Limiting Magnitude in Binoculars
edz
"star magnitude and binoculars". This link goes back to the article,
which includes links to many sites and formula that relate to this
topic.
A great deal concerning the subject Limiting Magnitude in Binoculars
has been published by various noted individuals addressing theoretical
LM values that might be reached. Based on previous studies I
performed in July 2002 and again in winter 2003 relative to what can
be seen in binoculars and the influence of magnification and aperture
on various star fields, I questioned some of the results that were
being predicted. After many nights of recording field notes testing
binocular performance, I was not convinced these limiting magnitude
predictive formula were truly representative of results that could be
achieved in the field, at least not under all conditions.
Based on the questions remaining in my mind after this discussion, I
set out to find some answers. But without the proper data and
analysis, I could not clearly see where the differences might be. It
took a lot longer and a lot more work than I thought before I was
convinced I had enough information to answer these questions for
myself.
After the collection of the field data, it took considerable
additional effort to sort it all out and make sense of it. The end
result will soon be a published article on CN addressing Limiting
Magnitude in Binoculars.
Based on testing eight binoculars on many different nights
representing a range of conditions, this is some of what I found:
Binocular Limiting Magnitudes for a given size aperture are
significantly less, nearly one full magnitude lower, than a scope of
equal aperture. This is due, among other reasons, to the inability of
the aperture in binoculars to reach full potential because of low
magnifications in use.
Two-eyed viewing vs. one-eyed viewing contributes only a small
fractional gain in magnitude. There is no 40% gain realized because
you have two apertures of the same size versus a similar sized scope.
gain may be more like 15% to 20%.
Naked Eye Limiting Magnitude does not act linearly on Binocular
Limiting Magnitude. BLM does not increase in step equally as NELM
increases. For the tested range with a variance of 1.5+ mag NELM,
Binocular Limiting Magnitude varied by less than 0.5 mag.
When binocular magnification and binocular aperture are each tested
separately, for various sizes and powers of binoculars, magnification
produces results about twice what Carlin's formula predicts and
aperture produces results about half of what Carlin's formula
predicts.
When binocular magnification and binocular aperture are each tested
separately, by incremental changes in magnification and aperture, it
is found for each equal increment that magnification has approximately
three to four times the influence as aperture on increases in limiting
magnitude.
In binoculars much more limiting magnitude gain is realized from
increases in magnification than from aperture. This is also related
to the fact that aperture is under-utilized in binoculars. Unless
optimum magnification is employed, the abilities of the aperture to
put an image in the focal plane are never fully delivered to the eye.
Based on my results, for commonly used binocular magnifications in mag
6.5+ skies, I approximate the maximum limiting magnitude for a 100mm
binocular at mag 12.0, for a 60mm binocular at mag 11.0 and for a 40mm
binocular about mag 10.0. For mag 5.0 skies, all limits are about
0.5 mag lower.
The ultimate limiting magnitude reached for any given aperture is
significantly dependant on the magnification in use.
The full article that has been submitted should be available within
the next week or two.
edz
Victor
> The end
> result will soon be a published article on CN addressing Limiting
> Magnitude in Binoculars.
Pardon me, but "CN" has no meaning to me. Is it a journal, a website or
a magazine?
> Based on my results, for commonly used binocular magnifications in mag
> 6.5+ skies, I approximate the maximum limiting magnitude for a 100mm
> binocular at mag 12.0, for a 60mm binocular at mag 11.0 and for a 40mm
> binocular about mag 10.0.
Am I correct to assume that 50mm binoculars were not included in your
study? If they were, were their performance closer to the 60mm or to
the 40mm instruments? Halfway?
Maybe I should just upgrade from a 50mm to a 60mm and save the money I
would have spent on a 100mm for something else, like a decent focusser
for my Dob! The mag 1 improvement from a 60mm to a 100mm instrument
makes one feel the extra weight and money is not worth it.
> The full article that has been submitted should be available within
> the next week or two.
I do look forward to your findings!
Jon Isaacs
>Pardon me, but "CN" has no meaning to me. Is it a journal, a website or
>a magazine?
>
The mag 1 improvement from a 60mm to a 100mm instrument
>makes one feel the extra weight and money is not worth it.
1 magnitude improvement, a factor of 2.5, is about what one would expect when
moving from a 6 inch to a 10 inch telescope, proportionally the same change,
obviously a significant increase in capability. In my experience certainly
worth the effort and expense.
Note that going from 40mm binos to 50mm binos is a only a 1/2 magintude
improvement mathamatically but again, the difference is obvious.
jon isaacs
Victor
Oh yes of course! It should be compulsory reading for everyone on this
newsgroup.
>> The mag 1 improvement from a 60mm to a 100mm instrument
>> makes one feel the extra weight and money is not worth it.
>
>
> 1 magnitude improvement, a factor of 2.5, is about what one would expect when
> moving from a 6 inch to a 10 inch telescope, proportionally the same change,
> obviously a significant increase in capability. In my experience certainly
> worth the effort and expense.
Yes, I see what you mean. The difference from a 6" to a 10" puts it all
in perspective.
The 100mm is back on my to-buy list and a 60mm Pentax PCF WP is
tempting, but I must be strong! ;-)
edz
>
> Pardon me, but "CN" has no meaning to me. Is it a journal, a website or
> a magazine?
>
> study? If they were, were their performance closer to the 60mm or to
> the 40mm instruments? Halfway?
>
CN is CloudyNights.com, an astronomy website for publishing articles
and equipment reviews. Hundreds of excellent articles there.
I included 10x50s and 12x50s in these tests. I used eight binoculars
from 8x42 up to 20x80. I tested all my binoculars above 50mm also
masked down to 50mm. So, in effect, I've included results for six
differnt 50mm binoculars.
My 60mm binocular was a 16x60. The 10x50 is closer to 8x42. The
12x50 is closer to 16x60. The 12x50 is more difficult to hand hold
than the 10x.
A 10x50 sees 50% more stars than a 8x42. A 16x60 sees nearly twice as
many stars as a 10x50.
Each jump in magnification from 8x to 10x or from 10x to 12x or from
12x to 16x produces far more significant increases in limiting
magnitude than the jumps from 42mm to 50mm or from 50mm to 60mm.
edz
edz
> > 1 magnitude improvement, a factor of 2.5, is about what one would expect >
The 100mm is back on my to-buy list and a 60mm Pentax PCF WP is
> tempting, but I must be strong! ;-)
The jump from 60mm to 100mm aperture will provide only about 0.3 to
0.4 LM gain.
The jump from 10x to 16x and likewise the jump from 16x to 25x will
provide 0.4 to 0.6 LM gain each.
You will gain more from the magnification than you will from the
aperture. You don't need 100mm lenses in the binoculars to get the
gain.
edz
Theo Ker
from a 20x60 oberwerk to a 20x100 miyauchi fluo? Nearly none???? Hard
to believe!
TK
ezar...@gilbaneco.com (edz) wrote in message news:<90c65221.03100...@posting.google.com>...
edz
> So if that is true, what would be the difference for me when switching
> from a 20x60 oberwerk to a 20x100 miyauchi fluo? Nearly none???? Hard
> to believe!
>
> TK
Theo,
Well first it would be necessary to isolate the influence and obvious
benefit of jumping to flourite lenses. The benefits due to increased
contrast and reduced aberations could be substantial.
Another important aspect of this comparison would be to clarify, are
those the pre-2003 Oberwerk 20x60 or the new 2003 version with
significantly improved coatings. I've already written a short paper
on the differences in Oberwerk coatings old vs. new and have hours of
testing to prove the substantial benefit. There is about a 0.3 mag
difference between the coatings in the 15x70 size old. vs new. The
old 15x70s perform at a level just below Pentax 16x60s. the new 15x70
are getting closer to performance (but not quality) provided by
Fujinon 16x70s.
So, looking at the pieces you would get 0.3 mag gain over older
version of coatings to newer, 0.3 to 0.4 gain from 60mm to 100mm and
I'm guessing maybe another 0.3 gain by going to flourite. That's a
total of 1 full mag gain. That would be outstanding.
To get that kind of gain from any other changes, you would have to
increase magnification in 10% increarments by a factor of 6 or 7,
or have skies imprve by a difference of at least mag2 in Naked Eye Lim
Mag.
If you've got the newer version of the Oberwerk 20x60, your gain would
be more on the order of 0.7 mag. Even an increase of 0.7 mag would
allow you to see more than twice as many stars in an equivalent fov.
Maybe you thought at first glance that the data doesn't support your
personal experience, but I think it supports it very well. Of course,
you haven't stated what Lim mag you've reached with either binocular.
I have answered arguments already from several individuals on differnt
forums that each have posed a very unequal situation as basis of
argument. When you take the problem apart and look at all the pieces
the answer is always right there.
edz
Shneor Sherman
The views through my 25x100 Burgess Binos are a drasmatic improvement
over the views through my 26x70 Kronos. I thing the purported
relationship between aperture and magnification relative to limiting
magnitude is non-linear, and as aperture increases, it assumes greater
significance. As most of the relationship has been established with
relatively small aperture binoculars, the existing formula makes sense
over that restricted range.
Next month, if the weather cooperates, comparisons by a number of
experienced observers will be done to measure the effectiveness of 22"
binoculars versus a 30". This is scheduled for October 25, and I
expect to report on the results within a couple of days of the event.
Clear skies,
Shneor Sherman
Tony Flanders
> Based on testing eight binoculars on many different nights
> representing a range of conditions, this is some of what I found:
>
> Binocular Limiting Magnitudes for a given size aperture are
> significantly less, nearly one full magnitude lower, than a scope of
> equal aperture.
Yes, certainly. I would have thought that the difference would be
more than one magnitude. Magnification helps an awful lot, especially
moving from ultra-large exit pupils like 5mm or 7mm down to respectable
exit pupils like 1mm or 2mm.
> Two-eyed viewing vs. one-eyed viewing contributes only a small
> fractional gain in magnitude.
