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Limiting Magnitude of the eye

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#1 FlorinAndrei

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Posted 08 December 2012 - 05:55 PM

To calculate the limiting magnitude of a scope, the Stellafane calculator assumes a limiting magnitude for the eye equal to 8.5

http://stellafane.or...scope-calc.html

This seems to overstate the results a little. E.g. for a 12.5" it estimates the limiting magnitude around 16.4 (after a transmission factor of 70%).

What are your thoughts? Is the formula a bit optimistic?

#2 frito

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Posted 08 December 2012 - 06:27 PM

well probably depends a lot on the individual to be honest and their age. it does sound a bit optimistic to me or at the very least it assumes a person who is younger and has good vision.

i know at public outreach party's we do between my experience, 20/20 vision and young age of 29 years allows me to see things the majority of the public, esp older and eye glass wearing public are unable to see in my telescope. i also know at least one person in our club who wears glasses and is a more experienced observer than myself also has a lot of trouble seeing extremely faint fuzzy's that myself as well as others have seen at the very same time. its a big factor for sure.

#3 BrooksObs

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Posted 08 December 2012 - 06:43 PM

Well, a limiting magnitude of 8.5 for the unaided eye does seem a bit ambitious in my opinion. I can admit to having seen magnitude 7.5-7.6 stars on many occasions under excellent skies myself, but over my own long observing career I have rarely ever encountered anyone willing to actually claim verifiably seeing fainter than about 8.0 with the naked eye and I've been acquainted with some of the best visual observers of the 20th century.

On the other hand, I will admit to having definitely viewed a magnitude 18.6 star from a very reliable visual sequence using a excellent 20-inch Newtonian working at about 300x while at the Texas Star Party near McDonald Observatory about twenty years back...which admittedly far exceeds what the Stellafane forumala seems to call for! So just where does that leave us?

BrooksObs

#4 Starman1

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Posted 08 December 2012 - 06:51 PM

To calculate the limiting magnitude of a scope, the Stellafane calculator assumes a limiting magnitude for the eye equal to 8.5

http://stellafane.or...scope-calc.html

This seems to overstate the results a little. E.g. for a 12.5" it estimates the limiting magnitude around 16.4 (after a transmission factor of 70%).

What are your thoughts? Is the formula a bit optimistic?

Optimistic for NELM, but not for the scope limit. I'll explain:

Magnification, cleanliness of optics, visual acuity, experience, extinction, color index of the star, and several other factors affect the limiting magnitude of a telescope. Without trouble, I regularly see to magnitude 16.8 with a 12.5", and have seen as deep as 17.35. [And a 2-mirror newtonian will have a transmission around 79% if the mirrors are recently cleaned.]

Yet, visual acuity limits my zenithal naked eye limit to 6.8 or so, which wouldn't predict what I see through a scope.
The Stellafane calculator is one of those that does not rely on any vision studies to determine what may or may not be visible.

NELM varies a lot (we got a spread of nearly 2 magnitudes on 20 observers at my site) due to visual acuity.
Limiting magnitudes through a scope have less variation, when all factors are taken into account. Most of the observers at my site got within a few tenths of a magnitude of predicted when using high powers in their scopes.

I recommend this calculator, which is corrected for typical transmissions in various scopes:
http://scopecity.com...-calculator.cfm
If you have enhanced coatings or an odd scope, you can go into the html code for the page and change the number of elements and transmission percentages. Otherwise, if that's beyond you, just use it as is.

It is based on the work of one Larry Bogen who based the calculator on the research of Bradley Schaefer.

#5 BillFerris

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Posted 08 December 2012 - 09:58 PM

Heber Curtis made some naked eye limiting magnitude measurements at Lick Observatory early in the 20th Century and was able to detect some quite faint stars. The photometry available at the time wasn't accurate and he wasn't able to claim more than seeing to 8th magnitude. Some years ago, Curtis' observations were compared against more current, accurate photometry and it was determined he'd seen stars as faint as 8.1, 8.3 and 8.5 magnitude. Though not a common occurrence, seeing that faint is possible.

Bill in Flag

#6 GlennLeDrew

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Posted 08 December 2012 - 11:51 PM

No one limit applies to a telescope if differing exit pupils are used. At higher powers one sees significantly fainter point sources than at low power. Perhaps the eye limit of 8.5m is chosen to represent a 'mean, corrected baseline' only for the purposes of calculation, which should apply to middling exit pupil diameters (of, say, 2.5-3mm?).

