FreeLists / az-observing / [AZ-Observing] Re: Aperture vs. Dark Sky

[BillFerris@xxxxxxx]

Jack Jones wrote:
>Neat article in the Aug 2003 S&T by Bill Ferris about Aperture vs. Dark
>Sky. I'm going to check all those test galaxies on our sites. I agree
>with the conclusion and always felt small scopes especially were truly
>transformed by a good dark sky site.

Thanks, Jack, glad you enjoyed the article. I think the naked eye offers a 
great illustration of the role a dark sky plays in observing extended objects. 
This past weekend, numerous observers around Arizona were treated to some truly 
spectacular naked eye views. The North America nebula, just to borrow one 
example, is an obvious naked eye object from a true dark sky site. But under 
urban skies, forget about seeing this object in any aperture.

>Nobody ever explained to me properly I guess just what exactly is a
>square arcsecond of light? Magnitudes per square arcsecond (MPSA) 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.

One of the challenges I've been mulling over is finding a way to convert a 
naked eye limiting magnitude estimate to a sky surface brightness value. It's a 
challenge because, even among a group of experienced observers, naked eye 
limiting magnitude estimates can vary significantly. See Bradley Schaefer's 
1990 PASP paper for an illustration of this.

At best, I suspect such a conversion would get an observer in the ballpark. 
Here's a table I'll throw out for commentary. I'd be interested in knowing if 
this at all reflects the real life experiences of observers:

NELM.(+/- 0.5)..===>..Sky Brightness (mag./sq. arc sec.)
.......8.0............22.0                
.......7.0............21.0                
.......6.0............20.0                
.......5.0............19.0                
.......4.0............18.0

The scale assumes a naked eye limiting magnitude of 8.0 (+/- 0.5) under a dark 
country sky and logarithmic relationship between sky brightness and NELM: for 
every full magnitude change in sky brightness, there is a full magnitude change 
in NELM.

The faint limit is actually well-established. The darkest the sky gets anywhere 
on the planet is 22.0 mag. per sq. arc second. Observers with acute vision have 
been known to go as faint as about 8.0 mag. (+/- 0.5 mag., depending on the 
observer) under such conditions. Hence, the choice of an NELM of 8.0 with a +/- 
0.5 magnitude range.

The sky surface brightness over Mars Hill on the west side of Flagstaff, 
Arizona, has been measured by Brian Skiff at 20.3 magnitude per square arc 
second on a moonless, clear night. His NELM from this site is about 6.4 
magnitude. It's only one data point but it falls within the NELM range for a 
sky brightness of 20.0 mag. per sq. arc second.

A sky brightness of 18.0 mag. per sq. arc second corresponds to conditions at 
Mars Hill during full Moon. From my home in north-central Flagstaff, about 3 
miles east of Mars Hill, I can still see most of the stars in the Little Dipper 
asterism under a full Moon. Also from home, I've seen M44 (3.1 mag, 95' 
diameter) with the full Moon just 25-degrees away in Leo. A naked eye limit 4.0 
magnitude (+/- 0.5) seems a reasonable choice for a bright sky with an 18.0 
mag. per sq. arc second surface brightness.

Regards,

Bill Ferris
Flagstaff
--
See message header for info on list archives or unsubscribing, and please 
send personal replies to the author, not the list.