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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 2047
Loc: Cambridge, MA, USA
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Here's a semi-dark site that's particularly useful for callibration purposes, because many people go there: Stellafane, just south of Springfield, VT.
The Clear Sky Chart page for Stellafane places it in the yellow zone, but I think that's its much more typical of the green zone. I attribute the discrepancy to two causes. First, this is an area with a sharp skyglow gradient, and a trivial change in callibration would put Stellafane on the green side of the border. Second -- a more telling problem with the color-zone map -- Stellafane's light pollution is strongly asymmetric. It's worst to the north -- where nobody needs to look and not nearly so bad to the south, the direction that really matters.
Even so, Stellafane has plenty of light pollution; there's light welling up from the horizon in every direction. And disturbingly, the light dome of Greater Boston, more than 100 miles to the southeast, is quite obvious. The southern Milky Way is seriously affected; for instance, the Pipe Nebula, though visible naked-eye, is by no means easy to see, as it is at any truly dark site.
Near the zenith, though, the sky's not half bad. I get SQM readings around 21.5 there, and telescopic observing isn't seriously compromised. I've never looked for the zodiacal light at Stellafane, but I assume that it's pretty easy to see -- at the right time of year.
All in all, Stellafane fits the criteria for Bortle class 3 pretty well.
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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
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
.....
The Clear Sky Chart page for Stellafane places it in the yellow zone, but I think that's its much more typical of the green zone. I attribute the discrepancy to two causes..... ....
Tony,
Could this be another case where the the light source is mostly in a nearby valley and thus valley fog limits the impact up at Stellafane? It seems 'green' to me there, not 'yellow'.
I've also observed a number of times from Ascutney State Park that is just a few miles to the north and there the light pollution is worse.
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George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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MikeRatcliff
Pooh-Bah
Reged: 06/12/04
Posts: 1105
Loc: Redlands, CA
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Quote:
Here's a semi-dark site that's particularly useful for callibration purposes, because many people go there: Stellafane, just south of Springfield, VT.
The Clear Sky Chart page for Stellafane places it in the yellow zone, but I think that's its much more typical of the green zone.
All in all, Stellafane fits the criteria for Bortle class 3 pretty well.
Did you mean Bortle 4? That corresponds to the green color.
Mike
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16" f/4.9 dob
Tele Vue Plossls 32,25,20,15,11
13 Nagler T6
10.5 Pentax XL
Brandon 32, 16
12.5 UO ortho, 9 Circle T ortho
2x TV Barlow
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
BTW, I implore people not to state their color zones in terms of Bortle classes. The proper names are black, gray, blue, green, yellow, orange, red, and white. The correlations between these color zones and the Bortle classes listed in the Clear Sky Chart are known to be innacurate in some cases, are hypothetical at best, and are liable to change.
Tony,
I like numbers rather than colors...... colors remind me of the useless 'national terrorist threat level'.... 
I had hoped that the SQMs would provide a way to compare areas, but at the recent Cherry Springs Star Party I found that my meter reads consistently a little 'brighter' than another person's SQM. Both seem to be consistent with themselves, but there is a slight difference between SQMs. Of course arguing over which is darker, Cherry Springs or the Adirondacks, is like arguing over who is richer, Bill Gates or Paul Allen!
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George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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Tony Flanders
Carpal Tunnel
Reged: 05/18/06
Posts: 2047
Loc: Cambridge, MA, USA
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Mike Ratcliff said:
Quote:
Did you mean Bortle 4? That corresponds to the green color.
And George N said:
Quote:
I like numbers rather than colors...... colors remind me of the useless 'national terrorist threat level'....
OK, let me say this all again in different words.
The statement "Bortle 4 corresponds to the green color" is flat-out wrong. Bortle classes and the color zones do not correspond to each other; any similarity is coincidental.
Bortle classes are a way of evaluating light pollution based on a collection of visual observations. The color zones are a mathematical abstraction created by Pierantonio Cinzano applying a hypothetical model of light spread to images taken some 10 years ago by the U.S. Defense Mapping satellites.
The correspondence between the colors and the classes was created for convenience by the North Virginia Astronomy Club during a study they performed, and was never intended to be taken seriously. Unfortunately, it got picked up by Wikipedia and the Clear Sky Chart, and has come to be taken as gospel by large numbers of people. Not only is it not gospel, it's demonstrably wrong in many cases.