But the best way to approach the problem is to first determine your own NELM, from which the telescopic limit can be fairly reliably calculated when the exit pupil diameter equals your eye's pupil. Then an empirical relation between sky darkening with diminishing exit pupil and the concomitant increase in limiting magnitude can be applied.

No one limit can possibly apply to all observers over a range of exit pupils and under varying sky darkness/transparency.

#7 FlorinAndrei

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Posted 09 December 2012 - 01:00 AM

http://scopecity.com/limiting-magnitude-calculator.cfm


Bookmarked, thanks.

At higher powers one sees significantly fainter point sources than at low power.


Is that solely due to the darker background at high magnification?

#8 Starman1

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Posted 09 December 2012 - 01:40 AM

http://scopecity.com/limiting-magnitude-calculator.cfm


Bookmarked, thanks.

At higher powers one sees significantly fainter point sources than at low power.


Is that solely due to the darker background at high magnification?

Slight clarification:
When the Airy disc (well, actually, the spurious disc in the center) is large enough to be resolved (varies, but 25X/inch of aperture is often cited), with increasing magnification after that it behaves like an extended object so no improvement in contrast with the background sky is possible.

So the improvement in contrast with the darkening background only occurs up to a point.

Obviously, seeing conditions and the transparency of the lens in the eye will influence how small an exit pupil can be used.
My own experience indicates I achieve my faintest limiting magnitude with about a 1mm exit pupil in excellent seeing. Others report smaller exit pupils.

Not mentioned in the calculator is coma correction, which can make fainter stars visible by concentrating the light into small points rather than smearing it out radially.

Also not mentioned in the calculator is visual acuity, which has an influence, albeit less intensely at high powers.

Also not mentioned in the calculator is the transparency of the viewer's eye's lens. I suppose it is influenced by the NELM plugged in. I would rather the calculator have had an entry for SQM level, which is objective rather than subjective. If I am observing in magnitude 22.0 skies, and I only see to mag.7 NELM, while another observer sees to mag.8, the calculator implies he will see fainter stars in the scope, and that is not necessarily the case.

Nonetheless, the calculator has worked well for a lot of people here on CN, so it might be on the right track.

#9 BrooksObs

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Posted 09 December 2012 - 09:06 AM

Heber Curtis made some naked eye limiting magnitude measurements at Lick Observatory early in the 20th Century and was able to detect some quite faint stars. The photometry available at the time wasn't accurate and he wasn't able to claim more than seeing to 8th magnitude. Some years ago, Curtis' observations were compared against more current, accurate photometry and it was determined he'd seen stars as faint as 8.1, 8.3 and 8.5 magnitude. Though not a common occurrence, seeing that faint is possible.

Bill in Flag


Indeed, Bill, this supposed accomplishment by Curtis has been cited repeatedly in recent years. However, I would note that there is scant evidence that it has ever been verifiably equaled by any of the best and most experienced visual observers since.

Over many years I've conducted numerous similar experiments in the company of the supposed best eagle-eyed observers of my time (Leslie Peltier, Steve O'Meara, Wayne Lowder, et al.) under excellent skies with quite interesting results. One of the resulting implications has always been that there really aren't any actual individuals with superhuman visual ability, as the observers in question all had virtually the same visual limits in their youth, give or take just a couple of tenths of a magnitude and that limit was essentially +7.8 magnitude . While from time to time one does see some rather extraordinary claims of faint detections reported on Internet forums I have the uncomfortable feeling that averted imagination tends to play a more significant role in visual observing today than ever before. A latent urge for one-upmanship in discussions all too often seems evident.

Now perhaps if one were to observe from some rather unique location, such as atop Mauna Kea, above much of Earth's atmosphere then some addition gain might well be possible. But Curtis wasn't there and to my knowledge he wasn't even recognized as a truly preeminent visual observer in his own time. That to me puts a decided question mark on his limiting magnitude results, especially after reading exactly how he reportedly accomplished his sightings.

But regardless of whether one accepts Curtis' results, or not, employing them as the basis of a limiting magnitude formula seems to me rather silly given that similar results seem essentially impossible today. Just how and why the Stellafane folks ever came to employ them in their formula escapes me.

