Limiting Magnitude in Binoculars

Tony, this does not answer your latest post but provides additional
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

(edz) wrote in message . com...

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

Tony,

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