Yes, that is my experience too. But small is not nothing, and
also the effect may be greater for diffuse objects than for stars.
> Naked Eye Limiting Magnitude does not act linearly on Binocular
> Limiting Magnitude. BLM does not increase in step equally as NELM
> increases. For the tested range with a variance of 1.5+ mag NELM,
> Binocular Limiting Magnitude varied by less than 0.5 mag.
That surprises me immensely, although I cannot claim to have studied
the subject rigorously. I have certainly found the limiting mag in
telescopes to track the NELM modestly well, much better than 0.5 for
1.5, and one would expect binoculars to track it even better. It is
hard to imagine a theoretical explanation for this result.
> When binocular magnification and binocular aperture are each tested
> separately, by incremental changes in magnification and aperture, it
> is found for each equal increment that magnification has approximately
> three to four times the influence as aperture on increases in limiting
> magnitude.
Again, this surprises me greatly, although I suppose it might depend
what range of mag and aperture you tested. I am generally very
sceptical about the results that you report, but I eagerly await
the details.
- Tony Flanders
edz
> The views through my 25x100 Burgess Binos are a drasmatic improvement
> over the views through my 26x70 Kronos. I thing the purported
> relationship between aperture and magnification relative to limiting
> magnitude is non-linear, and as aperture increases, it assumes greater
> significance.
None of the available formula provided by Sidwick, Schaefer or Carlin
would support that. Nor does the results of my testing. All predict
linear gain with incremental increase.
But then again, some of what I'm saying is not supported by existing
formula either!
As most of the relationship has been established with
> relatively small aperture binoculars, the existing formula makes sense
> over that restricted range.
35mm to 80mm is not necessarily a small range. It might be considered
as such if you have the following at your disposal.
>
> Next month, if the weather cooperates, comparisons by a number of
> experienced observers will be done to measure the effectiveness of 22"
> binoculars versus a 30". This is scheduled for October 25, and I
> expect to report on the results within a couple of days of the event.
22inch? and 30 inch? binoculars??? What will you test on. Will you
attempt masking tests to incrementally reduce aperture and observe
effect? Will you attempt numerous magnifications at the same aperture
and record differences?
AAnd will you be able to test same magnifications and apertures on
several occasions of differing sky conditions? That should provide an
interesting set of data. I would imagine you could vary
magnifications all the way from minimum to optimum without varying
aperture. That should either support or disput what I'm stating.
edz
edz
> > Binocular Limiting Magnitudes for a given size aperture are
> > significantly less, nearly one full magnitude lower, than a scope of
> > equal aperture.
>
> Yes, certainly. I would have thought that the difference would be
> more than one magnitude. Magnification helps an awful lot, especially
> moving from ultra-large exit pupils like 5mm or 7mm down to respectable
> exit pupils like 1mm or 2mm.
The results of this test simply show support for what Sidgwick stated
and what Schaefer showed us ten+ years ago in his graphic
representation. Increases in magnification provide for significantly
better results in a given aperture.
> > Two-eyed viewing vs. one-eyed viewing contributes only a small
> > fractional gain in magnitude.
>
> Yes, that is my experience too. But small is not nothing, and
> also the effect may be greater for diffuse objects than for stars.
Barry Simon and I have both performed several tests to try and put a
value on this. I agree, there would appear to be more improvement in
brightness than in magnitude limit.
>
> > Naked Eye Limiting Magnitude does not act linearly on Binocular
> > Limiting Magnitude. BLM does not increase in step equally as NELM
> > increases. For the tested range with a variance of 1.5+ mag NELM,
> > Binocular Limiting Magnitude varied by less than 0.5 mag.
>
> That surprises me immensely, although I cannot claim to have studied
> the subject rigorously. I have certainly found the limiting mag in
> telescopes to track the NELM modestly well, much better than 0.5 for
> 1.5, and one would expect binoculars to track it even better. It is
> hard to imagine a theoretical explanation for this result.
This was a difficult measurement. I felt that NELM determination (at
lower values i.e., 4.0, 4.2, 4.5, but not at higher 5.6, 5.8, 6.0) was
so unreliable that I reported a more conservative variance. My
notebooks show wider variances than I reported here. I reduced my
spreads of NELM and increased my variance of LM. This has the affect
of making the overall differences even smaller.
I have no theoretical explanation either, only actual field data to
support it. But I propose this; What if the binoculars, operating at
4D to 6D, and not at optimum of 24D to 30D, are operating so far below
optimum they could never realize the full potential of not only the
aperture but also because they do not utilize the full potential of
the aperture they also do not realize the full affects of NELM.
>
> > When binocular magnification and binocular aperture are each tested
> > separately, by incremental changes in magnification and aperture, it
> > is found for each equal increment that magnification has approximately
> > three to four times the influence as aperture on increases in limiting
> > magnitude.
>
> Again, this surprises me greatly, although I suppose it might depend
> what range of mag and aperture you tested.
I believe this can be explained entirely by the fact that operating at
4D to 6D, and not at optimum of 24D to 30D, they are operating (with
the exception of brightness) so far below optimum they could never
realize the full potential of the aperture. Aperture is so
underutilized that there is so much more potential for improvement
from magnification increases. When the two are isolated from each
other it seems to become readily apparent. This is the third time in
the past 18 months I have tested and published similar results. It is
the first time I have measured the limiting magnitudes to put a real
value on it.
I am generally very
> sceptical about the results that you report, but I eagerly await
> the details.
>
> - Tony Flanders
I hope you find the documentation supports the claim.
edz
Daniel A. Mitchell
number of years back at Astrofest (two years running?) when somebody
brought a big pair of 14" or 16" dobsonian binoculars. LOTS of people
wanted to look through them, but every time I, and many others, went
over to have a peek, they were having collimation problems, or some
such. The thing was HUGE.
Moving and setting up (IF they are portable), and even just using 20" or
30" binos must be a BEAR! Gad, though, once they are working the view
must be fabulous!
My two cents worth: I have both 20 X 80 and 20 X 125 binoculars. Without
getting quantitative, the difference in image quality and brightness is
remarkably in favor to the bigger binos.
This year at astrofest I got to look through both Jim's Mobile (JMI) new
reflex (reflector) binoculars ... the 6" and the 10" versions. The 6"
(now available) was working well, and was not hard to use. The view was
wonderful, with M-13 resolving VERY nicely (at perhaps 50X). It was
noticeably better in this regard than my 5" conventional binos (they
resolve M-13 only with difficulty ... mostly due to only 20
magnifications). The JMI 10" was having problems maintaining collimation
(it's still a prototype, a "work in progress"). The views through it
were transitory, but promising.
Dan Mitchell
==========
Shneor Sherman
22" binoculars vs. a 30" monocular. Testing will be on various DSOs:
limiting magnitude comparison, detail, contrast, etc. Sky condition
will be the same for both scopes, obviously, and good to very good
conditions are expected at that site at that time of the year;
elevation is approximately 4,000 feet with dark skies and little
skyglow, especially after midnight. Will try to obtain the same
magnification set for each scope per object. No attempts will be made
to reduce aperture. I'm not doing the testing, at best, I'll be one of
the observers. But I expect to spend most of my time with my own 22"
(monocular).
Clear skies,
Shneor Sherman
edz
> limiting magnitude comparison, detail, contrast, etc. Sky condition
> will be the same for both scopes, obviously, and good to very good
> conditions are expected at that site at that time of the year;
> elevation is approximately 4,000 feet with dark skies and little
> skyglow, especially after midnight. Will try to obtain the same
> magnification set for each scope per object. No attempts will be made
> to reduce aperture. I'm not doing the testing, at best, I'll be one of
> the observers. But I expect to spend most of my time with my own 22"
> (monocular).
> Clear skies,
> Shneor Sherman
I take it this 22" binoculars is manufactured to diffraction limited
specs, unlike most all other except a few models of binoculars?
edz
edz
> My two cents worth: I have both 20 X 80 and 20 X 125 binoculars. Without
> getting quantitative, the difference in image quality and brightness is
> remarkably in favor to the bigger binos.
>
Dan,
I wouldn't doubt for a moment that the views through 125s are brighter
with better image quality. That's the general basis that binoculars
are produced for. But this is not comparing brightness or image
quality.
Based on my results I would predict the gain in limiting magnitude
from 80 to 125 is only about 0.3 to 0.4 mag. If there is any
difference in quality (optics, coatings, baffles) there may be
additional gain attributed to that. Try it and see. When the article
comes out, there will be a chart with 50-60 stars labeled to mag12.
You'll have an opportunity to see if what I say holds up, or to see if
you get different results. Be prepared to expend considerable effort
to capture mag 11.5-12 stars, if you can even reach that far. My
guess is 5 to 10 minutes each, and probably averted only.
thanks
edz
edz
> Some distinction should be made in this thread regarding the effect of aperature on the limiting magnitude of stellar point objects,
> which I believe is what edz is mostly referring to with respect to limiting magnitude, and extended objects, like emission nebulae,
> globular clusters and some open clusters, which may be more sensitive to the increased light grasp from increased aperature. I am
> waiting to read ed's more detailed article on Cloudy Nights before forming an impression.
>
I'm expecting that distintion will continue to be blurred, since the
attributes of brightness and image clarity are what people most
perceive as increased performance in binoculars. This and comparisons
between unequal quality instruments have been the most prevalent
argument on every forum. It will be difficult for many to accept this
even if it cannot be shown incorrect, because it is not what we have
been led to believe.
Having yourself taken part in the last discussion, you know the
prevailing thoughts on predictive formulae. They are not questioned,
just manipulated for best fit. This may be the beginning of true
field tests that will either corroborate or dispute existing formula.