Considering that the vast majority of experienced observers these days have difficulty exceeding a limiting magnitude of 7.0 even under relatively good skies (although probably no longer truly pristine), I should think that an extreme baseline of 7.5 would seem far more realistic a value to employ in any such formula, especially considering that naked eye limit magnitudes don't necessarily have a direct relationship to instrumental ones! Just my thoughts on the subjects.

BrooksObs

#10 Starman1

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Posted 09 December 2012 - 10:56 AM

Considering that the vast majority of experienced observers these days have difficulty exceeding a limiting magnitude of 7.0 even under relatively good skies (although probably no longer truly pristine), I should think that an extreme baseline of 7.5 would seem far more realistic a value to employ in any such formula, especially considering that naked eye limit magnitudes don't necessarily have a direct relationship to instrumental ones!
BrooksObs

My point exactly.
Visual acuity is all-important when evaluating naked eye limits, but not very important at high powers in a telescope.
What I've seen in a scope backs down to a NELM of about 8+, yet I've never reliably detected any star fainter than 6.8, even in pristine skies. I see stars in the scope as tiny points. I see stars in the sky as multi-point blobs. It's all about exit pupil and acuity at that exit pupil.
We've gotten a spread of nearly 2 magnitudes for NELM among 20 observers participating in an NELM test at my local site (mag.21.4), yet I'm certain that wide a spread wouldn't have been maintained in the scope.

#11 BillFerris

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Posted 09 December 2012 - 02:30 PM

Heber Curtis made some naked eye limiting magnitude measurements at Lick Observatory early in the 20th Century and was able to detect some quite faint stars. The photometry available at the time wasn't accurate and he wasn't able to claim more than seeing to 8th magnitude. Some years ago, Curtis' observations were compared against more current, accurate photometry and it was determined he'd seen stars as faint as 8.1, 8.3 and 8.5 magnitude. Though not a common occurrence, seeing that faint is possible.

Bill in Flag


Indeed, Bill, this supposed accomplishment by Curtis has been cited repeatedly in recent years. However, I would note that there is scant evidence that it has ever been verifiably equaled by any of the best and most experienced visual observers since.


Dave Nash has visually detected multiple mag. 8 stars from Merritt Reservoir, site of the Nebraska Star Party, including one star of 8.15 magnitude. So again, not a common occurrence but naked eye detection of stars to magnitude 8.0 and fainter has been done and is, therefore, possible. So, while naked eye limits among observers with access to pristine skies--they're still out there--typically reside in the magnitude 7.5 +/- 0.3 range, we should not make the mistake of simply tossing the historical record out the window when it comes to discussing upper-most limits of visual acuity.

But regardless of whether one accepts Curtis' results, or not, employing them as the basis of a limiting magnitude formula seems to me rather silly given that similar results seem essentially impossible today. Just how and why the Stellafane folks ever came to employ them in their formula escapes me.


Whether one chooses to accept Curtis' observations is irrelevant. If one has objective evidence that he didn't or couldn't make the observations, that would be another matter. But so far, all that has been offered is skepticism, which is evidence of nothing.

As for the baseline naked eye limiting magnitude assumed in a telescopic limiting magnitude formula, the reasonableness of that figure depends on a couple of factors. One, is how the formula makes adjustments for relevant factors such as age, experience, etc. Another is the proof in the pudding, so to speak. How do the predicted results match real life experience?

Considering that the vast majority of experienced observers these days have difficulty exceeding a limiting magnitude of 7.0 even under relatively good skies (although probably no longer truly pristine), I should think that an extreme baseline of 7.5 would seem far more realistic a value to employ in any such formula, especially considering that naked eye limit magnitudes don't necessarily have a direct relationship to instrumental ones! Just my thoughts on the subjects.


I'm surprised to be reading comments to the effect that there is no relationship between naked eye limiting magnitude and telescopic limiting magnitude. Paricularly, in light of the fact that the telescopic limiting magnitude formulas being recommended in this thread are both built around assumptions about naked eye limiting magnitude.

If you accept that there is a relationship between aperture and limiting magnitude and that the relative abilities of two instrucments to show faint stars will be dependent, at least in part, upon their relative apertures, then you must accept that there is a relationship between naked eye limiting mag and telescopic limiting mag. The human eye is an aperture.