I don't like colors either, but that's what Cinzano chose to call his zones, so that's what we're stuck with. If you want to give them mnemonic names, call them "Joe," "Bob," and so on, or maybe A, B, C, etc., to avoid any possible confusion with the Bortle classes.
Quote:
I had hoped that the SQMs would provide a way to compare areas, but at the recent Cherry Springs Star Party I found that my meter reads consistently a little 'brighter' than another person's SQM. Both seem to be consistent with themselves, but there is a slight difference between SQMs. Of course arguing over which is darker, Cherry Springs or the Adirondacks, is like arguing over who is richer, Bill Gates or Paul Allen!
Yes, there seems to be a variation of around +-0.1 mag among multiple SQMs, and perhaps more in some outliers. Also, there may be an issue of long-term drift over a course of years.
Small discrepancies like that don't really matter for measuring suburbs, but they matter a lot in dark skies. Actually, my hunch is that SQMs aren't very useful for comparing different sites in the grey zone or darker, because the variations in natural skyglow (Milky Way, zodiacal light, airglow, etc.) outweigh the variations in artificial skyglow.
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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
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John Fitzgerald
In Focus
Reged: 01/04/04
Posts: 1218
Loc: AR
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Not using the colors, but using subjective observational data, I would rate my site in N Arkansas between Bortle 1 and 2.
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?
Observing since 1966
Messier Cert #898
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MikeRatcliff
Pooh-Bah
Reged: 06/12/04
Posts: 1105
Loc: Redlands, CA
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Quote:
... Bortle classes and the color zones do not correspond to each other; any similarity is coincidental.
Bortle classes are a way of evaluating light pollution based on a collection of visual observations. The color zones are a mathematical abstraction created by Pierantonio Cinzano applying a hypothetical model of light spread to images taken some 10 years ago by the U.S. Defense Mapping satellites.
The correspondence between the colors and the classes was created for convenience by the North Virginia Astronomy Club during a study they performed, and was never intended to be taken seriously. Unfortunately, it got picked up by Wikipedia and the Clear Sky Chart, and has come to be taken as gospel by large numbers of people. Not only is it not gospel, it's demonstrably wrong in many cases.
Thanks for the explanation, Tony.
Mike Ratcliff
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16" f/4.9 dob
Tele Vue Plossls 32,25,20,15,11
13 Nagler T6
10.5 Pentax XL
Brandon 32, 16
12.5 UO ortho, 9 Circle T ortho
2x TV Barlow
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MikeRatcliff
Pooh-Bah
Reged: 06/12/04
Posts: 1105
Loc: Redlands, CA
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As an update, I was able to see M33 with averted vision, at a dark site last weekend. Site was the Grandview Campground in Eastern California, elevation 8600ft, that attracts many viewers. In fact at times it seems like direct vision was working once in a while, maybe 5% of the time.
The secret for me was first being able to detect the star HD10348, magnitude 5.99, which is to the east of (below) M33 and closer to the tip star in Triangulum (alpha Tri). I could eventually see this star with direct vision as the constellation passed 30-40 degrees in altitude.
Then M33 was evident as a "something" just above. Sighting the Telrad on that got M33 easily.
By the way, that night was one of my best.
I found for the first time IC 1613, the Cetus galaxy that is one of the toughest of the Caldwells. In fact I saw it in the 60mm finder too! There was no doubt in my mind. After realizing that this might be controversial, I did some internet searching and found a couple of reports in finders. So I believe it wasn't imagination. I also repeated an observatin of IC 312 (averted but definite glow, instead of the typical averted imagination at other times for me).
I feel like I'm still climbing the learning curve too much to contribute substantially to this quest to redo the Bortle scales, especially without an SQM. But the endeavor to see M33 naked eye was fun and educational.
Mike Ratcliff
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16" f/4.9 dob
Tele Vue Plossls 32,25,20,15,11
13 Nagler T6
10.5 Pentax XL
Brandon 32, 16
12.5 UO ortho, 9 Circle T ortho
2x TV Barlow
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Hi Tony and all,
First something about my experience with Cinzano's maps and the SQM. I live in North-East Italy and my preferred site (two hours drive) is Casera Razzo, in the north eastern alps.
In the "classical" maps by Cinzano it is "green". SQM typically reads 21.5 (sometimes 21.3 and sometimes 21.7). You my find my readings under the nickname "maudal" in the unihedron database. http://unihedron.com/projects/darksky/database/?allreadings=true There also is the google landmark http://maps.google.com/maps?q=46.4792+12.6097 .
