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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1241
Loc: Estonia
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I also agree. I'm not a fan of the bortle scale at all, since none of the descriptions depict reality, at least for me. Usually I find that if I mix the descriptions of 3-4 different classes, I get what I experience, which ends up being a useless metric.
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8285
Loc: Beatrice, Nebraska
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I also have problems with the Bortle classes (too many levels, not consistent characteristics for each level, etc.). It can be really tough to give a judgement 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 may be 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!
David Knisely
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3469
Loc: Cambridge, MA, USA
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Quote:
I also have problems with the Bortle classes (too many levels ...
Don thinks the Bortle system has too few levels, you think there are too many, I think there are about the right number. Who's to say?
The beauty of a rating based on numbers, like the ones you get out of an SQM, is that you can cut it as coarse or fine as you like. When I say my sky is 18.4 at the zenith, I'm stating a fact, not making a judgment.
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Tony Flanders
First and foremost observing love: naked eye.
Second, binoculars.
Last but not least, telescopes.
And I sometimes dabble with cameras.
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8285
Loc: Beatrice, Nebraska
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Quote:
Quote:
I also have problems with the Bortle classes (too many levels ...
Don thinks the Bortle system has too few levels, you think there are too many, I think there are about the right number. Who's to say?
The beauty of a rating based on numbers, like the ones you get out of an SQM, is that you can cut it as coarse or fine as you like. When I say my sky is 18.4 at the zenith, I'm stating a fact, not making a judgment.
I can say. I use the ZLM figures alone when I do my logging because I do not have an SQM (and might not be able to completely justify the $120 for something I might use only once or twice a night). My personal ZLM reading isn't precise, but it does tell me enough of what the night was like in terms of conditions, which is all I really need. The Bortle scale bothers me much more for its inconsistent or inaccurate judging factors than it does the number of scales it has. I don't usually use my 5-level scale because, as Don indicates, it (and the Bortle scale itself) doesn't really describe what the actual observing night is like. However, for rating observing sites, my 5-level scale offers some consistency and simplicity. Hyde Observatory is generally in the "mild" category, as is the front driveway of my home. Rockford Lake, where I do most of my deep-sky observing, is, well 'dark sky' (surprise, surprise), while the south dunes above Merritt Reservoir is definitely in the "Pristine" category. Using the Bortle scale, my driveway might be anywhere from 5 to 6 to 7. Rockford Lake would be a 3 or 4, while Merritt Reservoir would be a 1 or 2, both depending on what characteristics you chose to emphasize. I feel at least one or two of these levels could have been combined to make a more consistent and useful scale, with perhaps no more than seven levels. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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Well, David, using ZLM is only relevant for YOU at your sites. Because a different individual might have a different result on the ZLM. It is, after all, a subjective assessment.
That is why I do not lend any weight to an individual's assessment of the Naked Eye Limiting Magnitude.
At Mt. Pinos, when we've conducted tests among individuals with stars near Polaris, we get a range of 5.7 to 6.7 on the same night at the same time.
I do agree that such a rating would have relevance for you because you are you and what you see is what you see. Extrapolating to others might be more problematic.
The Bortle scale, for me, seems to mix classes when it comes to what I see personally. The Dark Horse area of the Milky Way is visible and I see the bulge of the Milky Way all the way to Alpha and Beta Librae and dark nebulae on the N.edge of M24 on a night where I would definitely rate the night sky far from pristine (maybe Bortle Class 3), yet I can also see a ton of Messier objects with the naked eye.
Since I've been keeping track, nights at that site have varied from mpsas 21.15 all the way to 21.89. My vision didn't allow any fainter a naked eye star to be seen on the darker night (the fainter stars are smeared to invisibility by my vision), yet there was a profound difference in the appearance of the night sky and the limiting magnitude of the telescope. Bortle would have said there was a difference in class between those nights, but perhaps only one class.
I'm with Tony. Measure it and look up. That's what XX.X looks like. TO YOU, and not necessarily to someone else.
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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Hrundi
Pooh-Bah
Reged: 02/06/08
Posts: 1241
Loc: Estonia
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There's also added complications by those wearing glasses. I would estimate I lose quite a bit of light by my scratched and foggy glasses. They're a bit weak as well.