Of course, there is currently no formula capable of predicting either naked eye limiting magnitude or telescopic limiting magnitude for all observers with high precision in each instance. One of the best discussions of this can be found in Bradley Schaefer's 1990 paper, Telescopic Limiting Magnitudes. Figures 2 and 3 are bar graphs illustrating the spread of results in comparison with predicted limiting magnitudes for the 314 participants in Schaefer's study. Figure 3, which corrects for experience, shows that Schaefer's formula predicted results to within 0.5 magnitude for a majority of participants, and to within 1.0 magnitude for the vast majority.

This illustrates both the efficacy of science and the challenge of forecasting human behavior. We know the relevant factors affecting the ability of a human to see faint stars and we understand how to model the effect those factors have. But any time you toss human imperfection and subjectivity into the equation, you're going to get a range of results. This does not disprove the relationship between naked eye limiting magnitude and telescopic limiting magnitude. It confirms the inherent imprecision of the human factor.

Bill in Flag

#12 Starman1

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Posted 09 December 2012 - 02:41 PM

Bill,

Comparing naked eye limiting magnitude with telescopic limiting magnitude, at least in my case, is comparing an astigmatic, slightly-out-of-focus, spherically aberrated star image with a sharp, perfectly-focused, star image containing no aberrations.

The whole-eye aberrations can wipe out a lot of the faintest stars. I see the same limiting naked eye stars at SQM=21.4 as I do at SQM=21.9.

Schaefer's studies, and Bogen's calculator, work well for me telescopically. But I have to back down from SQM readings to plug in a realistic NELM that I can't always see.

I use the German formula
NELM = [(SQM-8.89)/2] + 0.5 and plug that in for NELM

How accurate does that make the calculator? I'm seldom off by more than 0.1-0.2 magnitudes telescopically.

#13 BrooksObs

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Posted 10 December 2012 - 08:27 AM

"Dave Nash has visually detected multiple mag. 8 stars from Merritt Reservoir, site of the Nebraska Star Party, including one star of 8.15 magnitude. So again, not a common occurrence but naked eye detection of stars to magnitude 8.0 and fainter has been done and is, therefore, possible. So, while naked eye limits among observers with access to pristine skies--they're still out there--typically reside in the magnitude 7.5 +/- 0.3 range, we should not make the mistake of simply tossing the historical record out the window when it comes to discussing upper-most limits of visual acuity. -
Bill"

Bill, I cannot speak to Dave Nash's observations as I am unfamiliar with any facet of his observing record, but I must contend that the limiting magnitude determined by H.D.Curtis has no place in establishing any formula for visual limiting magnitude. Specifically, the approach that he employed in his efforts was so unusual and downright foreign to what the typical observer might do to evaluate his NELM that there can be no comparison whatever. I would even go further pointing out that Curtis, himself, states that without his more-or-less laboratory-like experimental setup that the faintest star he could detect with his unaided eye by conventional means was a rather paltry +6.5 magnitude! This 2.0 magnitude difference alone, in my opinion, invalidates Curtis' results from use in any subsequent limiting magnitude formulae.

For any purposed limiting magnitude algorithm to have true value it needs as its basis a reliable real-world mean value representative of what a cross section of the best observers are reporting. Employing a single individual's results as a standard, when those figures are obtained through a decidedly bizarre approach that will never be replicated in the field by observers, I would assign even less than marginal value, or meaning, to.

BrooksObs

#14 blb

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Posted 10 December 2012 - 09:01 AM

...For any purposed limiting magnitude algorithm to have true value it needs as its basis reliable real-world mean values representative of what a cross section of the best observers are reporting...


Even among a group of experanced observers, on the same night and from the same viewing sight, you will get a difference of more than a magnitude. This is due to the differences in visual acuity. There is no standard eyesight. No mater what figure is used, if the results given by the formula match most peoples observations then that is what matters.

#15 BrooksObs

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Posted 10 December 2012 - 09:22 AM

To be quite honest, Buddy, your impression is quite the opposite of what I have found at some gatherings of mainly AAVSO observers over the years, folks whose observing programs center around the seeing very faint stars in the course of their work. As I pointed out previously, the most experienced at such gatherings seem consistantly to see very nearly to the same limiting magnitude when employing just their unaided eyes.

I think that perhaps most of the divergence seen among more general observers, even experienced ones, may be governed by the specific type of observing their eyes are used to doing, not a range in visual acuity, as so many folks believe.