Thus, surprisingly, a "green" place reads 21.5 on average. Why?
1) The classical maps of artificial night sky brightness, as Cinzano explains, provide only a evaluation of how much artificial light is introduced in the atmosphere. Not how much is seen back as light pollution. http://www.lightpollution.it/dmsp/artbri.html
2) Cinzano produced also maps for the total sky night brightness. These account for elevation and other factors that affect the sky brightness. My site in those maps is "light green", which corresponds to 21-21.5. http://www.lightpollution.it/dmsp/totbri.html Thus, this kind of maps is more suited to represent how much light "appears" as sky brightness (the former, at point 1 is how much light is injected into the atmosphere).
I made a Google layer of maps 2 and found rather good agreement with SQM readings in many places.
However there is a remaining factor that those large scale maps do not account: "mountain screening". In fact my site would be 21-21.5 due to elevation. But, it is surrounded by montains that screen the lights from the surrounding valleys and the padana plate. In practice a point above my site, 2.5 km height is not yet in the line of sight from the most light polluting areas. "screens" protect the atmosphere above my site up to about 3 km, and above there is little that can reflect light back. Clouds are black voids if they are low enough to be screened. Only high clouds vaguely glow (illuminated by the padana plate 64 km far away). That explains why I read between 21.5 and 21.7. Not only there is little atmospher above the site (due to elevetion) but nost of the remaining atmosphere is screened.
I wrote an article in a blog to try to explain the importance of screening (also Cinzano mention this fact). It is in italian but you may get the meaning by using some online translation tools:
original
http://visualsky.blogspot.com/2007/09/inquinamento-luminoso-quanto-contano-le.html
There are other points that I will post later. Translation
Edited by Tom L (08/24/08 08:49 AM)
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s58y
Post Laureate
Reged: 12/12/04
Posts: 4830
Loc: Eastern NY
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Quote:
Thus, surprisingly, a "green" place reads 21.5 on average.
Aren't "green" places supposed to read between 21.25 and 21.69 on the SQM? I'd think 21.5 would be typical.
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Hutech 30D, SBIG ST-402 autoguider
SV80S, SV66 guidescope
AP900, G-11, Barndoor tracker
http://www.pbase.com/s58y
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Quote:
Aren't "green" places supposed to read between 21.25 and 21.69 on the SQM? I'd think 21.5 would be typical.
The "classical" maps gives the amont of lihgt polluted into the atmosphere. The actual sky brightness, as explained by Cinzano, depen ds on a numebr of other factors like elevation, atmopshere and screening. For this reason Cinzano porduced dofferent types of maps. One (which I linked) reports the stimated sky brighntness. Incidentalkly this type of masp uses light green for 21-21.5 and dark green for 20.5-21 (the kaki for 20-21.5). However the contur leveò and meaning are completely differente and the comparison opf these masp with the "cvlassical" (yello-green-blòue-gray-black) shows that the same "green" may be differente in the maps of total sky brightness. Not to mention that there are also maps that account for stella extinction and provide the stellar visibility estimation.
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Now some of may experience about the correlation between the SQM and the Bortle degree (as said the SQM fairly matches the maps of "total sky brightness, not the "classical" one, except for the screening effect).
The point is that the SQM measures the TOTAL light coming from the sky. On the other hand the Bortle grade means visibility of object, that is the ratio between object's light and background. The former (object brightness) depends on the extinction. Therefore I think that what matters is the SQM minus Extinction (which is the contrast).
In pretty dark skyes I have experienced that, often, transparent atmosphere produces somewhat brighter reading of the SQM. In one night the atmosphere was extremely clear. RThe SQM marked 21.55 and IC 1296 was easy seen (16"). In other occasions, even with darker SQM (21.6-21.65) IC 1296 was difficult.
Although the Bortle scale is carefully described, I still have difficulty in assessing the grade based on the descriptions. M33 is always difficult for me and it seems I am not able to go above mag 6.5 whatever the darkness: I have little progress starting from 20.8. What happens is that the region of uncertain detection expands somewhat, but the direct vision sticks at somewhat 6. So, some my sky has some lemengts that seems to be grade 2 (darkness and difficulty to see objects, black clouds) and others that appears to be grade 3 or even 4 of the scale (difficulty to see M33).