You all need to keep in mind this entire study is absent of any tests
relative to diffuse extended objects. That is not the point.
thanks
edz
edz
> 22" binoculars vs. a 30" monocular. Testing will be on various DSOs:
> limiting magnitude comparison, detail, contrast, etc. Sky condition
> will be the same for both scopes, obviously, and good to very good
> conditions are expected at that site at that time of the year;
> elevation is approximately 4,000 feet with dark skies and little
> skyglow, especially after midnight. Will try to obtain the same
> magnification set for each scope per object. No attempts will be made
> to reduce aperture. I'm not doing the testing, at best, I'll be one of
> the observers. But I expect to spend most of my time with my own 22"
> (monocular).
> Clear skies,
> Shneor Sherman
This would be the perfect opportunity to attempt capturing all manner
of data.
By keeping magnification constant and masking aperture to various
sizes, you could capture all the data needed to plot an aperture
influence curve. it would keep all other aspects of instrument
performance and quality constant. I can't think of any better way to
get realistic data.
By keeping aperture constant and recording limits at any number of
magnifications, you could plot a magnification influence curve.
The only possible way to get the necessary data to determine influence
of NELM is to repeat the same tests on the same objects at the same
apertures and magnifications on nights of different conditions.
Without that you cannot determine the influence of NELM.
I suspect that this particular instrument is one of superior quality,
diffraction limited, and not in any way comparable to products on the
common binocular market. This is probably a mirror binocular without
some of the light losses common in usual binoculars. Do you think any
results obtained with this instrument would even be relevant to this
discussion? Would the results of such an instrument be better
compared to the diffraction limited specifications of higher quality
telescopes?
No of that makes it any less interesting what results might be
achievable with such an intrument!
thanks
edz
Tony Flanders
> I have no theoretical explanation either, only actual field data to
> support it. But I propose this; What if the binoculars, operating at
> 4D to 6D, and not at optimum of 24D to 30D, are operating so far below
> optimum they could never realize the full potential of not only the
> aperture but also because they do not utilize the full potential of
> the aperture they also do not realize the full affects of NELM.
I assume that by 4D, 6D, etc. you mean 4X, 6X, etc per inch of aperture,
i.e. exit pupils of 6mm or 4mm being less optimal than 1mm.
But that's just the problem! No matter how bad the exit pupil of
binoculars may be, it can never be worse than the naked eye. So
if binoculars do poorly for that reason, the naked eye should do
even worse.
That's exactly why this result is so hard to stomach -- low-power
binoculars are the closest possible mimic of naked-eye observing.
- Tony Flanders
Shneor Sherman
Correct, I think they are Swayze optics.
Shneor
Shneor Sherman
Clearly, this is a mirror binocular. The results will hopefully
demonstrate how much the processing using input from both eyes by the
brain' optical cortex affects visual perception, especially in very
low light conditions - e.g., faint galaxies, Abell clusters, and the
like. That is the major purpose for using binoculars.
The eye's rods are sensitive to a single photon - but the brain
interprets one photon as noise; it takes two for the brain to treat it
as information. This way, each eye will get a photon (as a minima).
Clear skies,
Shneor Sherman
edz
> I assume that by 4D, 6D, etc. you mean 4X, 6X, etc per inch of aperture,
> i.e. exit pupils of 6mm or 4mm being less optimal than 1mm.
that's correct.
> But that's just the problem! No matter how bad the exit pupil of
> binoculars may be, it can never be worse than the naked eye. So
> if binoculars do poorly for that reason, the naked eye should do
> even worse.
Didn't you answer you own supposition here. There is obvious gain
from the binocular over the naked eye, by every attribute. There is
every indication that binoculars show gain. Certainly they do better
than the eye in every respect ( don't interject brightness here, it's
not the point of this analysis). The point is how much gain.
>low-power binoculars are the closest possible mimic of naked-eye
observing.
Maybe it would help to think of this in loss rather than gain. if I
can see mag 11.0 stars in 6.0 NELM skies, it may be possible that
because i am observing point sources, I may not lose a full two
magnitudes of point source visibility if skies drop to NELM 4.0.
Certainly that is what was observed in the field. (On the contrary as
far as the brightness issue goes, I would bet if skies dropped from
NELM 6.0 down to 4.0, I would lose sight of most every faint extended
object, potentially indicating more than a 2 mag drop, but I did not
study the brightness of extended objects). It's possible that NELM
does not have a linear affect on point sources. I do not have a
difinitive answer for this. I only know it to be accurately
represented by field data.
edz
PrisNo6
"edz" <ezar...@gilbaneco.com> wrote in message news:90c65221.03100...@posting.google.com...
> "PrisNo6" <fish...@csolutions.net> wrote in message news:<3f7c77b8$0$85022$3a2e...@news.newshosting.com>...
<snip>
Tony Flanders
> I may not lose a full two magnitudes of point source visibility
> if skies drop to [from NELM 6.0 to] NELM 4.0...
> It's possible that NELM does not have a linear affect on point sources.
No, no, no, no, no!
It is *not* possible that NELM has a non-linear effect on point sources,
because NELM is *defined* in terms of point sources. You are talking
as though NELM were some objective phenomenon; it is not! NELM *is*
the visibility of point sources, as seen to the naked eye. Or, if you
prefer to think of it that way, through 1x10 binoculars.
Sky brightness and extinction are objective and objectively measurable
phenomena; NELM is not. If you tell me that under sky A you can see
mag 11 stars through your binoculars and mag 6 stars with the unaided
eye, but under sky B you can see mag 10.5 stars through binoculars
and mag 4 stars with the unaided eye, my first question will not be
with respect to the binocular observation but rather with respect
to the naked-eye observation. What is it that prevents you from
seeing mag 5.5 stars with your unaided eye, as theory suggests that
you should?
Were you, perhance, using star clusters to measure the limiting
magnitudes of your binoculars? If so, I have a suggestion about
why your results might be biased.
- Tony Flanders
edz
>
> > Naked Eye Limiting Magnitude does not act linearly on Binocular
> > Limiting Magnitude. BLM does not increase in step equally as NELM
> > increases. For the tested range with a variance of 1.5+ mag NELM,
> > Binocular Limiting Magnitude varied by less than 0.5 mag.
>
> That surprises me immensely, although I cannot claim to have studied
> the subject rigorously. I have certainly found the limiting mag in
> telescopes to track the NELM modestly well, much better than 0.5 for
> 1.5, and one would expect binoculars to track it even better. It is
> hard to imagine a theoretical explanation for this result.
>
Tony, I support my claim very well with actual data and reference to
others who have found the same. You may want to do a little rersearch
on this subject to determine if you still disagree. Quotes from my
article;
"A review of the article "Telescopic Limiting Magnitudes" by Bradley
Schaefer"..."referencing Schaeffer's article, he shows clearly that as
NELM drops from 6.0 to 4.0, for magnifications at 100x, telescopic
limiting magnitude would drop only 0.3mag for a 2" lens and 0.7mag for
a 5" lens. As aperture increases, the instrument experiences a
greater affect from NELM. The entire 2.0 mag drop in NELM would not
be realized in the instrument until up in the range of 15" to 20"
aperture."..."Field results indicate, and Schaeffer's article
supports, the optical limiting magnitude does not vary equally as NELM
varies."
Schaefer, Bradley E., 1989, Telescopic Limiting Magnitudes, A.S.P.
102:212-229, February 1990, http://www.astrosociety.org/index.html
I printed out a copy of this article years ago. I could not access it
currently at the ASP site. This article is the subject of great
in-depth discussion by Carlin (his discussion also referenced in my
article). Based on that I'm quite surprised that the non-linearity of
the affect of NELM was not brought up as an very important determinant
in any recent discussions on this BLM subject. I am also quite
surprised that it does not appear to be addressed in predictive
formula. Perhaps it has and I missed it, but I don't think so. I'm
very happy to have my results falling in step with the claims made by
Bradley Schaefer more than 10 years ago!
edz
Paul Schlyter
Tony Flanders <tony_f...@yahoo.com> wrote:
> That's exactly why this result is so hard to stomach -- low-power
> binoculars are the closest possible mimic of naked-eye observing.
viewing?
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stockholm dot bostream dot se
WWW: http://www.stjarnhimlen.se/
http://home.tiscali.se/pausch/
edz
> If you tell me that under sky A you can see....
> my first question will not be
> with respect to the binocular observation but rather with respect
> to the naked-eye observation.
>
> Were you, perhance, using star clusters to measure the limiting
> magnitudes of your binoculars? If so, I have a suggestion about
> why your results might be biased.
>
> - Tony Flanders
Tony, before you go any further with this, you need to take time and
ang go read my work. It should answer all your questions.
edz
Jon Isaacs
>ang go read my work. It should answer all your questions.
>
>edz
Since you opened the discussion here and have been discussing it here I think
it is appropriate for you to address this issue that Tony has raised.
Tony is not the only one interested in knowing your answer.
jon
edz
explanation, only a hypothesis, I will default to Schaefer's article.
My data seems to clearly support what Schaefer shows and that is
smaller apertures are not as affected by NELM as some might believe.
In fact Schaefer clearly shows the affects of NELM are not linear.
Since aperture in fixed mag binoculars can never be utilized to its
optimum, as it can in variable magnification telescopes, I would
suspect that is even more true and a stronger case in binoculars, not
less.
I don't know if this hypothesis is correct. But as I said I do have
clear and accurate data to support what I'm saying. The affects of
NELM are non-linear in overall outcome of binocular LM performance.
edz
edz
My target study area was Cr399, the coathanger. The maximum NELM
reached was 5.8. It has only 3 stars that would have been naked eye
visible throughout my weeks of observations. The NE stars are
separated by about 15arcmin. I explain clearly in my work the
difficulty of using these closely grouped stars and the unreliability
of NELM from them. On about half the nights, these three were not
even NE visible.
Other NELM targets were nearby constellations with a variety of star
magnitudes; Sge, Del, Psc, UMi. In every case I tried to confirm NELM
from more than one target location. In every instance at least one of
those locations was either my target or as close as I colud get to my
target.
I included a full written section on the determination of NELM that
should explain my procedure. there were several variables taken into
consideration in the determination of NELM, i.e., integrated
magnitudes.