BrooksObs

#16 Starman1

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Posted 10 December 2012 - 11:04 AM

To be quite honest, Buddy, your impression is quite the opposite of what I have found at some gatherings of mainly AAVSO observers over the years, folks whose observing programs center around the seeing very faint stars in the course of their work. As I pointed out previously, the most experienced at such gatherings seem consistently to see very nearly to the same limiting magnitude when employing just their unaided eyes.

I think that perhaps most of the divergence seen among more general observers, even experienced ones, may be governed by the specific type of observing their eyes are used to doing, not a range in visual acuity, as so many folks believe.

BrooksObs

Sorry, but I have first-hand evidence that assertion is false. The 20 individuals who participated in our test of limiting magnitude had a strong correlation between sharpness of vision and the faintest star viewable. The faintest limits in several sections of the sky were always the same individuals, who saw stars as tiny little points with their naked eyes. We did star counts in particular sections of the sky and we did specific stars in specific places to determine who could see the star in question. Our spread, at a site with SQM = 21.4 (about NELM 6.8) ran from magnitude 5 to a little past magnitude 7.
A few individuals had such poor correction with and without glasses that they couldn't reliably detect stars fainter than mag. 4, and we excluded them from the test.
The rest of the individuals were all telescope owners and familiar with the views through different scopes.

I could go along with the idea that a combination of visual acuity (which would include a lack of astigmatism), transparency of the cornea, lens, and humors in the eye, and a density of rods in the retina, coupled with experience, all add up to what an individual can see at the limit.

But, the biggest factor is acuity of vision. I went from barely able to see the mag. 6.2 star near Polaris to being able to reliably see mag. 6.8 when I got really well-corrected distance vision glasses. No difference other than the glasses. But the star images went from multi-point blobs down to slightly flared small dots.

#17 BrooksObs

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Posted 11 December 2012 - 08:58 AM

Don, I would have to say that your annecdote really only serves to confirm the well known fact that individuals suffering some sort of uncorrected visual impairment (and sometimes even when supposedly "corrected") don't see as faint stars as those with normal, or artificially fully corrected, vision do. For the purposes of this discussion when I speak of visual "acuity" in posts upstream I'm referring to the ability to see faint objects, not addressing uncorrected visual defects of the eye. I would also hasten to add that blind star counts only result in determining a minimum upper limit, not the defining of an absolute detection limit cutoff.

In conducting any experiment to determine visual limits the participants most definitely need to have normal visual capabilities, else the testing will be meaningless. Likewise, sky conditions must be very good to excellent. The situations I noted previously all involved individuals (AAVSOers) whose involvement in the hobby regularly, or at least very often, involved the observing of very faint stars so they were well experienced at it.

Further bolstering my previous statements let me relate a further instance in which it was demonstrated that even a very "average" observer, given the proper coaching, can see just about as faint as the supposed best of us.

Some years ago Steve O'Meara and I were at the TSP and had been discussing this very subject. Steve was having a conversation with a young lady (in her 20's) attendee relatively new to the hobby. Although Steve and I were finding the night's NELM a bit beyond 7.5, the lady stated she had never been able to see stars fainter than about 6.2 with the unaided eye and felt that was her ultimate visual limit.

Steve set down next to her and began pointing out star patterns, starting from a given bright star. It was a matter of, "You see delta Lyrae over there? Now look just a little to its right...you see that little star?" This proceeded onward step-by-step in descending order of brightness until the young lady, to her great amazement, found she could consistently detect stars in the 7.2-7.5 range, just like Steve and I.

This situation was hardly unique either, as I've conducted the same sort of "experiment" a number of times at Stellafane over my many years of attendence, always with the same results. The conclusion is that under guidance no one with good normal, or artificially fully corrected, vision ever distinctly exhibited a bright cutoff given excellent skies. The only possible exceptions are those individuals who have significant known visual deficiencies. Seeing very faint stars is thus much more a matter of experience/familiarity in the techniques, together with the proper approach to the experiment.

BrooksObs

#18 Tony Flanders

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Posted 11 December 2012 - 09:12 AM

To be quite honest, Buddy, your impression is quite the opposite of what I have found at some gatherings of mainly AAVSO observers over the years, folks whose observing programs center around the seeing very faint stars in the course of their work. As I pointed out previously, the most experienced at such gatherings seem consistantly to see very nearly to the same limiting magnitude when employing just their unaided eyes.


I strongly suspect that there's a selection effect at play here.

If you take the top ten runners in the world, their speeds will all be very close to each other.

