes a larger image, this is all science so it is not magic.

 

But when I point my 22 inch scope at Chi UM, I see nearby NGC3877 as a beautiful, 11th magnitude edge on spiral, 5.2' x 1.2'  Or point it at M51 and the spiral structure that looks like the photos.   Then I scan from M51 over and catch some nearby 13th-15th magnitude galaxies.. 

 

When compared to what is visible in say a 4 inch, one has to be mighty jaded to not see the magic in those views.  

 

In terms of concepts, one concept that I see rarely discussed is spacial brightness gradient, which at the same exit pupil becomes spacial contrast gradient.  This is one place where the smaller aperture can have an advantage.  Since the surface brightness at maximum exit pupil is not a function of aperture but image scale is, this means the spacial gradient of a small aperture scope can be greater than the large scope.  If one is hunting down something large like Barnard's loop, the smaller scope is superior because the spacial brightness and contrast gradients are greater.

 

There is magic in that as well.  

 

Jon

 

P.S:  "This impression I get is augmented by the supposition that formerly colorless extended objects can be made to exhibit color by virtue of a larger aperture, which would imply a boost to surface brightness."

 

We know that the surface brightness is a function of the exit pupil.  At the same exit pupil the object will be the same brightness.  At the same image scale, a large scope provides a brighter image.  Rarely does one view at the largest possible exit pupil because it is the combination of image scale and exit pupil that provide the best image.  

 

So.. one has a 60mm aperture operating at 20x, that's a 3mm exit pupil.  The guy next door has a 150mm operating at 20x, that's a 7.5 mm exit pupil.  If the guy next door has a dark adapted pupil that's at least 7.5mm in diameter, the object will be 2 magnitudes brighter.  If the 60mm is used with a 7.5 mm exit pupil, that would be 8x and the object may be too small to see.

 

At some point, one has to reconcile what is seen in the eyepiece with the science. On the same object, am I more likely to see color in a larger scope?  I have to say that as someone who regularly observes with apertures ranging from about 8mm up to 560mm, my experience is that I tend to see more color in extended objects with larger aperture scopes.  It goes back to the eye liking a brighter image, liking a larger image.  Sometimes that bright image in a small scope is just too small to detect that color.  Think M42 naked eye.. 

Jon,

I certainly am aware of the considerable "choir" to whom I preach here. wink.gif

 

But we do a disservice to the less knowledgeable--including non-members or just lurkers--if we assume almost uniform book learnin'. After all, we've all encountered hereabouts on CN more than one person who's been surprised to learn that no telescope can supply an image having surface brightness higher than seen by eye alone.

 

What I describe here is related to that, and one reason why I mention that there is no 'magic' involved.


Edited by GlennLeDrew, 25 June 2018 - 03:50 PM.


#18 GlennLeDrew

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Posted 25 June 2018 - 03:47 PM

A big scope sees more because:

--at the same exit pupil, the magnification is higher

--at the same magnification, the exit pupil is larger

--at any magnification, the resolution is greater (post #2 is a good illustration).

And, that works for faint, extended objects.

If the faint object is 0.1 magnitude brighter than the background, it will be at a much higher magnification the background sky appears the same to the eye.

Same contrast and larger = more visible.

If a faint object is the same size, the overall image will be brighter because of a larger exit pupil (this only works to a point).

Overall image brighter = more visible.

Typically, in the field, the larger scope will be being used at both a higher power AND a larger exit pupil, so people will see faint objects easier and better than in the small scope.

 

But, this breaks down when analyzing small points (stars) in the field.

(we'll consider, for simplicity, only a star on the center axis).

Every star image will be created by the entire aperture.  As a result, a 4x larger aperture will result in a 4x brighter star image because a point is a point.

[I'm not considering the size of the Airy disc in the scope, here].

And brighter = more visible.

So the larger scope will see fainter stars because the brightness has increased.

But, did the contrast between the star and the background increase?

The "noise" in the image--the background sky brightness--will have increased just the same as the star image, so the contrast should remain the same.

 

And yet, the larger scope sees fainter objects and fainter stars.

