Return to Home Page
>> The studies used a 60% detection rate as the threshold of detectability.
QED. You're basically quoting the venerable lab stuff that was done
50+ years ago by Blackwell. It's still good, but needs to be put in the
astro-viewing context. Roger Clark and Brad Schaefer have done a lot of
work in this area that you should read. Try these for starters:
Brad Schaefer 1989, "How faint can you see?", Sky & Telescope, volume 77,
page 332 (March 1989).
Brad Schaefer 1989, "Your telescope's limiting magnitude", Sky & Telescope,
volume 78, page 522 (November 1989).
Brad Schaefer 1990, "Telescopic limiting magnitudes", Publ. Astron. Soc.
Pacific, volume 102, page 212.
Roger Clark 1994, "How faint can you see?", Sky & Telescope, volume 87, part
4, page 106 (April 1994).
Roger Clark 1990, "Visual Astronomy of the Deep Sky", Cambridge University
Press
Once you get to Roger's book, which is out-of-print and maybe hard
to find in libraries, skip to Mel Bartels Web site:
http://vsnet.efn.org/~mbartels/aa/visual.html
...where you can find excellent discussion about visual thresholds and
so forth, and where you can find out the errors Roger made in his book
(which he admits to). This page contains Roger's threshold vs detection-
probability table from the 1994 S&T article. The 50 percent detection
limit for 10cm (4-inches) aperture is indeed 13.7, as Mike quoted.
My 14.5-to-15 figure also indeed corresponds to 10 percent by his table.
There's a link at Bartels' site to the very long exchange about this
problem between he, Nils Olof Carlin, Harald Lang, and Roger Clark. The
math gets very serious in that one. Likewise there's a link to my not-so-long
description of how-dark-is-dark re best observing sites. Not included
are my brief notes about the Heber Curtis mag limit tests from Lick in the
early 1900s nor about Dave Nash's double-blind test at the Nebraska Star
Party some years ago. Both the latter indicate reasonable limits of V=8.2
or so for typical eyes (Nash says his vision is not particularly sharp).
If you look at the papers cited in my sky-brightness article, the ones
from the 1950s cited near the beginning should convince you that the range
of retinal sensitivity is very small among normal individuals. This and
the pupil-diameter frankly have rather little to do with visual thresholds,
instead physio- and psychological factors weigh heavily. Basically, the
experienced old observers with 4-5mm pupils see just as faint as younger
folks with 7-8mm pupils. And of course telescopically pupil size doesn't
enter into it at all (you want to use exit pupils of 1mm or smaller).
I agree that the photographic V sequences around the active galaxies
could be problematic (the ones from Asiago that were commonly used have
large scatter and scale errors). Also I have trouble with quoting
magnitudes for variable objects, when you don't actually have reliable
contemporaneous V-band photometry. It is much better to use fields with
stable ordinary stars. The one produced by Arne Henden around M57 that
we published in the September 2001 issue of Sky & Telescope (page 102)
is widely used by northern deep-sky observers, but there are plenty of
others all over the sky. (I can supply some if you want to compare with
V-based AAVSO charts.)
\Brian
Return to the Powerful Daisaku Nogami
vsnet-adm@kusastro.kyoto-u.ac.jp