#19 BrooksObs

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Posted 11 December 2012 - 09:22 AM

To be quite honest, Buddy, your impression is quite the opposite of what I have found at some gatherings of mainly AAVSO observers over the years, folks whose observing programs center around the seeing very faint stars in the course of their work. As I pointed out previously, the most experienced at such gatherings seem consistantly to see very nearly to the same limiting magnitude when employing just their unaided eyes.


I strongly suspect that there's a selection effect at play here.

If you take the top ten runners in the world, their speeds will all be very close to each other.


If that be true, Tony, how do you explain the results from the TSP and Stellafane experiments, where in the individuals involved in the tests were simply "average", or even in the TSP instance novice, observers?

BrooksObs

#20 galexand

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Posted 11 December 2012 - 10:03 AM

Quite the debate!

For me the ergonomics of it, and probably what is called here "visual acuity", define the situation. With my glasses off, I can barely see Jupiter naked-eye. With my glasses on, I can see a pretty wide variety of stars, but probably nowhere near as good as some of my friends. But through the telescope, I can choose the right eyepiece and I can play with averted vision and I can move the scope around and I can study one piece of sky for an hour trying to get the details. I can squeeze out at least a whole magnitude over what I saw when I first casually glanced through the EP. I can do all those tricks naked-eye, I guess, but it's not natural for me to use averted vision that way.

So what I'm saying is naked eye I have all these drawbacks keeping me from coming close to any theoretical perfection, but through the telescope I feel like I can get pretty close to ideal, after a fashion anyways. So it makes sense to me that a telescope limiting magnitude calculator might assume perfection even if the naked eye perfection isn't really attainable...

#21 Starman1

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Posted 11 December 2012 - 12:10 PM

Don, I would have to say that your annecdote really only serves to confirm the well known fact that individuals suffering some sort of uncorrected visual impairment (and sometimes even when supposedly "corrected") don't see as faint stars as those with normal, or artificially fully corrected, vision do. For the purposes of this discussion when I speak of visual "acuity" in posts upstream I'm referring to the ability to see faint objects, not addressing uncorrected visual defects of the eye. I would also hasten to add that blind star counts only result in determining a minimum upper limit, not the defining of an absolute detection limit cutoff.

In conducting any experiment to determine visual limits the participants most definitely need to have normal visual capabilities, else the testing will be meaningless. Likewise, sky conditions must be very good to excellent. The situations I noted previously all involved individuals (AAVSOers) whose involvement in the hobby regularly, or at least very often, involved the observing of very faint stars so they were well experienced at it.

Further bolstering my previous statements let me relate a further instance in which it was demonstrated that even a very "average" observer, given the proper coaching, can see just about as faint as the supposed best of us.

Some years ago Steve O'Meara and I were at the TSP and had been discussing this very subject. Steve was having a conversation with a young lady (in her 20's) attendee relatively new to the hobby. Although Steve and I were finding the night's NELM a bit beyond 7.5, the lady stated she had never been able to see stars fainter than about 6.2 with the unaided eye and felt that was her ultimate visual limit.

Steve set down next to her and began pointing out star patterns, starting from a given bright star. It was a matter of, "You see delta Lyrae over there? Now look just a little to its right...you see that little star?" This proceeded onward step-by-step in descending order of brightness until the young lady, to her great amazement, found she could consistently detect stars in the 7.2-7.5 range, just like Steve and I.

This situation was hardly unique either, as I've conducted the same sort of "experiment" a number of times at Stellafane over my many years of attendence, always with the same results. The conclusion is that under guidance no one with good normal, or artificially fully corrected, vision ever distinctly exhibited a bright cutoff given excellent skies. The only possible exceptions are those individuals who have significant known visual deficiencies. Seeing very faint stars is thus much more a matter of experience/familiarity in the techniques, together with the proper approach to the experiment.

BrooksObs

Well, then, over 90% of us have some form of visual deficiencies, meaning that any calculator that starts with the assumption that the limit of human seeing is held in common between any two individuals is doomed to failure.

Yet, the calculators work. And they seem to predict well the limit that can be seen through a telescope.

I am an advocate of getting away from naked eye tests of limiting magnitude. An SQM says a lot more to me about the quality of someone's sky than their estimate of NELM.

#22 Starman1

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Posted 11 December 2012 - 12:11 PM

Quite the debate!

