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Naked Eye Limiting Magnitude

One of the most striking things about the Nebraska Star Party was the gloriously dark sky conditions. Having grown up in the Rockies I've seen my fair share of dark skies, and the NSP can compare with the finest places in the country. On the morning of July 28, 1995, during the second NSP, I selected a region of the sky near the zenith, without prior consultation of star charts, and drew all the stars that I could see in a reasonable area of the sky. The region I selected was the "head" of Draco, a.k.a. the "lozenge". A rendition of the sketch in my notes is given below:

In the chart above, Beta, Gamma, Nu, and Xi are the traditional "lozenge" stars. The various Roman letter labels are arbitrary, and used only for my own reference. On checking the star field in Sky Atlas 2000, I was surprised to find that the faintest stars I had seen had the smallest dot size in the atlas, that is, they were 8th magnitude! (I knew that I had to upgrade to Uranometria for telescopic observing from sites like this, but I had never considered the possibility that this might be necessary for naked eye observing :-) The faintest star plotted has a photometric magnitude of +8.15.

The magnitudes of the stars labeled with Roman letters are given below, using values obtained from the Yale Bright Star Catalog for stars brighter than approximately +6.5 and the SAO Catalog otherwise. The magnitudes in the SAO are known to be in error by several tenths in many cases, so more accurate measurements are needed to get a precise value for the limit. In 1996, Brian Skiff obtained high-precision photometric data for five of these stars; these measurements are also listed.

Star     Magnitude (Catalog)    Magnitude (Photometric)

A        5.67
B        6.6
C        7.8                    8.06
D        5.99
E        8.0                    8.15
F        7.2
G        5.02
H        5.75
I        7.5                    7.609
J        6.6
K        7.2                    7.027
L        (*)
M        7.5                    8.01
N        6.10
O        7.1
P        7.1
Q        5.95
R        6.9

In a few cases, there were two or more stars near the plotted position; in this case, the magnitude of the brightest of these stars was used. The star labeled "L" could not be reliably identified with a single star in the area (several faint candidates were nearby, but none could be identified unambiguously).

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