To summarize, here are my indications:
Location Casera Razzo (Italian Alps) http://maps.google.com/maps?q=46.4792+12.6097
1. DISTANCE TO METRO AREAS.
To the south east 64 km to Udine (some million people urban area). To the south south east the Padana plate at 80 km (a dozen million people).
Lightdomes: SE only some mild brightness in the distance between the mountains. Some diffuse light from N and W in very wet nights.
Elevation: 1740 meters.
2. COLOR ZONE IN LIGHT POLLUTION ATLAS
Green in the !"classical" Cinzano's maps. Light green (21-21.5 in the total sky brightness maps).
3. SQM.
21.4-21.6 on average (see also http://visualsky.blogspot.com/2007/11/airglow-fluctuations.html )
4. VISIBILITY OF MILKY WAY. Winter MW is visible. Summer MW is strutured with many many grades of gray (and visble eve in spring when low at North). Arm into Ophiucus obvious. North america si naked eye.
5. VISIBILITY OF NAKED EYE DSO'S. M33 is visible with adverted vision. Limiting magnitude 6.5 (16.1 or q16.6 at the telescope 16"). M13 visible. M31 is easy (shape clearly visible).
6. CLOUDS BRIGHT OR DARK? Low clouds are black voids. Only very high clouds faintly glow illuminated by the Padana plate 80 km apart. See also http://visualsky.blogspot.com/2007/10/linquinamento-luminoso-di-cr.html
Edited by Mauro Da Lio (08/17/08 04:09 PM)
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
Quote:
Thus, surprisingly, a "green" place reads 21.5 on average.
Aren't "green" places supposed to read between 21.25 and 21.69 on the SQM? I'd think 21.5 would be typical.
Stellafane is shown on Clear Sky Chart as Yellow, but when you look at the map it's right on the Green/Yellow border. At Stellafane 2008 I got SQM readings (early morning) of 21.00 and 21.25, on Friday and Saturday mornings respectively. the local weather reported heavy fog for Springfield VT which is just in the valley below. This was also with the Milky Way nearly overhead and the SQM website still does not have a recommended adjustment for subtracting the Milky Way from the readings.
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George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
......
However there is a remaining factor that those large scale maps do not account: "mountain screening".....
OK..... that's it!
I’ve often wondered why at my location in northeast New York there is no sign of a light dome from Montréal, Canada, one of the world’s largest cities – it’s got to be the high mountains in that direction (including the site of the 1980 Olympics).
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George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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s58y
Post Laureate
Reged: 12/12/04
Posts: 4830
Loc: Eastern NY
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Quote:
This was also with the Milky Way nearly overhead and the SQM website still does not have a recommended adjustment for subtracting the Milky Way from the readings.
With the SQM-L, readings are totally bogus if you point it straight up while the Milky way is overhead. Where I am, you'll get 21.1 to 21.2 if you point the SQM-L straight up at Cygnus this time of year. If you point the SQM-L to the darkest area between the Milky way and the light domes near the horizon you might get 21.5, so the affect on the readings is significant.
With the old SQM, you might get 21.4 to 21.5 pointing straight up at Cygnus (and there's no real way to point it very far from the zenith without getting a bad reading because of the wide FOV of 120 degrees or more).
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Hutech 30D, SBIG ST-402 autoguider
SV80S, SV66 guidescope
AP900, G-11, Barndoor tracker
http://www.pbase.com/s58y
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
Quote:
This was also with the Milky Way nearly overhead and the SQM website still does not have a recommended adjustment for subtracting the Milky Way from the readings.
With the SQM-L, readings are totally bogus if you point it straight up while the Milky way is overhead. Where I am, you'll get 21.1 to 21.2 if you point the SQM-L straight up at Cygnus this time of year. If you point the SQM-L to the darkest area between the Milky way and the light domes near the horizon you might get 21.5, so the affect on the readings is significant.
With the old SQM, you might get 21.4 to 21.5 pointing straight up at Cygnus (and there's no real way to point it very far from the zenith without getting a bad reading because of the wide FOV of 120 degrees or more).
Well, I don't think "totally bogus" is the right term, but the SQM does include the Milky Way when reporting the average sky brightness - it includes both the 'good stuff' and the 'bad stuff'. The website sez that they are working on an "adjustment" and there are those who say that if the Milky Way is high you need to add .3 to the SQM readings to get a value that corresponds to a reading minus-Milky Way. Your findings seem to support that .3 is at least in the ball park.