Our language about it is inexact.  Apparently, the fainter targets are more visible because contrast with the background has somehow improved, so we inexactly say contrast has improved.

We see intellectually that, while objects have increased in brightness, so has the background, so contrast remains the same as in the smaller aperture.

 

So could it be that it is not contrast that is making the fainter stars and objects more visible, but simple brightness?

Let's take the example of a star image 1" in size at magnitude 19.0 in a background sky of brightness magnitude 20.0 per square arc second.

The star is 1 magnitude brighter than the background sky (I'll ignore the contribution of the background in the star image).

Now, increase the scope size by 2.5x.  Each has increased in brightness by a magnitude, and the difference is the same.

If contrast were the only factor, the star might not be any more visible in the larger scope.  And yet, it is more visible.

What has increased?  Brightness.  Size and contrast have not.

But the faint star has suddenly come out of the background at the exact same exit pupil, so surface brightness has not increased.

 

No amount of language can erase the fact that fainter targets, both stars and extended objects, are more visible in larger scopes.

As are colors.  I've seen it too many hundreds of times to disbelieve it.

Yet, if contrast did not improve, some other factor did to make the objects more visible.  And it isn't size, because that doesn't explain the improved visibility of faint stars.

and it isn't exit pupil because at the same exit pupil as in the small scope, fainter stars are visible.

So it must be brightness in terms of photons.

 

Does improving contrast help?  Sure, or nebula filters wouldn't work.  The nebula did not get brighter, yet it became more visible because the background darkened.

The effect is very similar to making the aperture a lot larger.  Hmm.  Could it be the larger aperture somehow improves contrast?

If so, how?  I follow the arguments that it does not, and yet.......

 

The serious question is whether the eye sees only the "contrast ratio", which does not change, or "brightness difference" which is not the same thing.

If the object has a brightness of 2 and the background a brightness of 1, the contrast ratio is 2:1.  Double the brightness in a larger aperture and the object becomes a 4 and the background a 2.

The contrast ratio is the same, 2:1, but the brightness difference has gone from 1 to 2.  Does the eye see the difference rather than the ratio?

 

I frankly don't know, but I do know that the explanation of larger size and larger exit pupil in the larger scope simply doesn't add up to make faint stars more visible in the larger scope.

Or colors more visible at the same exit pupil (indeed, colors are more visible even at SMALLER exit pupils).

Until I read a rational explanation that doesn't defy the evidence of my senses, I am going to say that this is one of those examples where I follow the arguments, but that they contradict what I see.

As Galileo supposedly muttered, "Eppur si muove".

Don,

When you invoke point sources and tinier objects, you're detouring down that road I expressly avoided.

 

In the case of point sources, of course contrast is improved with increasing aperture, or increasing magnification (until the Airy disk is involved.)

 

And small, dim objects below the minimum size for detection in a smaller scope become visible in a larger one in part because they are made to cross this threshold. Yes, total brightness plays a role too, which in a certain regime of size and surface brightness can be ascribed to a contrast change. But this often is more the result of our own visual system than it is the way the telescope works. To understand this, compare images of most any type of DSO taken with a pretty small aperture with visual impressions by even pretty sizeable scopes. Particularly in the realm of low surface brightness, our own visual limitations are most forcibly impressed upon us.

 

Which is why I stressed to limit to more sizeable, detail-less objects well resolved as such by all apertures when conceptualizing things.

 

As I hinted to Jon in my previous post, this treatise is probably better aimed at the relative tyro who might think that a telescope can deliver an image having higher surface brightness than seen by eye alone...


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#19 Redbetter

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Posted 25 June 2018 - 03:52 PM

Organic Astrochemist,

 

You neglected to include the important sentence/paragraph following what you quoted:

 

 

Other things being equal (basic instrument desgn, optical quality, transmission efficiency, control of unwanted light and exit pupil), threshold contrast for visual detection is invariant with aperture. If a 2" scope just permits to perceive [object + sky] surface brightness of, say, 0.07m brighter than sky, a 20" scope will do no better.