For me the ergonomics of it, and probably what is called here "visual acuity", define the situation. With my glasses off, I can barely see Jupiter naked-eye. With my glasses on, I can see a pretty wide variety of stars, but probably nowhere near as good as some of my friends. But through the telescope, I can choose the right eyepiece and I can play with averted vision and I can move the scope around and I can study one piece of sky for an hour trying to get the details. I can squeeze out at least a whole magnitude over what I saw when I first casually glanced through the EP. I can do all those tricks naked-eye, I guess, but it's not natural for me to use averted vision that way.

So what I'm saying is naked eye I have all these drawbacks keeping me from coming close to any theoretical perfection, but through the telescope I feel like I can get pretty close to ideal, after a fashion anyways. So it makes sense to me that a telescope limiting magnitude calculator might assume perfection even if the naked eye perfection isn't really attainable...


My point exactly.

#23 BillFerris

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Posted 11 December 2012 - 04:10 PM

Steve set down next to her and began pointing out star patterns, starting from a given bright star. It was a matter of, "You see delta Lyrae over there? Now look just a little to its right...you see that little star?" This proceeded onward step-by-step in descending order of brightness until the young lady, to her great amazement, found she could consistently detect stars in the 7.2-7.5 range, just like Steve and I.


Brian Skiff has done the same on several occasions at Texas Star Party and his experience is that observers didn't start having problems detecting faint stars until they got close to Vmag = 8.0. This, the Dave Nash and Heber Curtis observations demonstrate it is possible to see stars to a Vmag of 8.0 and fainter.

Until about 10 years ago when the Hipparcos/Tycho photometry was published, most observers didn't have access to accurate photometry for stars in the 7-8 magnitude range. Traditional catalogs were off by as much as 0.5 magnitude. You may already have done so but, if not, it might be worth the effort to look up the modern photometry on the faint stars you've used to determine a limiting visual magnitude.

Bill in Flag

#24 David Knisely

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Posted 11 December 2012 - 04:48 PM

BrooksObs wrote:

Bill, I cannot speak to Dave Nash's observations as I am unfamiliar with any facet of his observing record,


Well, I can speak for Dave Nash's observations, as I was set up about 20 feet from him when he was doing them. It was at the 2nd annual Nebraska Star Party high in the Sandhills of northwestern Nebraska (3100 ft elevation and over 90 miles from the nearest large town (North Platte, pop. 24,600). He did his estimates as a sort of blind test by just sketching an area and noting which stars he saw in the area of the head of Draco. He later sent the star ID's to Brian Skiff at Lowell Observatory to ask him what precise magnitudes they were. It turned out that one of the ones Dave saw was magnitude 8.15 (and Dave wasn't wearing glasses). Since then, I have used the newer photometric data to see how faint I have been seeing at NSP. At NSP-2, I hit 7.8, but since then, I haven't quite gotten that faint. One night at a much later NSP, we were taking a break, so while I was sitting back in my lawn chair with a friend of mine, I looked up at the head of Draco and began to see how faint I was going. Once we finished our break, I went over to my laptop and fired it up to get the photometric data on the stars I had seen. It turned out that was seeing a number of stars fainter than 7th magnitude, and one of them was magnitude 7.60 (HD 162131), so I considered it a fairly typical 'decent' night for NSP and went back to using my scope. I wasn't too concerned about pushing the very limits of naked-eye observing, but this quick experiment confirmed once again that the old magnitude 6.5 "limit" for unaided eye work is definitely overly conservative, especially at a true dark sky site. Clear skies to you.

#25 Tony Flanders

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Posted 11 December 2012 - 04:49 PM

Brian Skiff has done the same on several occasions at Texas Star Party and his experience is that observers didn't start having problems detecting faint stars until they got close to Vmag = 8.0.


For what it's worth, I've never seen a star significantly fainter than magnitude 7.0 -- and it's certainly not for lack of trying!

Moreover, and rather distressingly, even when I have seen stars around (say) mag 7.1 or 7.2, there are also numerous considerably brighter stars in the same part of the sky that I try and fail to see. In other words, my cutoff is quite ragged. I haven't been able to correlate it clearly with star color, which might well affect a star's visibility.

It's also interesting that my NELM is only about 0.5 mag better at a truly dark site than at my country home, which has blatant light pollution and SQM measurements around 21.2 at the zenith. Light pollution has much less effect on my ability to see faint stars than on my ability to see faint nebulosity.






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