Re: Ranking light pollution

From: David Knisely (ka0czc_at_navix.net)
Date: 06/25/04 

Date: Thu, 24 Jun 2004 23:39:59 -0500

Craig Frank posted:

> Is there a standard system for ranking light pollution based on
> limiting magnitude? The Southern exposure of my apartment
> complex lets me see stars down to 4.5. I was wondering what
> this would correspond to as light pollution goes.

I would say that this is probably in the moderate category of light pollution.
  There are worse, but there are also considerably better conditions.
I came up with an idea of gauging sky quality a while back which divides it
into several levels: Severe, Moderate, Mild, Dark Sky, and Pristine. I would
call Severe light pollution a situation where (even in the absence of direct
lighting), only stars magnitude 3.5 and brighter were visible to the unaided
eye with considerable skyglow in all parts of the sky. Areas like some places
in New York City or central Los Angeles could fit this classification.
     Moderate light pollution is even more difficult to quantify, but some
limited deep-sky observations are possible in such an environment
(double stars, brighter open clusters, brighter planetary nebulae, ect.). I
would probably put the moderate light pollution at a limiting magnitude of
between 3.6 to 4.5 or so with varying amounts of skyglow in different
directions, although no part of the sky would appear very dark. A "mild"
light pollution level would probably be at the point where a person could see
4.6 to 5.5 or so in some areas of the sky, and where the brighter portions of
the Milky Way high above the horizon might be visible at times. Various areas
around the horizon would show skyglow, but overhead and perhaps in a few other
areas, the sky brightness would appear noticably darker. I think that
probably most of the Messier Objects would be visible in a good telescope over
4 inches in aperture with mild to moderate light pollution levels, although the
detail visible in them might be a bit limited. The number of faint galaxies
visible in a 6" or 8" takes a big jump when you get down to the mild light
pollution level. Some light pollution filters can help combat the effects of
low to moderate light pollution to a degree.
    I would consider a "dark sky" site to be a place where you can see stars
around 6th magnitude or so (occasionally even fainter) with averted vision
routinely, but where there may be a few limited light domes from nearby
cities visible. At such sites, detail in faint deep-sky objects becomes a
good deal easier to see, and the Milky Way shows up prominently. My dark sky
site routinely gets me to +6.5 naked eye, and occasionally fainter. Skies
which routinely allow naked-eye sightings fainter than +6.5 I would consider
"pristine". Usually, these places are located well away from any major
cities, and at higher altitudes. Little or no light pollution is visible
over the entire sky at such pristine sites. For example, the Nebraska Star
Party site is deep in the sparsely-populated Sandhills at 3100 feet
elevation (the nearest sizable "city", North Platte, Nebr., pop. 24,509, is
100 miles to the south). At that site, I have seen stars as faint as +7.5,
and others have gone to 8.0. Similar such feats are possible in many areas,
especially in the Rockies, and in the high desert areas of the American
southwest.
    In summary, the following are the approximate unaided-eye visual magnitude
limits (Zenith Limiting Magnitude) and overhead light-pollution ratings for
judging light pollution:
SEVERE: only mag. 3.5 or brighter stars are visible with fairly bright skyglow
over most of the sky.
MODERATE: mag. 3.6 to 4.5 visible (variable skyglow).
MILD: mag. 4.6 to 5.5 visible (some notable darker areas visible).
DARK SKY: mag. 5.6 to 6.6 visible (dark with occasional light domes).
PRISTINE: mag. 6.7 and fainter visible (little or no light pollution).

If you want to get the most out of Deep-Sky with a telescope, get to as dark
a site as you reasonably can (even if there is some light pollution), and
then work with what you have. You may be surprised as to how well you do,
even from a non-optimal location! Clear skies to you. 

-- 
David W. Knisely  KA0CZC@navix.net
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/
**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
*    July 18-23, 2004, Merritt Reservoir     *
*      http://www.NebraskaStarParty.org      *
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