I think the major difference is thru the galss and not in the
determination of NELM, as I usually had a second observation to
confirm.
edz
Tony Flanders
> Tony, before you go any further with this, you need to take time and
> ang go read my work. It should answer all your questions.
I apologize profusely; I hadn't realized that the article had
already been published. I hate it when people ask me to write
something that I have already written!
I think that I can explain the discrepancy in our results w.r.t.
NELM vs. BLM, but before I get into that, let me make a few
comments about other aspects of the article.
As I am sure you are well aware, the entire topic, although
interesting, is only one aspect of what people want from
binoculars, and probably not the most important. When I use
binoculars, I am interested primarily in seeing nebulosity
and secondarily in seeing moderately bright stars better.
Seeing faint stars near the limit of vision is rarely very
important to me. But of course, that should not be taken
as a criticism of your article. The subject that you explored
is very interesting in its own right, is certainly relevant
even if it not the whole story, and it has the virtue of
being much easier to quantify than seeing faint nebulosity.
I have some doubts about the methodology of the study of
aperture and magnification; in particular, I disagree with
the statement that masking binoculars down to 50mm is unlikely
to improve aberrations in a way that bears on LM. In fact,
the statement that your 20x80 binoculars generally do little
better than your 16x70 suggests to me that the 20x80 *do*
have serious aberrations. What else could account for
such a huge discrepancy in performance? Certainly not light
loss alone, unless there is some kind of internal mask that
makes the effective aperture much less than 80mm. And why
shouldn't masking them down to 50mm reduce those aberrations
dramatically? Unfortunately, the only truly reliable way
to study this subject would be with a binocular telescope
with interchangeable eyepieces and with known high quality.
Having said that, I certainly agree with your general
conclusion that the benefits of higher magnification in
binoculars have been understated dramatically in the past.
Without a doubt, binoculars with abnormally small exit
pupils, such as my own Canon 15x45 IS binoculars, perform
much better than formulas weighted heavily towards
aperture would predict.
---------------------------------------------------
In the interest of full disclosure, I must admit that I have
spent very little time rigorously measuring the LM of
binoculars. That is because binoculars lose most of their
charm for me once they are placed atop a tripod, and of course
the subject cannot be studied truly rigorously if the binoculars
are hand-held. However, I *have* spent many, many hours
measuring telescopic LM, a subject which is discussed in
my website at http://mysite.verizon.net/vze55p46.
I should mention three points where my experience differs
wildly from yours. First, you say that you find it much
easier to determine instrumental LM than NELM. I agree
with you in finding NELM very hard to estimate; like you,
I have spent 2-3 minutes trying to determine the visibility
of a single star. But I find it every bit as difficult when
looking through a telescope. Second, you say that once you
have found a star in an instrument, you can then see it with
direct vision. Again, that is wildly discordant with my own
experience. Indeed, I have always found globular clusters
extremely frustrating to sketch precisely because most of the
stars that I can see with averted vision disappear when I look
at them directly, making it extremely hard to pinpoint their
location.
Third, and most relevant to the question of NELM vs. BLM, you
say that you find NELM much harder to estimate under brighter
skies than under darker skies. Again, that is just the opposite
of my own experience. It is true that it is frequently hard to
find a sequence of stars with adequate granularity under
heavy light pollution; this can be an insuperable obstacle
for NELMs lower than 4.0. However, for any given marginal
star, I find the question whether it is or is not visible
much easier to answer under bright skies than under dark
skies. Another way of saying that, perhaps, is that the
benefits of averted vision over direct vision decrease as the
skies get brighter.
--------------------------------------------
But on to the subject of NELM vs. BLM. When you chart BLM on
the Y axis and NELM on the X axis, you get a dramatically
lower slope than I do for ultra-low-power telescopic LM
versus NELM. I believe that the discrepancy lies almost
entirely in your estimates of NELM, and very little in your
estimates of BLM. In other words, under the same skies as
you, I think I would have found only an 0.5 mag variation
in NELM, just as you found only an 0.5 mag variation in BLM.
It is hard to tell that for sure, because you omitted one
absolutely critical fact from your report, namely the nature
of your observing site. I have attempted to deduce that
from your reported NELM and other observations, but I put
*very* little credence in NELM, since I have found estimates
of NELM for the same site at the same time to vary more
than one full magnitude from one experienced observer to
another. It sounds as though all of your observations
were done under what most people might call good suburban
conditions, right?
Incidentally, the points graphed on Chart 2 do not seem to
agree with the numbers quoted in the individual observations.
For instance, I see a number of points charted at NELM = 6.0,
but no recordings of mag 6.0 stars being seen naked-eye.
Or did I miss something?
With respect to the individual observations of NELM, you say
that on Aug 20 you saw a mag 5.6 star in Cr 399, but that the
Cygnus Milky Way was only faintly visible. For what it is
worth, any time that Cygnus has been overhead and I have been
able to see a mag 5.6 star, I would call the Cygnus Milky Way
"very bright". I suspect that the difference is in our NELM
estimates; my NELM estimates for dark and semi-dark skies seem
to be consistently on the low side for experienced observers.
But it is possible that the difference is in our notions of
"faintly visible" versus "very bright". It is impossible to
tell in lieu of objective measurements of sky brightness and
extinction.
Much more important, almost all of the data for your NELM vs.
BLM correlation comes from the three points on Sep 10, 11, and 12.
In lieu of the usual careful report of NELM, you note "improving
from about 4.0 ... to 5.0". You seem to have arbitrarily fudged
the lowest figure from 4.0 to 4.4 in your chart, but I suggest
that you still underestimated the NELM dramatically -- unless,
perchance, this was a very hazy night (you don't say).
I have a huge amount of data of all kinds proving that at my
latitude of 42N, the zenith is always considerably darker on a
reasonably clear night at Full Moon in the absence of light
pollution than it is at my own city home on a moonless night.
Yet I consistently measure the NELM here at anywhere from 4.4
to 4.8, depending on the transparency and the time of night.
That suggests that if I had been with you on Sep 10, I would
have estimated the NELM at 5.0 or better.
Why the discrepancy? I can suggest various explanations. First,
perhaps, experience. I do a lot of observing under heavy light
pollution, so I am accustomed to estimating NELM in those
conditions. You, clearly, have much more experience estimating
NELM under semi-dark skies, which is a rather different problem.
Second, sky brightness at Full Moon varies tremendously from one
place in the sky to another; it is even more critical than usual
to measure the BLM and the NELM in the same part of the sky.
Third, even a quick glimpse of the Full Moon wreaks havoc with
one's dark adaptation. Obviously, this won't be an issue as
long as one's eyes are glued to binoculars, but it is hard
to avoid when observing naked-eye. I am particularly good at
avoiding looking directly at bright lights because of my
extensive practice at urban observing.
Another factor worth considering is the pupils of our eyes.
You mention that an optometrist measured your pupils at 6.5mm
to 7mm in subdued room light. That is probably abnormally
large; most people report 7mm or less under fully dark skies,
with considerable decrease under suburban night skies, which
in turn are surely much darker than subdued room light.
My own pupils always seem to measure from 5.0mm - 5.5mm
anywhere from very dim room light to total blackness,
depending how I measure them.
Now let's imagine that under suburban skies, your pupils
typically open to 8mm, but that under the Full Moon, they
close down to 5mm. That could conceivably account for
a full magnitude loss in NELM due entirely to your pupils,
quite ignoring issues of contrast of stars against the sky.
But you would not experience that with any of the binoculars
that you tested, all of which had exit pupils of 5mm or less.
Likewise, I would not experience it for NELM due to my
relatively constant pupil size.
Frankly, I am rather sceptical of that explanation, because
there are numerous reports that people with 8mm pupils do *not*
see a full magnitude improvement in naked-eye observing over
people with 5mm pupils. Some people say that there is no
improvement at all, atttributing this to the massive
aberrations of the human eye at unstopped 8mm aperture.
Yet another thing to remember is the effect of focus, something
I am keenly aware of due to my myopia. The optimal prescription
for my eyes varies considerably from one time of day to another,
and also depending on the altitude of my target. The eyeball
compresses as one tilts one's head up, with a fairly large
effect on focus. The eye has some degree of accomodation to
this, varying greatly from one individual to another, but
nowhere near as great as binoculars, where you can tune focus
to your heart's content. Indeed, I find that I am constantly
fiddling with the focus on my binoculars, and frequently change
it from one target to another, probably due to differences
in their altitude.
Finally, let me stress once more that you should get out of the
habit of saying "NELM has an effect ...". NELM is itself an
effect; it is sky brightness and extinction that are causes.
NELM has been used traditionally as a surrogate for sky
conditions for lack of any better substitute, but it is a
truly terrible surrogate in many, many ways. Without a
doubt, NELM is heavily non-linear w.r.t. sky brightness.
Indeed, I am pretty sure that insofar as NELM and telescopic
LM are non-linear w.r.t. each other, it is because telescopic
LM is much *more* linear than NELM w.r.t. the underlying causes.
In summary, let me say that I have spent a lot of time talking
about areas where we disagree and very little about areas where
we agree, which is an awful lot. It's more efficient that way,
but it can be a little disconcerting to read. Despite all of
my criticisms, this was a very impressive study. Now I feel
obliged to try to replicate findings myself!
- Tony Flanders
Phillip Coker
in this group. Great posts!!
Phillip
Anton Jopko
another factor to consider is vignetting of the edge of the field of view. I
notice in even premium binoculars that the edge of the field of view is not
fully illuminated. some calculations i've done suggest you could lose about 0.3
magnitudes at the edge of the field compared to the centre. I often wonder why
this is so when you pay big bucks for well corrected binos. must be easier to
stop the view down i guess. where did you publish your research on bino's?