Then there's the whole concentration factor. I find that sometimes, if I know how to focus, I can get mag 6.5 stars to be sharp dots, and can see up to mag 7, but other times, stars start to smear into foggy dots at mag 5, which makes apparent NELM lower than it is.
And none of these considerations seem to matter at the eyepiece, since the strain is gone mostly, as well as the glasses.
Also, 1000 posts 
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rob0260
super member
Reged: 01/31/09
Posts: 116
Loc: Teulon, Mb., Canada
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I was out observing Friday night from my home. I live out in the country with an estimated NELM of about 6.3 at the zenith. M31 and the double cluster are easy naked eye objects. I easily saw M33 with my Meade 8 inch S/N. It reminded me of the core of m31 from the city with binoculars. I tried to see it naked eye but I could not. Next time, I think I'll get out the etx80 and give it a try with it.
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SN8, LXD75, ETX 80, 4 1/2 inch Newtonian, DSI-C, Meade series 4000 eyepiece set, Antares ub2s barlow, antares 5x barlow, 10x50 binoculars.
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8285
Loc: Beatrice, Nebraska
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Quote:
Well, David, using ZLM is only relevant for YOU at your sites. Because a different individual might have a different result on the ZLM. It is, after all, a subjective assessment.
That is why I do not lend any weight to an individual's assessment of the Naked Eye Limiting Magnitude.
At Mt. Pinos, when we've conducted tests among individuals with stars near Polaris, we get a range of 5.7 to 6.7 on the same night at the same time.
I do agree that such a rating would have relevance for you because you are you and what you see is what you see. Extrapolating to others might be more problematic.
The Bortle scale, for me, seems to mix classes when it comes to what I see personally. The Dark Horse area of the Milky Way is visible and I see the bulge of the Milky Way all the way to Alpha and Beta Librae and dark nebulae on the N.edge of M24 on a night where I would definitely rate the night sky far from pristine (maybe Bortle Class 3), yet I can also see a ton of Messier objects with the naked eye.
Since I've been keeping track, nights at that site have varied from mpsas 21.15 all the way to 21.89. My vision didn't allow any fainter a naked eye star to be seen on the darker night (the fainter stars are smeared to invisibility by my vision), yet there was a profound difference in the appearance of the night sky and the limiting magnitude of the telescope. Bortle would have said there was a difference in class between those nights, but perhaps only one class.
I'm with Tony. Measure it and look up. That's what XX.X looks like. TO YOU, and not necessarily to someone else.
I never claimed the ZLM was strictly applicable to anyone but myself (and I would have been wrong if I had). It should not be given any *absolute* weight, but it would be incorrect just to throw such an observation out altogether. However, when I (and others) see stars in a certain average range of magnitudes, chances are that the observing location in general could fit into a certain "class" of site using a simple 5 to 7 level classification scheme. Such a classification would be useful and possible using a mean stellar ZLM range along with other well-defined characteristics (sky color, light domes, visibility of Milky Way, M31, M33, etc.). This the Bortle scale does not do as well as it should, which was the point I was trying to make. For one example, M33 is visible to me from my dark sky site with my unaided eye, but I have *never* been able to see it with absolutely dead-on direct vision. Oh, if I avert even slightly, it may be visible, but straight on, I just can't do it. With extended tests, I have determined that my faintest direct vision star tends to be in the 4.8 to 5.0 magnitude range (my averted limit, however, *does* range from 6.7 to 7.6 depending on local observing conditions). However, Bortle in his descriptions of his class 2 sky claims M33 can be seen with direct vision and uses this naked-eye direct vision claim to define that class. This I can't duplicate, not even from the truly pristine dark site of the Nebraska Star Party. Not many people I have had contact with over the years have seen M33 even with averted vision, let alone directly. If M33 were to be used in such a classification scheme, it should be used only at the point of being visible at all (and as a secondary characteristic among others), and not exclusively with direct vision. Similarly, references to "color" in things like the Milky Way or the Zodiacal Light are similarly flawed as indicators of sky quality, as many people *never* can see that color no matter how dark the sky is. Classes 1 and 2 should at the very least be merged. The others could be adjusted to assign consistent objective indicators. As such, the Bortle scale is something that I consider less useful than it really should be. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Starman1
Vendor - Scope City
Reged: 06/24/03
Posts: 12230
Loc: Los Angeles
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David's comments ring very true. I have found that skies darker than a certain darkness vary a bit in transparency and seeing has a lot of influence on the faintness of naked eye stars seen.
