Telescope Reviews: Ground Truth for the Bortle Scale

It's always been tough to tell other people how dark or light-polluted your favorite observing site is. Descriptions like "pretty dark" or "very bright" convey nothing at all. Skies that appear dark to a city dweller may seem horribly light-polluted to someone from Wyoming.

Traditionally, people have described their skies in terms of naked-eye limiting stellar magnitude (NELM), which conveys something -- but still not much. NELM estimates of identical skies vary by at least one full magnitude among experienced observers, and by much more if novices are included.

To my mind, the best subjective measure of sky brightness is the Bortle Scale. This incorporates NELM and several other subjective indicators such as the visibility of the zodiacal light, the Milky Way, and certain key deep-sky objects. Bortle ratings still vary a lot depending on the individual, but less so than NELM estimates.

However, I and many other experienced observers think that the Bortle Scale has some serious internal inconsistencies. Truth be told, it's very easy to pick holes in the Bortle Scale, but a lot harder to fix them. Only someone with intimate knowledge of all 9 brightness levels is really qualified to judge the system as a whole. And there may be nobody on Earth -- certainly not me and probably not Bortle -- who can claim such knowledge. This makes me wonder if a community like Cloudy Nights can accomplish what no individual can.

So I will describe the four observing sites where I've done the lion's share of my observing in as much detail as possible. And I encourage other people to do so two.

I've incorportated two objective measures which I find quite valuable. First, the color zone assigned to the site in the World Atlas of Light Pollution. If you live in North America, you can find your zone using Jonathan Tomshine's Dark Sky Finder. One of the things that I'd like to find out by this exercise is how reliable and/or meaningful these zones are.

Second, readings taken by Unihedron's Sky Quality Meter (SQM) provide an objective measurement of skyglow at the zenith. But this needs some caution; it's quite easy to get invalid readings with this instrument. Dark obstructions (e.g. trees) more than 30 degrees above the horizon make the readings darker than they should be, and ambient lights more than 10 degrees above the horizon (including the Moon) can make the SQM go completely haywire. In some places, it's extremely hard to find sites that don't have obstructions and/or lights. The new SQM-L has a built-in lens giving it a much narrower field of view, and making it much less prone to these problems.

Also, artificial skyglow varies depending on atmospheric conditions, and it falls steadily over the course of a night as lights are turned off and car headlights thin out. And in areas with deciduous trees and/or snow, there's a huge seasonal variation. Early evening after a fresh snow is more than 1 magnitude brighter than 3 a.m. in the summer. To normalize for all of this, I propose that SQM measurements be taken on Moonless nights with good transparency around 11 p.m., and only when the trees are fully leafed out. Just about all stargazers are up and about when conditions are like that.

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My closest site is Danehy Park, in Cambridge MA, deep inside the white zone, SQM 17.9. It's about 4.5 miles from the center of Boston, a city of 700,000 in the center of a metro area of some 5 million. I can typically see stars to mag 4.7 here. M31 and the Double Cluster are just barely visible naked-eye when they're well placed in the sky, but I've never seen the Milky Way. This is a pretty good match for Bortle Class 8 aside from Bortle's ridiculous statement about only the brightest Messier objects being detectable. In fact, I've seen almost all of the Messier objects here with my 7-inch scope, and most of them with my 70-mm scope.

My second closest site is Robbins Farm, in Arlington, MA, also deep inside the white zone, SQM 18.4, 7 miles from the center of Boston. I can typically see stars around 5.1 here. I can just see the Cygnus Star Cloud when it's directly overhead, but for all practical purposes, the Milky Way is invisible. Robbins Farm is a fairly good match to Bortle Class 7 -- one of the few really plausible fits that I've found between Bortle's descriptions and my own experience.

Although Robbins Farm is quite a bit darker than Danehy, I think they clearly belong in the same category, with Robbins Farm at the dark end of the category. Both naked-eye and telescopic views are qualitatively quite similar at both sites, though everything is a smidge or two easier at Robbins Farm. But I'd say that Robbins Farm is near the dark edge of its category. As soon as sites gets significantly darker, the Milky Way becomes visible, deep-sky observing really starts to open up, and the whole experience becomes qualitatively different.

My club (ATMOB) has an observing field in Westford, MA, on the red/orange border, SQM 20.0. This is 30 miles from the center of Boston, still well inside the Consolidated Metropolitan Area defined by the Census Bureau. There are several distinct light domes here, with Boston to the ESE by far the biggest. S is mediocre, NW is relatively good. I can make stars out to mag 5.8 or thereabouts. The summer Milky Way is obvious when it's reasonably high in the sky, but severely washed out and shows little detail. M31 and the Double Cluster are really obvious naked-eye, and many fainter DSOs are visible with some effort, including M34, M35, and maybe M13. Clouds everywhere are much brighter than the background sky. If forced to assign a Bortle Class, I'd call this 6 or possibly 5, though neither is a very good fit.

My darkest regular observing site is my country home in New Lebanon, NY, deep in the yellow zone, SQM 21.1. The brightest light domes are Albany (900,000 people 20 miles WNW) and Pittsfield/Lenox/Lee (150,000 people 10 miles E and NE). The New York City metro area, 20 million people 120 miles distant casts a broad low glow along the southern horizon. I can see most 6.5-mag stars, and occasionally fainter ones. The Milky Way is immediately obvious, and shows in considerable detail. However, the extension into Ophiuchus is barely visible and seeing the Pipe Nebula naked-eye is out of the question. Clouds are usually brighter than the background, but not always. The zodiacal light is visible, but only when it's not tangled up with the light domes of Albany or Pittsfield, which happen to be in the worst possible spots for this particular purpose. I can see M33 with averted vision when it's reasonably well placed in the sky. If forced to assign a Bortle Class, I would call this 4, though it's well ahead of that by some measures and well behind by others.

I would say that ATMOB and my country home are clearly in different categories. Bright galaxy features, like M51's spiral arms, are visible in big telescopes at ATMOB, but M31's dust lanes are probably out of the question. At my country home, I more or less expect to see everything described by dark-sky writers such as Steve O'Meara, though I'm occasionally disappointed. M31's dust lanes are easy, Pickering's Wedge in the Veil is visible with an O-III filter, and the California Nebula is easy in a small scope with an H-Beta filter.

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Tony Flanders

eyeglasses
6x15 and 8x32 monoculars
8x25, 7x35, 10x30 IS, 10x50, and 15x70 binoculars
70mm and 100mm achromatic refractors
4.5", 7", and 12.5" Dobs