Thanks,
my 2 cents,
anton
edz
> another factor to consider is vignetting of the edge of the field of view. I
> notice in even premium binoculars that the edge of the field of view is not
> fully illuminated. some calculations i've done suggest you could lose about 0.3
Anton,
I measured for loss of magnitude across the fov. All binoculars lost
from lack of sharpness in the outer portions of the fov. Even the
best binoculars, by 80% out from center had already lost 0.4mag and by
85% out had lost 0.5mag or more. Well known moderately wide angle 6°
10x50 binoculars had lost a full magnitude by 70-80% out from center.
I never even got close to the edge where there might have been loss of
illumination due to vignetting, if any were present. The fov in
almost every binocular in the outer 10% is so poor as to make stars
distorted that probably a full 2 mag is lost in the outer 10% fov. I
stop measuring at 1 mag loss.
edz
edz
> aperture and magnification; in particular, I disagree with
> the statement that masking binoculars down to 50mm is unlikely
> to improve aberrations in a way that bears on LM.
I provided a link to a discussion of the affects of masking
binoculars. See binocular performance. Having discussed this method
with others, I think it is valid.
> the statement that your 20x80 binoculars generally do little
> better than your 16x70 suggests to me that the 20x80 *do*
> have serious aberrations.
I clearly stated that the 20x80shave some problems. The exit pupils
cannot be made round. When compared to Fujinon 16x70s, they often are
not significantly better. I stated that in my results.
> Unfortunately, the only truly reliable way
> to study this subject would be with a binocular telescope
> with interchangeable eyepieces and with known high quality.
But this would be avery different test. It would pass right over the
performance deficiencies of low mag binoculars. So It probably
wouldn't be relevant.
> However, I *have* spent many, many hours
> measuring telescopic LM, a subject which is discussed in
> my website
I’ve been to your site.
>
> I should mention three points where my experience differs
> wildly from yours. First, you say that you find it much
> easier to determine instrumental LM than NELM. I agree
> with you in finding NELM very hard to estimate; like you,
> I have spent 2-3 minutes trying to determine the visibility
> of a single star.
> But I find it every bit as difficult when
> looking through a telescope.
That is probably true for me also. As I think about it, sometimes I
need a great length of time at the eyepieces to acquire a star and
many times it means coming back several times before finding it.
> you say that once you
> have found a star in an instrument, you can then see it with
> direct vision. Again, that is wildly discordant with my own
> experience.
Often! Once I have found a star by averted vision, I can come back to
or pass over the area and catch many more glimpses of that star, even
looking directly at it.
> I have always found globular clusters
> extremely frustrating to sketch precisely because most of the
> stars that I can see with averted vision disappear when I look
> at them directly, making it extremely hard to pinpoint their
> location.
I find globulars and clusters like M37 difficult because of the
density, the close proximity of stars, not necessarily because of the
magnitudes of the stars.
>
> Third, and most relevant to the question of NELM vs. BLM, you
> say that you find NELM much harder to estimate under brighter
> skies than under darker skies.
Seems that way to me. But not sure this has any relevance, it just a
perception, not a measurement. it's not used anywhere.
> But on to the subject of NELM vs. BLM. When you chart BLM on
> the Y axis and NELM on the X axis, you get a dramatically
> lower slope than I do for ultra-low-power telescopic LM
> versus NELM. I believe that the discrepancy lies almost
> entirely in your estimates of NELM
Actually, I think I took great care in estimating NELM and I think I
thoroughly explained my procedures. But if you think that I
underestimate NELM, you would not have any reason to say I
underestimated selectively at either the high end or the low end.
Typically, when someone develops a process and follows it regularly,
if the procedure produces results that are off, they are all off,
usually by the same amount.
If I underestimated all my NELM readings by 0.2 or 0.3 or even 0.5mag,
the slope of my observations would not change. All my observations
would just move further to the right with the same slope. The slope
of the plot of my observations is one of the most relevant findings of
this entire study. It’s what shows that BLM does not act
linearly with NELM.
> It sounds as though all of your observations
> were done under what most people might call good suburban
> conditions, right?
I'd say rural suburban. I've got no street lights within a quarter
mile. 15 miles north of Prov. probably only 40-50 miles south of
you.
>
> Incidentally, the points graphed on Chart 2 do not seem to
> agree with the numbers quoted in the individual observations.
> For instance, I see a number of points charted at NELM = 6.0,
> but no recordings of mag 6.0 stars being seen naked-eye.
> Or did I miss something?
>
> With respect to the individual observations of NELM, you say
> that on Aug 20 you saw a mag 5.6 star in Cr 399, but that the
> Cygnus Milky Way was only faintly visible. For what it is
> worth, any time that Cygnus has been overhead and I have been
> able to see a mag 5.6 star, I would call the Cygnus Milky Way
> "very bright". I suspect that the difference is in our NELM
> estimates.
I've seen the Milky Way, very recently, under mag 6.5 skies. The
Milky Way was so bright, I mistakenly though I was looking at clouds.
That's bright. In my backyard, it sometimes gets a lillte bright
after midnight. I expect how I percieve the Milky Way has no bearing
on what stars I can site for NELM.
> Much more important, almost all of the data for your NELM vs.
> BLM correlation comes from the three points on Sep 10, 11, and 12.
> In lieu of the usual careful report of NELM, you note "improving
> from about 4.0 ... to 5.0".
First you need to realize that all of this data is not a word for word
transcribe of my field logs. On different nights when I sat down to
type, it appears I had a different style. I have probably 3 or 4 more
nights of data that is not even shown in the daily reports in the
paper.
Tony, I must say you seem to be taking on the qualities of Quincy.
Any particular reason why. Or do you just like being forensic?
> You seem to have arbitrarily fudged
> the lowest figure from 4.0 to 4.4 in your chart, but I suggest
> that you still underestimated the NELM dramatically -- unless,
> perchance, this was a very hazy night (you don't say).
I wrote several paragraphs explaining why I did this. Go back and
read again the sections on why I felt I under-estimated some nights.
> I have a huge amount of data of all kinds proving that at my
> latitude of 42N, the zenith is always considerably darker on a
> reasonably clear night at Full Moon in the absence of light
> pollution than it is at my own city home on a moonless night.
> Yet I consistently measure the NELM here at anywhere from 4.4
> to 4.8, depending on the transparency and the time of night.
> That suggests that if I had been with you on Sep 10, I would
> have estimated the NELM at 5.0 or better.
I observed just tonight under very clear skies with a moon similar to,
not quite as full as last month. I recorded NELM 3 or 4 times
throughout the night tonight at 4.4 to 4.7, it varied throughout the
night. Two sample 5.0 stars could not be seen, even after many
attempts. Looking completely away from the moon towards the north I
observed NELM of 5.0, but this was not in the vicinity of my study
observations, so I would not use it.
> it is even more critical than usual
> to measure the BLM and the NELM in the same part of the sky.
As I stated above and in the article, I also recommend this procedure.
> Third, even a quick glimpse of the Full Moon wreaks havoc with
> one's dark adaptation.
I try to hide from the moon.
> Likewise, I would not experience it for NELM due to my
> relatively constant pupil size.
I even close my eyes when infrequent cars go by to eliminate reflected
light off the house. Why would you assume other’s pupils change
but yours remain constant. isn't that odd. Anyone, with any exposure
to light will experience a pupil change, no matter how large they
start out.
> I find that I am constantly
> fiddling with the focus on my binoculars, and frequently change
> it from one target to another, probably due to differences
> in their altitude.
I seldom find that I need to change focus, but yes I do occasionally,
and it is sometimes due to differnet altitudes. Usually it's because
the dials have been touched. I don't think I've refocused my Fujinons
in the past week or so.
>
> Finally, let me stress once more that you should get out of the
> habit of saying "NELM has an effect ...". NELM is itself an
> effect; it is sky brightness and extinction that are causes.
As NELM is a value (a result) used in the application of Carlin's
formula, it has an affect on the outcome. Which brings us full circle
to why we are here. I attempted to determine the influence of various
pieces of the puzzle.
> NELM is heavily non-linear w.r.t. sky brightness.
> Indeed, I am pretty sure that insofar as NELM and telescopic
> LM are non-linear w.r.t. each other, it is because telescopic
> LM is much *more* linear than NELM w.r.t. the underlying causes.
There are probably a dozen different factors needed to measure NELM by
way of sky conditions, if you use Schaefer's method, but it's so
complex it's not practical. It's precise, but not practical.
Obviously star magnitude determination of NELM is the simplest,
crudest way to get at a factor, but it's something anyone can
understand. It may not be precise, but it's the best method I know.
AND, it's the factor used in the application of Carlin's formula. If
it's applied incorrectly, the predicted results will be wrong.
>
> In summary, let me say that I have spent a lot of time talking
> about areas where we disagree and very little about areas where
> we agree, which is an awful lot. It's more efficient that way,
> but it can be a little disconcerting to read.
Without that, no questions would be raised. And I've left one
unanswered.
Why did I plot some points at 6.0 instead of 5.8. I felt that on a
particular night when I observed TX Pisces (5.8) that its
instantaneous appearance indicated a darker sky than another night
when I needed to spend several minutes to acquire that same star with
much difficulty. It is an interpretation of my field notes to try to
more accurately reflect what I felt to be the conditions.
> Despite all of
> my criticisms, this was a very impressive study. Now I feel
> obliged to try to replicate findings myself!
That could only be beneficial. I'd appreciate that.
edz
Tony Flanders
> another factor to consider is vignetting of the edge of the field of view.
Ed covered that in his report.
> where did you publish your research on bino's?
Ed Zarenski's research, not mine. Take a look at
http://www.cloudynights.com/articles/LM-binoculars.htm
- Tony Flanders
edz
As a result of all the in-depth and detailed questions Tony raised, I
felt it prudent to post this summary of post-publish notes.
These additional notes should help provide understanding for some
apparent questions raised concerning the data in the "Limiting
Magnitude in Binoculars" article.
edz
Notes Oct. 6, 2003 supplement to "Limiting magnitude in Binoculars"
article.
I have since gone back thru my log to make note of those observing
nights not published, observing conditions not detailed and to note
some specific clarifications. This provides answers for some of the
questions raised post-publish on s.a.a. Also, this provides some
additional supporting documentation for the article "Limiting
Magnitude in Binoculars."