 

The preceding naturally assumes that the object in each case is easily larger than the minimum size for detection. And therein lies the key to this myth I'm knocking down.

 

 

 

Again, I don't see that there should be an argument about this.  Seeing a threshold object is perhaps best represented as a three-legged stool:  brightness, contrast, and image scale. 

 

There is an apparent size (image scale) that is required for detection as objects become closer and closer to the background brightness.  That size becomes tens of degrees when the difference is only slightly brighter than the background.  It is smaller if the difference in surface brightness is twice as great, etc.

 

Another factor not in the quoted sentence above, but understood, is that the further one gets above the minimum apparent size, the easier/more certain the detection will be.

 

There are of course various things that complicate matters such as the fact that most DSO's dim towards the edge and may be considerably brighter in the center.  Seeing the extent of an object can require some "margin" of background sky so that we can recognize the contrast. 


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#20 Redbetter

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Posted 25 June 2018 - 04:09 PM

Don,

 

The point sources part actually works against the argument of larger aperture improving contrast.   The limiting magnitude does not increase disproportionately with aperture (area).  Instead it begins to decline because seeing begins to spread point sources and small objects over a greater area.  This both enlarges the size and reduces the slope of the gradient with the background.  Contrast is reduced.   

 

Now if one had perfect seeing this would be less of a factor.  I say "less" rather than not a factor because extreme magnification levels for the same effective exit pupil introduce other problems such as holding the head/eye steady enough.  Breezes, vibration, etc. eventually become a problem as well.  Some vibration/movement is beneficial to detecting the faintest wisps (e.g. tube tapping) but some steadiness is also needed for orientation.

 

The Crumey article has been very helpful to me in appreciating how brightness, contrast, and image scale interact for point sources.  If only the black background cut off used had been accurate. 


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#21 GlennLeDrew

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Posted 25 June 2018 - 04:16 PM

“If a 2" scope just permits to perceive [object + sky] surface brightness of, say, 0.07m brighter than sky, a 20" scope will do no better.”
This is false. The 20” scope will do better by allowing one to perceive objects that are smaller and/or dimmer.
Consider the object at the limit of detection in the 2”. If one increases the magnification to increase the apparent size, the [object + sky] surface brightness dims below the threshold and the object can’t be seen (the sky is black and can’t be dimmed). If one reduces the magnification to increase the surface brightness, the apparent size falls below the threshold and the object can’t be seen.
The narrow range of this optimum magnification will make it difficult to observe such objects in practice.
With a larger scope one will be able to use both lower and higher magnification to see the object, hence increasing the likelihood of actually observing it in practice. Furthermore, objects which are smaller and dimmer would be invisible in the 2” but might be visible in the 20”.

Of course objects that can’t fit in the field of view of the 20” would be better observed in the 2”.

The opening statement of mine you quoted here was *intended*---but likely not clearly enough defined by me--in the context of an object already well enough resolved by the smaller scope, and having no detail to distract, and where both scopes were operating at the same exit pupil.

 

As I endeavored to make clearer in subsequent posts, my intention is to show that *at given exit pupil* a larger aperture cannot bring into visibility a splotch having a smaller magnitude delta w.r.t. to the surrounding sky.

 

To this end I invoke a hypothetical large splotch of featureless nebulosity that all apertures reveal as having a clear areal extent.

 

I specifically forbade to consider small objects for which the detection threshold for size is involved. That's complicating matters with yet another variable, and it distracts from the fundamental concept under discussion.

 

In such navel-gazings ;) as this, the reader tends to immediately try to relate to his own experience, which almost invariably involves the consideration of numerous messy variables which distract and confuse from the one principal phenomenon being isolated. It takes discipline to focus upon a restricted problem involving a hypothetical not normally or commonly encountered.

 

OK. A real life example.

 

Consider the North America nebula. Visually it exhibits what we could characterize as an almost featureless expanse, outside of its shape.

 

We apply an 9X50 finder, which magnifies the nebula's 3 degree height to an apparent 27 degrees. It fills about 1/2 the field by diameter.

