(IAAC) Fwd: [amastro] Re: limiting magnitude at WSP

[Forwarded from 'amastro' posting #1724:
Lew Gramer wrote:
>And in fact, for the same INSTRUMENT and conditions, I'd be interested to
>hear what relationship holds between unaided and telescopic LMs across a
>sample of several different observers. With all-important individual
>variations accounted for, I suspect these two numbers are well linked.
Are you asking about the correlation between naked-eye limiting magnitude
and telescopic limiting magnitude, among a selection of different
observers?. Here is some data from the same night, names changed to protect
the innocent:
            NELM      10"LM
Me:         6.8       15.9
ObsW:       5.9       15.7
ObsX:       6.3       15.9
ObsY:       6.6       15.8
ObsZ:       5.8       15.9
Scatter:    1.0        0.2
This was done at a rural Arizona site (my backyard) about six months ago. I
have a significant light dome to my NW, somewhat similar to the way
Flagstaff looks from Anderson Mesa for those who have been there. We took
the NELM from near the zenith and the altitude of our telescopic field
varied between 70 and 80 degrees during our observations. The seeing on
this night allowed us to split a 1 arcsecond double with a magnitude
difference of .2, so the seeing was pretty good. My entirely subjective
impression of the evening was that the transparency ran a little to the
hazy/dusty side.
The star magnitudes are in V, and come from Brian Skiff's faint star
photometry list (I forget what you call that, Brian - the one you plug on
MPML from time to time). The observers simply sketched the field, and I
identified the stars the following morning.
Its only one night and only five observers, but the numbers support a
belief that I have held on more subjective grounds for a dozen years. To
put it cautiously: there is no obvious relationship between naked eye
limiting magnitude and telescopic limiting magnitude at sites that approach
truly dark. I have a laundry list of reasons why I think this occurs. The
two biggest are the ability to control the apparent brightness of the
background through magnification, and the ability to use only the good
parts of your eye telescopically (specifically the good parts of the
cornea/lens apparatus, which for most of us is pretty bad even at 4 or 5 mm
My (also more subjective) observation is that scatter in this sort of thing
goes up in light polluted skies or heavily nontransparent skies, and at
bright sites there probably is a correlation between naked eye limits and
telescopic limits.
Jeff Medkeff
Hereford, Arizona
Have Laptop, Will Travel
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