I have a page in my log book that I developed shortly after the start
of this exercise. It has sketches of Delphinus, Sagitta, the circlet
of Pisces and Ursa Minor. All the stars, 48 stars in total, than can
be used for NELM determination are plotted in these sketches. Often
throughout the many nights of log notes, a NELM observation, if not
recorded in detail, will simply be listed as a single value. These
values are the magnitudes of the stars shown on my sketch page. There
are several values, such as 4.37, 4.43, 4.68 and 5.05, that are
unmistakable since they only occur in one location within the four
constellations.
Aug 23 was recorded as NELM 5.8. Log notes indicate I was able to
acquire TX Pisces (5.8) instantly. Never before and never since have
I seen Tx Pis that easily. This was undoubtedly the best NELM I have
ever recorded from my home.
Very late observing improved on this night. This was the night of all
objects listed in the section "What Else Can be Seen by the Best
Binoculars?" I observed the North America neb., portions of the Veil
neb. and I suspected seeing nebulosity around IC1805.
Based on these records, after the fact I elected to tabulate
observations at NELM mag 6.0. It is significant that although these
NELM sky conditions have not been repeated, I have since been able to
duplicate on several occasions the BLM observations recorded on this
night.
On Sept. 10, observations were recorded very early in the evening,
completing the viewing session around 8:30 to 9:00 PM. This was full
moon night and I ended my observing session before the moon completely
interfered. Log indicates viewing was difficult, sky was washed out,
Sagitta was barely visible. This would have indicated difficulty
seeing the back end of the arrow, two equal stars both of mag 4.37.
The 5.0 star in Sge was not seen as it is always specifically noted if
seen. This star is usually not seen until some other mag 5 star is
seen first. Observations of Delphinus do not indicate seeing the 5.05
star off the end of the tail or the mag 4.68 star just off the sw edge
of the diamond body. This is a difficult star and is always noted if
seen. Maximum NELM seen Sept. 10-11 was 4.4 Del and 4.37 Sge.
On Sept 12, I wrote a note "better than last night, 5.0?". In my
notes, I questioned marked the 5.0. This would have been an
indication that I was questioning whether or not I saw the 5.0 mag
star in Sagitta, probably because I did not note seeing the 5.05 star
off the tail of Delphinus. Also I did not specifically note seeing
the mag 4.68 star just off the body of Del.
Sept. 14th includes a note to the effect "clearly better than any of
the three previous nights." Although that note in itself is not
definitive, it is a common practice of mine to sometimes write short
notes referring to improving conditions as it reflects a notation of
relief from previous difficult conditions, kind of like a written sigh
of relief. Sketches made on the 14th show NELM reached 5.0 direct and
5.2 averted, but these were well away from my target area and even
further away from the moon.
Sept. 25 notes were not included in the BLM Observations section.
They are the body of the work that was the basis for determining the
affect of changes in aperture. The observations are included in my
BLM analysis.
Skies started out around 8PM at NELM 5.0 Sge. Seeing was poor to fair
with a thin haze. NELM deteriorated by 9 PM to 4.68 Del and later
even lower to 4.4 Sge and 4.43 Del.
At NELM 5.0, 20x80s saw BLM of 10.76 and two stars of 10.83.
The16x60s saw BLM of 10.6. At NELM 4.7 to 4.4, 15x70s saw BLM of 10.6
and 16x70s saw BLM of 10.6 in worsening conditions.
These Sept. 25 observations support the observations of Sept. 10, 11
and 12 plotted for the lowest NELM of 4.8 and 4.4. Had these Sept 25
observations been used instead, the resultant slope of the actual
observations would be even flatter than that shown plotted.
Minor observation notes were entered on Aug. 31, Sept. 5 and Sept 6.
NELM varied from 5.8 to 4.8. Additional observations are recorded for
8x42, 10x50, 12x50 and 16x70 over various nights of conditions ranging
from NELM 5.0 to 5.8.
I stated that I find it easier to observe BLM thru the lens than I
find it to observe stars at the limits of NELM. Having reviewed these
notes, I believe there is reasoning behind this. There is so much
(50°) brightly lit sky pouring light into the eyes during the NELM
assessment that it makes observations difficult. During binocular
observation for BLM, even though stars are difficult to acquire at mag
10.0 to 10.8, the area of the sky lighting the eye is only 3° to 6°.
So much less light thru the lens provides for a much easier filed of
view for the eye to look at. This shows the affect of contrast
improvement by apparent darkening of extended background sky. I
believe this is what seems to make it easier to observe at the limits
of BLM vs the limits of NELM even though still it may take time to
acquire targets in both assessments.
edz
David Knisely
> Aug 23 was recorded as NELM 5.8. Log notes indicate I was able
> to acquire TX Pisces (5.8) instantly.
Be careful here, as TX Piscium is a variable star (runs from as bright
as 4.79 to as faint as 5.2 or so). It is also a *very* red star (a
"Carbon Star", among the reddest known). It may not be a good star to
be doing a NELM estimate on. 22 Piscium is not far away and it is
about 5.58, so that might fit the bill better. Clear skies to you.
David Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
edz
Hi David,
You say
> TX Piscium is a variable star (runs from as bright
> as 4.79 to as faint as 5.2 or so).
I checked TX Pis with AAVSO and got 5.8 as the current (August) value.
There's no question it is by far the faintest in the circlet, #7 Pis
next faintest is 5.2 and is easily seen in comparison. I can always
see the small group north west of the circlet, 55Peg, 57peg, 58Peg,
59Peg containing 5.3 and 5.4 stars before I can see the circlet. TX
is still fainter than these.
In the two years I've been using the circlet as a reference, up until
recently, I've never been able to see TX. I'm assuming it is now not
near minimum. However I still have never been able to see 16Pis,
21Pis or 22Pis, all nearby to TX, all very close in mag, all brighter
than 6.0.
Usually when I can see TX, I can also see the 5.7 mag star off the
east end of the tail of Delphinus.
edz
edz
> Ed, you've done a lot of great work, for which you should be
> commended. It's amazing how much it takes to collect 15 good data
> poinsts. >
> Regards - Kurt
>
Your statement that only 15 good data points can be found from all the
set is completely invalid as it is based on grossly improper
assumptions.
Your monumental analysis that you painstakingly developed, based on
incorrect assumptions, without having asked any questions is not what
I would expect from anyone in the astronomy community.
You clearly go down a path to reduce the number of valid points
without ever asking any questions from the original author. You could
not possible have believed that's all there was. In fact, the data
reported in my article is only the maximum point for every
observation. The complete data set includes logged records from
somewhere between 600 and 800 stars from over 50 binocular/nights of
observations.
The whole basis for my observations was to report maximums and work
with my own data set to get there. In fact every observation for
which I reported a maximum has at least 30 to 50 data points leading
up to the maximum with as few as 2 or 3 but possibly as many as 7 or 8
data points clustered in a very tight range of 0.1 or 0.2 magnitudes
around the maximum for each binocular on each night. So in essence, I
could have reported possible 5 or 6 times on average as many points
for over 50 binocular/nights. But I elected to not publish my entire
data set. I elected to work with the results of my own data, since I
knew the essence of my own data.
You on the other hand elected not to ask, and in so doing you
developed a critical analysis without all the information and you
reached unfounded conclusions based on improper assumptions.
You probably could have fostered a good working relationship by asking
any number of very important questions that would have been critical
to your beginning assumptions you should have started out with.
Instead, you proceeded down a a very narrow path, without all the
information, without any benefit of first hand information, without
checking, and apparently without consideration.
I'm sorry, I don't appreciate one bit what you did here.
edz
Tony Flanders
> Tony, I must say you seem to be taking on the qualities of Quincy.
> Any particular reason why.
Absolutely! I have three reasons.
First, this is a subject that interests me intensely. Actually, I don't
care all that much about binocular LM, but I hypothesize that telescopic
LM should track binocular LM quite closely, assuming identical exit pupils.
Second, your results w.r.t. correlation between NELM and instrumental LM
are wildly discordant with my own, and I am trying to figure out why.
Third, I hate to place theory above empirical data, but I find your
conclusion w.r.t. NELM vs. binocular LM absolutely impossible; it is
as much at odds with my notions of reason as (say) creationism.
Therefore, I am trying to poke holes in your observations in hopes
of rescuing my basic faith in science.
Why impossible? Well, I claim the following. Assuming:
1. Insignificant transmission losses, and
2. Insignificant optical aberrations, and
3. Identical fields of view, and
4. Identical modes, i.e. monocular vs. binocular
5. Identical effective pupils, i.e. the intersection of the
instrument's pupil and the eye's pupil
6. Identical sky conditions
then every 2.56 increase in aperture *must* yield a 1-mag increase in LM.
Why? Well, imagine a cluster of stars ranging from mag 4 - mag 9
placed at a certain distance, and you are observing it naked-eye.
You will get a certain LM. Now move that cluster 2.56 times as far and
observe it through an ideal 2.56x binocular with a very large aperture,
so that the effective exit pupil is determined entirely by your eye's
pupil and not at all by the aperture of the instrument. I claim that
the photons reaching your retina are *exactly* the same as in the
first case; the sky brightness will be identical, the actual star
magnitudes will be reduced by 2 mag, but the apparent star mags as
seen through the 2.56x binoculars will be exactly as in the first
case. Therefore, you will see precisely the same stars, yielding
a BLM 2 mag higher than the NELM.
Therefore, insofar as 1 mag increase in NELM does not yield 1 mag
increase in BLM, it must be due to a failure in one of the 6 conditions
that I listed above. But it is hard to imagine all of them together
adding up to such a whopping discrepancy as 1.5 mag vs. 0.5 mag.
Light loss and optical aberration in high-quality binoculars are
both very low. The reduced FOV of binoculars as compared to the
unlimited FOV of the naked eye may have an effect, but it is surely
small. This can, of course, be tested with the classic pair of
toilet-paper rolls. Binocular vs. monocular viewing may be an
issue with telescopic LM, but cannot be an issue with binocular LM.
