FreeLists / az-observing / [AZ-Observing] Re: Mag sequence was:(Saturn's Rings)

[Brian Skiff <bas@xxxxxxxxxx>]

     There are indeed a lot of factors going into the magnitude-sequence
testing and results.  Apart from observer actuity and skill, and hardware
circumstances, there are the atmospheric effects:  not only does higher
airmass (i.e. lower altitude) reduce brightness, but also the seeing is
inevitably worse, and the sky background itself is brighter.  These latter
two effects both reduce _contrast_, and my qualitative feeling is that
they reduce thresholds much more than the simple loss of brightness, which
is only a few tenths of a magnitude even at 60 deg from the zenith.
     Another point to make is that this sort of test benefits greatly from
experience, doing it on a bunch of nights on the same test field.  Back
in the late 80s when Brad Schafer had people looking at the M67 test field,
I found that by the third night of looking at it (under essentially
identical conditions at Anderson Mesa) I found I could see much fainter
than on the first night, probably simply from becoming familiar with the
field.
     My experience using my Pronto (recently) and my 15cm refractor (in the
past) is that galactic latitude also makes a differenece in threshold
limits.  This is just the loss-of-contrast thing again, but this time not
from Earth's atmosphere, but instead the bright background of the Milky Way
caused by all those mag. 18 and fainter stars you can't see in the eyepiece.
Thus it is much easier for me to get to mag. 14.0 with the Pronto on the
sequence around NGC 2419 (from the Luginbuhl & Skiff book) than the one
around M57.  That V=14.1 star southeast of the Ring is damn tough in the
Pronto, whereas similar stars in the little quadrangle around NGC 2419
(or photometrically-observed stars near galaxies in Virgo) are much easier.
     Frank's use of 400x on his 10-inch was not too much power.  I did best
with my 6-inch at 300x, although views of deep-sky objects were much more
satisfying at 150x-200x.     
     As has been pointed out, the value in making such tests is not the
number, per se, but that for most folks simply doing it regularly is one
exercise in helping you become a better observer, because you'll pick up
skills that will instantly let you see more in a quick glance than you
did previously.
     There plenty of other similar magnitude sequences available around
the sky (now that M57 is well past the meridian after dusk).  But I know
of few that are done around the most-observed objects like M57.  The ones
Chris and I put in our book were purposely around not-the-usual clusters
(M3 being an exception) since we were hoping to encourage readers to get
away from the same-ol' objects.  It would be nice, however, to have three
or four more spread evenly in RA around the sky.

\Brian
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