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[here are my two notes with results about naked-eye magnitude limits,
which have posted around the Net for about as long as there's been the Web.
Note that these results are entirely consistent with the limit Mike Linnolt
gave, but simply apply to lower detection probabilties. They are also
consistent with other personal anecdotal experience. For instance, several
times at the Texas Star Party, I would get different folks oriented with the
sequence in eastern Coma mentioned below, and giving them general directions
and not saying anything about magnitudes, have them try to spot progressively
fainter stars. Even the relatively inexperienced observers, people you
wouldn't think of as being hot-shot deep-sky observers, typically start
having problems only once you got close to V=8.0. Similarly, observing the
same region very casually at a dark site in southeastern Arizona, a friend
pointed out a star I hadn't mentioned---one he didn't know the location of and
certainly didn't know the magnitude of, but which I did: V=8.1. It's a star
I know the location of, but have never been able to see.
Anyway, on to Dave Nash and Heber Curtis... ]
In re naked-eye magnitude limits, here is a post I made to the
sci.astro.amateur newsgroup several years ago. At that time, the
Hipparcos/Tycho photometry had not been published, so the observer not only
did not know where the stars were but also could not have known their correct
V magnitudes, since they had not been measured, and traditional astrometric
catalogues contained values up to 0.5 mag. in error.
-------------------------------------------------------------------------------
A couple of months ago, in the s.a.a. thread of discussion comparing the
merits of sky quality and darkness of the Texas Star Party versus the Nebraska
Star Party, Dave Nash claimed to have seen some mag. 8 stars from NSP last
August. The test was "blind" in the sense that he made a sketch of a small
area of the sky without looking at an atlas to select stars. (As will be
seen, however, Dave is far from being blind.) The region is near the head of
Draco, the asterism sometimes called the "lozenge". Upon comparing with the
Tirion "Sky Atlas 2000", he was surprised to find that the most difficult
stars were marked with the smallest symbols on the chart, corresponding to
roughly mag. 8.
At my request, Dave supplied IDs for the specific stars involved. Only
the brightest star involved had any reliable published photometry, so I was
curious to find out just how bright these stars really were. I promised to
observe them photoelectrically at the first opportunity. I was able to do so
about 3 o'clock this morning, and the results are summarized below.
For the record, I used the Lowell 53cm photometric telescope, Stromgren
b and y filters, and a 29" diaphragm. I observed Dave's stars together with
nine primary and secondary Stromgren standards, whose colors extended beyond
the range of colors of Dave's stars. The y filter observations are
transformed to standard Johnson V magnitudes, as is usual in this sort of
work. The Stromgren y filter has the same central wavelength as a V filter,
but is only ~200A wide, instead of ~1000A wide. For what it's worth, my
several hundred observations of Landolt equatorial standards show that I match
his V system systematically to within a couple thousandths of a magnitude over
the color range -0.28 < B-V < 1.75. As a further historical aside, the
original primary standards for the UBV system were measured on the same 53cm
telescope in the mid-1950s by Harold Johnson and Daniel Harris. This morning
the rms errors of the linear fits to V magnitude and b-y color were +/-0.009
and 0.008 mag., respectively. I would have preferred to use more standards,
but these nine were entirely adequate for the purpose at hand.
During the course of a general survey of mag. 6-8 stars, I had previously
measured the two brighter stars three and two times each, respectively. This
morning's results agree with those observations well within their mutual
errors, so I am confident that the single observations of the fainter stars
are reliable to within about 0.015 mag. or better.
Name RA (2000) Dec V b-y spec SAO HD BD
HD 160520* 17 37 14 +55 44.5 7.027 0.710 K0 7.2(GC) 7.18 7.4
.006 .003
HD 162131 17 46 30 +53 34.7 7.609 0.088 A2 7.5(AGK1) 7.34 7.5
.005 .001
HD 163010 17 50 31 +57 26.6 8.01 0.01 B9 7.5(AGK1) 7.9 7.7
HD 161353 17 42 13 +52 03.4 8.06 0.58 K0 7.8(AGK1) 8.1 7.9
HD 161179 17 41 23 +51 28.3 8.15 0.77 K2 8.0(AGK1) 8.1 7.9
* for HD 160520, Kornilov et al. (1991) give: V = 7.050, B-V = 1.158, from
four observations.