That leaves only pupil. But can the difference between your 7mm
pupil and the binocular's 5mm pupil really cause such a vast
discrepancy? Hard to believe. I suppose that, too, could be
tested with 7x50 or 10x70 binoculars.
Note that if, as you describe for Schaefer, you increase the aperture
while holding the magnification constant, or increase the magnification
while holding the aperture constant, then you are changing the exit
pupil, and hence the background sky brightness, and then all bets
are off.
- Tony Flanders
edz
> Although it is important that you, unlike most other amateurs, made
> the effort to collect observations, a sufficient number of
> observations have not been collected to reject or accept, from a
> statistical viewpoint,
HAVING TAKEN A BRIEF LOOK AT MY LOG I ESTIMATE THAT I HAVE RECORDED
OVER 1000 OBSERVATIONS. FROM MY REPORT, YOU COULD HAVE READ THAT I
RECORDED OVER 500 VALID OBSERVATIONS.
> Boiling down the CN "Limiting Magnitude" article, 15 useful
> observations were recorded, as listed in Table 1:
>
YOU CLEARLY DID NOT READ MY ENTIRE ARTICLE AND MISREPRESNT SOME VERY
IMPORTANT INFORMATION THAT I DID INCLUDE.
> Table 3 lists 4 observations of "averted" limiting magnitude that were
> not applied for the purpose of this discussion on the grounds of
> consistency in data collection:
MY LOG INCLUDES WELL OVER 100 AVERTED OBSERVATIONS.
>
> ========================
> Table 3 - Averted vision readings - Deferred in favor of direct
> reading and where the averted reading is the lowest magnitude for that
> scope on that night
>
>
> As can be seen in Table 4, a cross-table summary of Table 1 above,
> an insufficient number of observations has been made within each
> binocular class to make any statistically valid conclusion rejecting
> or accepting the Carlin formula as a predictive tool for amateur
> binocular observers:
>
> ========================
> Table 4 Count of NELM-Mag CN
> observations by binocular class
> and magnitude observation
>
> Mag range
> Bino 5-5.5 5.6-6 Total
> 10x50 1 0 1
> 12x50 2 2
> 15x70 3 2 5
> 16x60 2 2
> 16x70 2 2 4
> 08x42 1 1
> Total 5 10 15
> For example, only 1 useful reading was taken in the 8x42 and 10x50
> classes; 2 in the 12x50 class; 2 in the 16x60. As discussed in the
> remainder of this post, the remaining observations regarding 70mm
> aperature binoculars are also of insufficient number to warrant
> reaching any firm conclusions at this time, although the data
> collected is as good start.
>
YOU ATTEMPTED TO WRITE YOUR OWN TECHNICAL ANALYSIS FROM MY WRITTEN
DESCRIPTIONS OF ONLY A SELECTION OF MY OBSERVING NIGHTS. IN REALITY I
HAVE ANYWHERE FROM 50 TO 100 OBSERVATIONS RECORDED FOR EVERY ONE OF
THESE 1'S AND 2'S YOU SHOW ABOVE.
YOU SHOULD HAVE KNOWN BETTER THAT I COULD NOT HAVE WRITTEN A COMPLETE
ANALYSIS SUCH AS I DID BASED ON A PALTRY FEW DATA POINTS.
What you did was take someone else's summary report data and
misrepresent it and manipulate it to try to make a defensive stand.
In my opinion you owe the astronomy community an apology.
edz
edz
> I generally recall that amateur observers using general star charts
> can estimate ZLM accurately to within +-0.5 magnitudes. (Citation
> omitted.) The +-0.5 variance in ZLM collection can be improved to
> between +-0.1 to 0.3 using special purpose magnitude finder charts.
> (While I have no doubt as an experienced observer that you can
> estimate NELM accurately to 0.2-0.3 mags, your NELM sources were not
> standardized and are not reproducible.) Two standard online sources
> providing such charts are:
I took great pains to develop NELM charts with all points in small
localized areas so estimates were more consistent. Charts that have
the eyes roving over very large wide areas performing star counts
leave the user open to far too much error. All my NELM charts are not
only reproducable, but are readily avaiulable, if you want to take the
time and effort to discover.
> An insufficient number of useful observations have been reported to
> date to make any interpretations outside the v5.2-v5.6 box of useful
> Cloudy Nights observations.
You have absolutely no idea how many useful observations were made as
this is not your data and I suspect you know this is a false
statement.
> 3) NELM should be estimated using charting areas in one of the two
> reference sources listed above. This provides a commonly available
> methodology for use by differing observers at different locations.
> Alternatively, amateur prepared NELM charts should be published online
> and first "vetted" by the amateur observing community.
Not only did I inform the reading community what I used but the
constellations and asterisms and charts I used are commonly available
to the entire astronomical community.
>
> 5) The cluster selected of estimating binocular limiting magnitude
> should be within or immediately adjacent to either a McBeath or IMO
> visual limiting magnitude charting area.
Not only did I ALWAYS attempt a reading exactly within my observation
area, or exactly adjacent to it, but I also made valiant attempts to
verify in duplicity all my observations.
Edz's reference to the chart for
> Brocchi's Cluster from Covington qualifies.)
As it turns out. if you read my article you would have known the
Covington Cr399 chart was inadequate to perform the task and I went
much further beyond that.
> some back of the napkin notes on potential clusters suitable of
> binocular limiting magnitude tests. My rough notes list the following
> candidates:
None of these are useful until someone takes the extra effort to
verify and label the magnitudes of all the stars in the cluster and
publish the information for others. I have no doubt you will run into
all the same discrepencies I did, that publish magnitudes from
different sources vary by so much as to be in some cases usless.
> Ed, you've done a lot of great work, for which you should be
> commended. It's amazing how much it takes to collect 15 good data
> poinsts.
> By discussing your article at some length and evaluting the
> strenght of analysis,
I can assure you I will not be discussing my article witrh YOU!
The more I read the way you bastardized my data, the more I am
infuriated. I do not want your personal commendation.
I believe you have manipulated my data for your own purposes and in
doing so have done a great dis-service to the community.
I believe you should retract what you have written.
edz
edz
above. This is how I feel about it.
His Method
Reworking my technical approach into a 400 line technical work of his
own. Discrediting my work with a grossly misrepresentative analysis
without ever asking a single question or requesting a single
clarification.
His Approach
He blindly analyzed my written work without full acknowledgement of
detail available. He selectively misrepresented data while obviously
overlooking pertinent facts provided in the original work.
His Argument
Not enough clarification in my source work to prevent him from making
all the wrong assumptions and improper misrepresentations he made, so
it's not his fault, it's my fault he misrepresented my work.
His Response
A defense behind supposed ambiguity. After the fact request for
clarifications. Continued claim of his valid approach while
continuing claims of supposed ambiguity in my data. Accusing me of
causing a flame war after so grossly misrepresenting my work.
Apologizing to the community for my outrage towards him.
How do you like that, I get totally misrepresented by someone with an
apparent motive to defend his own work, and by exposing his gross
misrepresentation of my work, I get accused of flaming. Of course, we
are to believe that what he did was just fine! Sounds almost like my
two teenagers defending their lack of wisdom in making inappropriately
wrong decisions in life, but not being able to admit they did anything
wrong. Do you want someone like this posting apparently informed
technical responses to your work? I don't.
I can take criticism. I can't take deceit or misrepresentation,
whether knowingly intended or not.
PrisNo6, if you thought you would be allowed to perpetrate such a
grossly wrong action and have it appear clearly thought out,
surrounded by professional reference, technically presented and
without question accurately critical, you are wrong. I am without
respect for your actions. In the corporate world, people lose their
jobs for what you did.
edz
edz
information about one critical topic. There is still plenty more to
review.
edz
More Clarifications
Methods I used for determining NELM in my Binocular Limiting Magnitude
Study
Certainly without complete knowledge of what my NELM chart sources
were,
any claim of non-standardization or that they are not reproducible is
unfounded.
I took great pains to develop NELM charts, some with points in small,
localized areas so estimates would be more consistent. In my opinion,
charts that require the eyes roving over large wide areas performing
star counts leave the user open to far too much error, not only
boundary error but count error. It may be difficult for some to
visually project boundaries, even across small areas of the sky, with
the intent to count or not count stars on either side of that
projected boundary. Probably more than anything else, unfamiliarity
with star fields in the charts would render the user prone to error.
I prefer not to use the star count method, as it is my opinion there
are better means to accomplish the task.
For purposes of this study, I have identified five constellation areas
or asterisms and by using sources such as published NELM charts,
computerized star charting programs and AAVSO charts, verified a wide
range of star magnitudes for use in NELM determination. These charts
include only stars identified by a Greek letter or Flamsted number in
and adjacent to the primary figure of the constellation or asterism.
I corrected for any integrated magnitudes and checked variable
magnitude for one important variable star, TX Psc, used in the study.
For this study I used Sagitta, Delphinus, the Circlet of Pisces, Ursa
Minor and the Cr399 asterism itself. The end result is I have at my
disposal, several charts allowing NELM determination near continuous
from 3.6mag to 6.4mag at 0.1 mag intervals or less with few gaps and
often with check stars nearby to verify. All my NELM charts are not
only reproducible, but contain readily available information. I
believe the charts I am using are equally as accurate if not more so
than any star count method of NELM determination.
I almost always use Ursa Minor, as it has a wide variety of
magnitudes, is almost always visible, provides for a good check even
though it may not be in the vicinity of my observations and provides a
good night to night comparison. I have learned it is relatively easy
to know what to expect from Ursa Minor even after a short time outside
to assess the sky darkness and transparency. Also after doing this so
many times, I can now tell during the evening if sky conditions are
improving, and I often am able to verify such by a re-observation of
Ursa Minor. "The Backyard Astronomer's Guide" by Dickinson/Dyer
provides very good LM charts for the Ursa Minor area.
