[AZ-Observing] Re: Aperture vs. Dark Sky
- From: Brian Skiff <Brian.Skiff@xxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Wed, 2 Jul 2003 19:00:36 -0700 (MST)
A lot of this stuff is dealt with 'in extenso' at Mel Bartels'
Web site, which I think is cited in Bill's article.
In re getting an at-the-eyepiece feeling for surface brightness,
a lot of people find it is easier to reckon in units of square _arcminutes_
rather than arcseconds. The main reasons are that an arcminute is easier
to actually see in an ordinary telescope, and also the numbers are
more in the range you're familiar with. (There are, of course, 3600 square
arcsec in a square armin, so [do the arithmetic] the numbers are 8.9
mag. "brighter" for the same brightness level.
An easy way to get a feeling for, say, mag. 14 per square arcminute,
is to find stars in a photometric sequence (such as in L&S, Roger Clark's
book, or the M57 sequence we had in S&T a couple years ago), and defocus
a mag. 14.0 star carefully until it's an arcminute across. _That's_ what
mag. 14 per square arcmin looks like (pretty faint!); it's also
equivalent to about mag. 23 per square arcsecond.
>> Magnitudes per square arcsecond...eludes me as a measurement of darkness.
>> Why is the surface brightness of light
>> polluted sky at 18th magnitude. How far can it go? What's daytime,
>> what's the limit? Stuff like that.
A lot of "stuff like that" is in the series of old s.a.a. responses
I did some yearrs ago, which Jerry Lodriguss has tidied up and posted
at his Web site:
http://www.astropix.com/HTML/L_STORY/SKYBRITE.HTM
...which came along following the every-couple-of-weeks discussion on
s.a.a. about "perfectly dark" skies and misconceptions about the topic.
(Looks like the link to Gordon Garradd's site has moved, sorry.)
\Brian
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