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George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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Mauro Da Lio
sage
Reged: 09/12/04
Posts: 223
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Quote:
... "adjustment" and there are those who say that if the Milky Way is high you need to add .3 to the SQM readings to get a value that corresponds to a reading minus-Milky Way. Your findings seem to support that .3 is at least in the ball park.
A fellow has been recently in Namibia which is absolutely perfect dark sky (http://www.ghhsnet.de/tivoli/enzwo/framestart_e.html). SQM read 21.93 without the Milky Way and 21.54 with the Sagittarius overhead.
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s58y
Post Laureate
Reged: 12/12/04
Posts: 4830
Loc: Eastern NY
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Quote:
there are those who say that if the Milky Way is high you need to add .3 to the SQM readings to get a value that corresponds to a reading minus-Milky Way. Your findings seem to support that .3 is at least in the ball park.
My readings were with the SQM-L, which is more of a spot-meter than the SQM. Any Milky Way correction factors would have to be dependent on how dark the site is. With the SQM-L, I might get 21.15 if I point the SQM-l overhead at Cygnus and 21.5 pointed to the darkest spot away from the zenith. At a really dark site, you might get something like 21.25 pointing at Cygnus and (ideally) 22.0 pointing to a dark patch of sky. The other night as the moon was rising, the Milky way was almost invsible, and I was getting about 20.25 pointing towards Cygnus or away from Cygnus.
--------------------
Hutech 30D, SBIG ST-402 autoguider
SV80S, SV66 guidescope
AP900, G-11, Barndoor tracker
http://www.pbase.com/s58y
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
My readings were with the SQM-L, which is more of a spot-meter than the SQM. Any Milky Way correction factors would have to be dependent on how dark the site is. With the SQM-L, I might get 21.15 if I point the SQM-l overhead at Cygnus and 21.5 pointed to the darkest spot away from the zenith. At a really dark site, you might get something like 21.25 pointing at Cygnus and (ideally) 22.0 pointing to a dark patch of sky. The other night as the moon was rising, the Milky way was almost invsible, and I was getting about 20.25 pointing towards Cygnus or away from Cygnus.
I’m not sure what your moonlight readings are saying because the rising moon certainly does not evenly brighten the sky. On some spring/fall (non Milky Way) moonlit night you will have to try the same ‘spot meter’ readings. It would be interesting. I know that when using a CCD camera with a very narrow field, it is possible to image say 45 degrees from a gibbous moon, but not say 15 degrees away because the sky is much brighter there.
I’d also be interested in what readings you get with the Milky Way at the zenith, and then again say 3 hours later, when a more sparse area of the sky is at zenith. I think this would give a more accurate idea what the ‘Milky Way correction factor’ should be, rather than comparing zenith readings to say 45 degree altitude readings.
The Milky Way is adding a steady .3 to .5 (or so) mag/sq-arc-sec amount of light to a large area of the sky. I’d not be too happy to learn that the SQM could not accurately record a difference that large. Of course none of this makes much difference if we are talking about readings from a down-town mall parking lot. We already know that it would not be the place to set up a big Dob! 
--------------------
George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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George N
sage
Reged: 05/19/06
Posts: 296
Loc: Binghamton & Indian Lake NY
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Quote:
Quote:
... "adjustment" and there are those who say that if the Milky Way is high you need to add .3 to the SQM readings to get a value that corresponds to a reading minus-Milky Way. Your findings seem to support that .3 is at least in the ball park.
A fellow has been recently in Namibia which is absolutely perfect dark sky (http://www.ghhsnet.de/tivoli/enzwo/framestart_e.html). SQM read 21.93 without the Milky Way and 21.54 with the Sagittarius overhead.
I bet that there aren’t too many SQM owners who have to deal with the center of the galaxy being near the zenith!
Your friend’s experience gives me some more confidence that I would get a generally accurate reading by adding .3 to the SQM reading with the Cygnus Milky Way at the Zenith. It might also explain my reading of 21.00 with Cygnus at the zenith, and 21.25 some 4 hours later, with the Milky Way moved over into the western side of the sky.
--------------------
George N
Obsession 20
Optical Guidance Systems 10" F/9 R-C Cass
6" F/5 & 8" F/8 home-made Newts
MI-250 mount
SBIG STL-1301E CCD
Member, International Dark-Sky Association
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