The suggested practice of using charts only in close proximity of 4°
to the area observed seems unrealistically applied and somewhat
impractical. This would require the observer to make critical
observations and NELM assessment in an area no bigger than the size of
a circle that would surround Sagitta. In fact, one website providing
NELM charts uses 14 charts of 10°x10° to cover the whole sky and
recommends observation from at least two charts. If one were to keep
critical observations within 4° of any one side or corner of those
charts, it would be possible to have a NELM observation within the
chart boundaries easily at a distance of 15° from the target critical
observation. Another website, which provides 30 charts for the whole
sky, recommends using observations from several charts. In so doing,
the observer may be making NELM estimates from an area easily as wide
as 10-20°. This seems practical advice.
My experience is this; areas of the sky in the same direction as wide
as 10-20° may not show a significant difference in NELM. This does
not say there will never be any difference. However, placing limiting
restrictions on what method must be used by the observer to accomplish
the task will do more to prevent attempts at observations than to
assist attempts. I still recommend an observation at the critical
location if at all possible and several others nearby to verify.
I have found quite often, both thru the lens and naked eye, faint
stars of a specific magnitude can sometimes be observed while
apparently equal or brighter nearby stars go unobserved. Due to this
phenomenon that I suspect can be attributed to color index, which I
address in my article, I elect most often to observe NELM from several
locations and in so doing often may record a variation of 0.2mag in
observed NELM.
Discordant advice is given to the user as to how much effort should be
expended to acquire stars for determination of NELM. While some
recommendations are given for observing stars at the averted vision
limits, other recommendations suggest spending no extra effort at all
to attempt acquiring stars by averted vision. The practice I use is
to spend as much time, several minutes if necessary, to acquire stars
at the limits of averted vision.
edz
Tony Flanders
> Tony, this does not answer your latest post but provides additional
> information about one critical topic...
> Methods I used for determining NELM in my Binocular Limiting Magnitude
> Study [and why I didn't use standard star-count methods].
> ...
> For this study I used Sagitta, Delphinus, the Circlet of Pisces, Ursa
> Minor and the Cr399 asterism itself. The end result is I have at my
> disposal, several charts allowing NELM determination near continuous
> from 3.6mag to 6.4mag at 0.1 mag intervals or less with few gaps and
> often with check stars nearby to verify.
A few thoughts.
I have made a few attempts to use the IMO star-count fields to determine
NELM, but inspired by PrisNo6's posting, I downloaded them again all
fired up to give them a fair trial. In a word, they are useless for
the purpose at hand. Like you, I have problems counting stars, but
perhaps that can be overcome with practice. I have seen meteor
observers use these charts to get very rapid counts indeed; presumably
in time one becomes familiar with all of the fields. Perhaps too
familiar ... I have great fear of seeing stars just because I know
that they are there. But that's another topic for another day.
The real problem with the IMO fields is that they are designed for
dark and semi-dark skies. Under typical suburban conditions, with
NELM around 5.0 - 5.5, many of the fields have gaps of 0.5 mag
between reference stars, and under urban conditions, with NELM
from 3.5 to 5.0, the gaps usually exceed 1 full magnitude.
Based on the sky-brightness measurements that I have recently done
with my digital camera, I would say that it is *not* necessary to
take NELM readings within 4 degrees of the area of interest --
and a good thing too, because as you point out, that would hardly
ever provide adequate granularity except under the darkest skies.
In fact, sky brightness tends to vary quite gently with azimuth;
being off by 20 or even 30 degrees in azimuth is no big deal.
However, sky brightness can vary quite rapidly with altitude,
so being at a similar altitude is much more important, particularly
as one approaches the horizon.
With respect to the fields that you have chosen, Vul, Sge and Del
are impeccable choices because they are very close to Cr399 in
altitude, fairly close in azimuth, and all pretty high at the
time of year and night in question. Psc and UMi, although
convenient, are not such good choices because of their relatively
low altitude. And given your location, you must have significant
light domes to the S and to the N, with significant effects on
Psc and UMi respectively.
A particular word of caution w.r.t. UMi. Just about everyone I know
uses UMi to get a quick reading of NELM, including me. However,
UMi has many shortcomings for this purpose. Even at latitude 42N,
Polaris is well down into the part of the sky that is seriously
affected by light pollution, and this problem is much more severe
in most of the U.S. And depending on the time of year, the
critical star Eta UMi varies anywhere from 30 to 55 degrees
above the horizon, which is a monumental range. The sky can
easily be 2-3 times as bright at 30 degrees as at 55 degrees.
The other comment, which you may be less glad to hear, is that
both PrisNo6 and I have the same basic criticism w.r.t. your
extrapolation of NELM vs. BLM. Because of the inherent
inaccuracy in *any* LM determination, you can only get a
decent curve for NELM vs. BLM if you have a fairly wide range
of NELM. In practice, your curve depends utterly and totally
on those two estimates of 4.0 and 4.5 near the full moon in
September, and relatively small inaccuracies in those estimates
(particularly the lower one) will alter the curve dramatically.
I suggest that before you can draw any conclusions about NELM
vs. BLM, you need to refine your bright-sky NELM estimation
methods, and you need a bunch more data from bright nights.
It wouldn't hurt to have some data from truly dark skies either,
to stretch the NELM range on the other end. Travelling to
brighter and darker sites would be a handy way to obtain
such data without having to rely on the Moon.
- Tony Flanders
edz
>In practice, your curve depends utterly and totally
>on those two estimates of 4.0 and 4.5 near the full moon in
>September
Well that's not entirely accurate, so this will help clarify. See the
clarifications dated Oct 6. I refer to the observations on Sept 25
that I have included in my analysis, but did not include any written
text for you to know the data exists. I clarified that in Oct 6
notes.
This is a summary of what I have for bright skies listed as <mag5.0,
total observations and total critical observations (stars no more than
0.3mag below max)
Sept 10. NELM 4.37 approx 40 observations 17 critical
Sept 12 NELM 4.43 approx 32 observations 13 critical
Sept 12 was mag 5.0
Sept 25 NELM 4.68 approx 80 observations approx 20 critical
Because the analysis uses only the maximums it may seem like there is
not a lot of data, but there is atleast 50 critical observations from
3 bright nights and that's not really a small amount.
>I suggest that before you can draw any conclusions about NELM
>vs. BLM, you need to refine your bright-sky NELM estimation
>methods, and you need a bunch more data from bright nights.
No question there is always room to refine. As I said, In reality I
already have included in the analysis the maximums from 50 critical
bright night observations, but only over three bright nights. I did
go out and collect some more data on another recent opportunity to
check viewing under bright sky. Here is that data.
Oct 5. conditions deteriorated from 4.68 at 8pm to 4.43 at 9:30pm
Recorded NELM several times throughout session. In vicinity of
observations started out with 4.68. No 5.0 or 5.05 or higher stars
were ever visible and NELM never got back up above 4.43Del and
4.37Sge. Cr399 (5.15) was not visible. In Ursa Minor I observed one
5.0 star.
Oct 5 NELM 4.43dir, 4.68avr, approx 60 observations 14 critical
Limits reached
8x42 = 9.5,
16x70 = 10.6(didn't try any higher)
Oct 5 NELM 4.43dir, approx 70 observations 15 critical
Limits reached
8x42 = 9.1(also 9.3/9.1 Pair),
12x50 = 10.23,
16x70 = 10.76 (tried many more for all three binocs)
For comparison, Best BLM observed under mag5.8 skies, reported in
article
8.42 = 9.75
12x50 = 10.5
16x70 = 10.83
For comparison, Best BLM observed under mag4.4 skies, data used in
article
16x50 = 10.1
16x60 = 10.3
15x70 = 10.3
16x70 = 10.5
Oct 5. NELM 4.43, 125mm/1250mm G5 SCT scope, testing for maximum Lim
Mag for the night. mag12.0 star in Cr399, not vis at any mag <100x,
tried (50x, 60x 70x 80x 100x), at 140x mag12.0 star glimpsed averted,
at 170x mag12.0 star seen constant but just barely. This scope has
been tested many times on the Brian Skiff M57 photometry, on a few
occasions of absolutely best local conditions (NELM 5.8, I have never
ever recorded a mag 6.0 NELM star locally) has reached 13.1mag, have
never recorded anything deeper.
Oct 5 NELM 4.43, 78mm/480mm AT1010 Ref scope, testing for maximum Lim
Mag for a 78mm scope at low power. Used a 15mm TV plossl for 32x
magnification. Stars of 10.5, 10.52 and 10.56 seen readily, stars of
10.76 and 10.8 seen but not easy, 10.86 barely seen, 10.93 only
glimpsed, 11.0 and 11.2 not seen after many tries.
The 78mm scope is closely equivalent to the 16x70 binoculars. In NELM
4.43 skies, It reached a max Lim Mag of 10.93 at a magnification of
32x (more powerful than binocs). In NELM 4.43 skies, the 125mm scope
reached 12.0 mag but only at 140x to 170x.
For comparison purposes in mag 5.8 skies, using the 78mm scope two
mag11.0 stars were seen at 40x. A mag 11.3 star was seen at 40x. A
mag 11.4 star was not seen at 40x, however it was just barely seen at
65x.
For comparison to the 125mm scope, when it was used in mag 5.8 skies
the same mag12.0 star was not seen at 50x, but was glimpsed at 76x and
was seen steady at 110x. Under mag best ever skies, the 125mm scope
reaches mag13.1 only at very high powers about 175x to 200x.
Scopes show a much wider range in Lim Mag vs NELM, but only at higher
magnifications. Scopes show scope Lim Mag follows much more closely
in line with NELM.
Binoculars show almost the same results recorded on earlier bright
nights. BLM does not drop nearly as much as NELM.
So from this it can be seen that scopes follow what you are saying Lim
Mag closely mimics NELM. But the binoculars still prove to not be
following that rule.
I'll keep trying, but I keep getting the same thing.
edz