Visual Night Rating
to be rolled down
charts from © Guide
7 & CE, 1998.
Some readers were interested so why not make these charts
available to every one? The charts above are destined to the observer who
wants to estimate the transparency of the night sky (a possibly useful
preliminary for an observing session). You will easily recognize Polaris
centered in the charts with the starting of Ursa Minor upward. My recommendations
are to use averted vision in order to find the faintest star and estimate
its visible time (eg. 50% of the time, 25%, etc.), and to limit the visual
exploration time to 5 minutes (15 minutes if you use the charts for the
first time and then less when you get used to it); this limit is proposed
as a kind of standard so that most of the observer are nearly equal regarding
that consideration (and also to allow for maximum observing time afterwards).
For exemple, from my usual observing site (in the french Alps, elevation
1650m ie 5500ft, good transparency and very little light pollution), I
often spot the "o" star of V6.31 for 25 to 50% of the observing
time. The values given for visual magnitude are from Tycho/Hipparchos data
(that means the most valuable and homogeneous data obtained at the present
time). The chart was drawn by Guide 7 software and the scale is 1mm=0.51°. Susan French (Sue_and_alan_french@msn.com) notes that star "t" is a suspected variable (photographic range: 6.6 to 7.1).
SAO 308 (Alpha UMa; Polaris)