 

Now we bring to bear a 27X150 scope, which is operating at the same exit pupil. Is the nebula any easier to see? Or more specifically, does it exhibit a seemingly brighter surface w.r.t. the surrounding sky? It cannot, if other variables are also the same.



#22 Redbetter

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Posted 25 June 2018 - 04:32 PM

In the case of point sources, of course contrast is improved with increasing aperture, or increasing magnification (until the Airy disk is involved.)

 

The crossover point for point source to extended source at the faintest detection limit is difficult to quantify from what I have seen testing smaller apertures.  With sufficiently good seeing I am able to barely detect limiting magnitude stars in known locations at exit pupils that would enlarge the airy disk to about 4 times the exit pupil at which I first detect it.  However, these sometimes don't appear all that "stellar" when pushing the limit.  I don't think they are bright enough at the scale and instead have a ghostly quality, but persistently/consistently in the right position as compared to other areas that don't contain visible stars.  It might even be less than perfect seeing producing the tiny blur at this scale/exit pupil.  Two tenths of a magnitude or so brighter and I still get that faint stellar impression flashing at times.  Two tenths to the other side (dimmer) and things are becoming random or simply unseen. 

 

With averted vision stars what we are likely to see is only the higher intensity portions of the spurious disk rather than where things rapidly fall off the shoulders of the PSF.  This might be 1/2 of the airy disk size or less--whatever is sending enough photons on average within a given time period that is sufficient for the eye/brain to register as some sort of signal.  This is a function of that image scale vs. brightness and contrast that we have been talking about. 



#23 Starman1

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Posted 25 June 2018 - 05:28 PM

You mean "higher intensity portions of the Airy disk", since the Airy disk doesn't shrink, but the spurious disk definitely does.

We always see all of the spurious disc, but its size changes with magnitude, whereas the Airy disk only changes with f/ratio.

 

I wish you had my seeing conditions.  I resolve to the limit of the 12.5" aperture quite often, and a 0.9-1mm exit pupil is usually rock solid.

Stars usually don't bloat at all until I get down to 0.6mm exit pupil or smaller.


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#24 Jon Isaacs

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Posted 25 June 2018 - 06:13 PM

In such navel-gazings ;) as this, the reader tends to immediately try to relate to his own experience, which almost invariably involves the consideration of numerous messy variables which distract and confuse from the one principal phenomenon being isolated.

 

Glenn:

 

There are two principle variables being discussed, aperture and threshold contrast.  

 

"Does increasing aperture improve threshold contrast: Nope"

 

That's a dramatic title and for me, problematic since it is generally under the navel-gazing restrictions of this "spherical cow" you are discussing that it holds true.  There are many situations out in the field where increasing the aperture does allow for low contrast object to be seen in a larger aperture telescope.    

 

Being involved in research one way or the other most of my life, it is my experience that dramatic titles lead to counterexamples and opposition.  As discussion of the subject with the conclusion as a conclusion rather than the title is more likely to get everyone on board. 

 

To me, what is interesting is understanding the various mechanisms that affect the what I see, why I see what I see.  This is indeed a complicated equation of many variables.  

 

So.. rather than stating that increasing aperture does not affect the threshold contrast, research paper might be titled: 

 

"The effect of aperture on threshold contrast:  A conceptual study. "

 

:shrug:

 

Jon 


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#25 Waddensky

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Posted 25 June 2018 - 06:15 PM

And if contrast is not varying, neither can threshold contrast.

Perhaps it's a definition thing, but there's a difference between contrast (the ratio of the surface brightness of an object and the background brightness, or, more precisely, the ratio of the "combined surface brightness and background brightness" and "background brightness") and threshold contrast, which is the ability of our eyes to detect an object. Contrast never changes when the conditions are the same, no matter how large the aperture is. Threshold contrast is a function of angular size and apparent background brightness: an object with a given contrast is detected easier on a brighter background, or with a larger angular size. The reason we can see diffuse objects like galaxies easier using larger apertures is because the background is brighter at increasing magnifications than in a smaller scope at the same magnification (larger exit pupil). The optimum is found when the object is not too small, and the background is not too dark. This is the optimum detection magnification.


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