Telescope Reviews: Light Pollution Rating

Well, the Bortle scale is rather inconsistent in the items it uses to measure the "levels", so I would rather go with a Zenith Limiting Magnitude figure coupled with a simple description (or better yet, the SQM readings). I once came up with this scale:

Re: Judging light pollution. It is really tough to give a judgment of the exact level of sky quality. I guess I could divide it into several levels: Severe, Moderate, Mild, Dark Sky, and Pristine.

SEVERE: Severe light pollution might be a situation where (even in the absence of direct lighting), only stars brighter than magnitude 4.0 would be visible to the unaided eye overhead, with considerable skyglow in all parts of the sky. The sky background would be bright (especially nearer the horizon) with perhaps just a slight dimming of the glow overhead. Such conditions maybe found in some areas in some of the central portions of major cities like New York or Los Angeles.

MODERATE: Moderate light pollution is rather difficult to quantify, but some limited deep-sky observations are possible in such an environment (double stars, brighter open clusters, brighter planetary nebulae, etc.). I would probably put the moderate light pollution at a limiting magnitude of from 4.0 to 4.9 or so with varying amounts of skyglow in different directions, although no part of the sky would appear very dark. The horizon would still be fairly bright overall, making deep-sky observations lower than about 30 degrees in altitude more difficult.

MILD: A "mild" light pollution level would probably be at the point where a person could see 5.0 to 5.9 or so in some areas of the sky, and where the brighter portions of the Milky Way high above the horizon may be faintly 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 noticeably darker, although perhaps not exactly fully dark. M31 would be visible to the unaided eye with averted vision when well above the horizon, although it would usually be perceived as just a small faint fuzzy spot. I think that probably all 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.

DARK SKY: I would consider a "dark sky" site to be a place where you can see typically see stars 6.0 to 6.8 with averted vision routinely and large parts of the sky appear quite dark, but where there may be a few limited light domes from nearby cities visible. The Milky Way would show up prominently with a sort of granular texture. M31 would appear noticably elongated to the unaided eye, and occasionally, M33 would be glimpsed with the unaided eye as well when well above the horizon. At such sites, detail in faint deep-sky objects becomes a good deal easier to see, and the number of objects visible in the telescope goes up dramatically. My dark sky site routinely gets me to +6.5 naked eye, and occasionally fainter.

PRISTINE: Skies which routinely allow naked-eye sightings of magnitude +6.9 and fainter 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, and the Milky Way shows some faint detail which is usually shown only in photographs. 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 my scale of judging light pollution:

SEVERE: only stars brighter than mag. 4.0 are visible with bright skyglow over most of the sky.
MODERATE: mag. 4.0 to 4.9 stars visible (variable skyglow depending on direction of observation).
MILD: mag. 5.0 to 5.9 stars visible (some notable darker areas visible).
DARK SKY: mag. 6.0 to 6.8 stars visible (dark, sometimes with a few light domes along the horizon).
PRISTINE: mag. 6.9 and fainter stars *consistently* visible (little or no light pollution in any direction).

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.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info