The Bortle scale is a subjective scale and mixes the visibility of faint contrasts with color sensitivity and visual acuity. I'm sure Bortle made a scale based on his impressions but, having viewed from a variety of sites where the mpsas ranged from 19.6 to 22.0, I definitely think there could be a subjective scale with a 1-5 rating starting at about magnitude 21.0.
How I would go about describing it subjectively so that someone else would be able to tell would take a while for me to define.
David's abbreviated scale certainly would be easier.
One thing I can definitely say with certainty. When the seeing conditions are excellent and the night is exceptionally dark (say mpsas 21.7 or darker), it's magic, and those nights are remembered for years. That was the case on that night of magnitude 21.89 for me. There was no air movement all night, near zero humidity, and superb seeing. It only lacked temperature to be totally perfect--it was 9F. But on that night, M33 was naked eye, and noticeable as a gray spot on the blackness of the sky. But, like David, with slightly averted vision. When you can use a Telrad on M33, I think that ranks as a dark night. 
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Don Pensack
12.5" Truss Dob, 5" Maksutov, Fujinon Binos
Sustaining Lifetime IDA member
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8285
Loc: Beatrice, Nebraska
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Quote:
David's comments ring very true. I have found that skies darker than a certain darkness vary a bit in transparency and seeing has a lot of influence on the faintness of naked eye stars seen.
The Bortle scale is a subjective scale and mixes the visibility of faint contrasts with color sensitivity and visual acuity. I'm sure Bortle made a scale based on his impressions but, having viewed from a variety of sites where the mpsas ranged from 19.6 to 22.0, I definitely think there could be a subjective scale with a 1-5 rating starting at about magnitude 21.0.
How I would go about describing it subjectively so that someone else would be able to tell would take a while for me to define.
David's abbreviated scale certainly would be easier.
One thing I can definitely say with certainty. When the seeing conditions are excellent and the night is exceptionally dark (say mpsas 21.7 or darker), it's magic, and those nights are remembered for years. That was the case on that night of magnitude 21.89 for me. There was no air movement all night, near zero humidity, and superb seeing. It only lacked temperature to be totally perfect--it was 9F. But on that night, M33 was naked eye, and noticeable as a gray spot on the blackness of the sky. But, like David, with slightly averted vision. When you can use a Telrad on M33, I think that ranks as a dark night.
Yes, M33 in the Telrad is one dark night. At the Nebraska Star Party last year, one of our young club members was fighting with her new 10 inch Dob trying unsuccessfully to find M33. As usual, I could see it pretty easily with averted vision, so I walked over to her scope, moved the center of her Telrad over the tiny faint fuzzy spot of M33, and stood back to let her take a peek in the scope. She looked in the eyepiece, saw the galaxy in all its glory, and turned to me with an incredulous look on her face and said, "How the heck did you *do* that??" I just smiled and said, "Experience", and then kind of under my breath I said, "Gad, I love these skies!" Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Dave Mitsky
Postmaster
Reged: 04/08/02
Posts: 10511
Loc: PA, USA, Planet Earth
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M33 was visible with averted vision without optical aid during the 2009 Black Forest Star Party. The elusive zodiacal band was fairly obvious, along with the gegenschein and, of course, the zodiacal light, on the night of Friday, September 18th. An SQM reading of 21.71 was obtained on Saturday morning.
Dave Mitsky
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Chance favors the prepared mind.
De gustibus non est disputandum.
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wfj
sage
Reged: 01/10/08
Posts: 259
Loc: California, Santa Cruz County
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Quote:
... so I walked over to her scope, moved the center of her Telrad over the tiny faint fuzzy spot of M33, and stood back to let her take a peek in the scope. She looked in the eyepiece, saw the galaxy in all its glory, and turned to me with an incredulous look on her face and said, "How the heck did you *do* that??" I just smiled and said, "Experience"...
-or-
Omne ignotum pro magnifico
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