The table lists the stars along with 0'.1-precision positions. The new
magnitudes and colors come next. For the two multiply-observed stars, I give
the rms scatter of the data on the second line of each entry. For the fainter
three stars, I give the results to just two decimals. The spectral types are
listed directly from the HD catalogue, which is the only source of types for
these particular stars. My b-y colors combined with the spectral types show
that the sample consists of two early-A type stars (probably dwarfs) and three
garden-variety K-giants.
The final three columns are "visual" magnitudes from three sources: the
SAO star catalogue, the HD catalogue, and the BD. The original source for the
SAO magnitudes is given in each case. As you can see, the magnitudes in the
early catalogues aren't _so_ bad, but nevertheless usually report the stars
to be brighter than they really are, and scatter around "truth" by up to half
a magnitude.
In summary, Dave saw three mag. 8 stars from Merritt Reservoir, which
shows not only that NSP has a great site, but that Dave is a skilled visual
observer and also probably has acute (sharp) vision. He probably can see
similarly faint from similarly-good sites, such as TSP, or other high and dry
dark places. I've been observing from "perfect" sites in the desert Southwest
US and Chile for twenty years and haven't been able to go fainter than about
V=7.8, so my hat's off to ya, pal!
===============================================================================
[posted to the 'amastro' list]
This second note deals with Heber D. Curtis' observations of some
faint stars from Lick at the turn of the (last) Century. In this case,
the stars were selected ahead of time, and a large black mask attached to a
telescope (the 12-inch refractor at Lick). The mask contained a hole through
which Curtis viewed from the observing floor, and swept across the star
location in search of it. This isn't a good test of finding faint stars at
unkown locations, but as with the Nash test the star magnitudes were very
poorly determined, so he could not know how faint he was seeing on the now
standard scale.
For some years Brent Archinal and I have wondered about the naked-eye
limiting magnitudes determined by Heber Cutis at Lick Observatory at the turn
of the Century. He claimed to see down below mag. 8, but we wondered what the
modern standard V magnitudes of those stars were. I've dug out the relevant
publication (1901 Lick Obs. Bulletin, 2, 67), and have looked up the stars
(fortunately a short and well-identified list). The names are given below
along with V and B-V from the Hipparcos/Tycho catalogues, and the magnitude
Curtis gave for the same stars. I use the H/T data for convenience; they
ought to be reliable to within a couple percent, and comparison with data
collected in SIMBAD from ordinary sources suggest these numbers are fine for
the present purpose. Curtis's comments are also shown for most of the stars.
As can be seen, the faintest star he saw reliably is V = 8.44, and two
others below V=8.0 and one at 7.98 were seen as well. As might be expected,
he did rather better overall on the near-overhead field around T UMa than on
the T Vir field, several degrees south of the Equator (Lick is at about
latitude +37.4). Just that alone tells you a lot about observing other than
close to the meridian.
These in essence reproduce the results of Dave Nash at the Nebraska Star
Party several years ago, when he did a double-blind test using stars in the
head of Draco, and saw down to about V=8.2. On winter and spring nights at
our Anderson Mesa site, I use a star in Coma at V=7.8 as a transparency test,
and usually see it.
group near T Vir
Star V B-V Curtis remarks
HD 106384 6.56 0.28 6.52 [FG Vir, sl var]
HD 107830 7.19 0.43 7.20 seen easily
HD 105654 7.23 0.40 7.31 seen quite easily
HD 106515 7.34 0.82 7.42 seen easily
HD 106622 7.47 0.93 8.1 seen without difficulty on last two nights
HD 106579 8.44 0.44 8.3 seen with considerable difficulty; perhaps
one-fifth of trials failed
group near T UMa
HD 110275 7.98 0.24 8.1 seen; one or two failures
HD 110408 8.08 0.53 8.2 seen
HD 110104 8.21 1.12 8.3 seen with difficulty
BD+60 1415 8.98 1.35 8.5 glimpsed at intervals; very doubtful
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