derangedhermit
sage
Reged: 10/07/09
Loc: D/FW, TX
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Estimating light pollution at a new site
#5371959 - 08/16/12 05:53 PM
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The next two nights I will try to estimate the light pollution at a potential observing site. I don't have the experience to just look up and say something meaningful.
The location is near Ranger, TX. Astronomical twilight is about 4:30 a.m. and 8:30 p.m on Aug 17 and 18.
Moonrise is about 6 and 7 a.m., and moonset is at 7:05 and 7:45 p.m. on Aug 17 and 18. So moonlight is not an issue.
I plan on photographing the horizon in all directions at 9 pm, 10 pm, 11 pm, and midnight each night, clouds permitting. I hope to find two locations close to each other, one with a clear view of the horizon, and one more like an observing field with trees surrounding it.
I expect to see obvious strong light pollution from D/FW covering ESE through NE. I expect to see the lights of Abilene to the west, much weaker. Other than that, I hope it's not too bad.
I have a Canon 40D and plan to use a 17-55 2.8 lens on a tripod. I'm not sure what exposure settings would be most helpful. That's the only objective tool I have at hand.
I will also do some basic naked eye observations, but my eyes are not so good, and not calibrated against anyone else's.
Advice most welcome on how to get data that can be shared in a meaningful way.
Lee
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Scott in NC
Mad Hatter
Reged: 03/05/05
Loc: NC
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5372183 - 08/16/12 09:27 PM
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Advice most welcome on how to get data that can be shared in a meaningful way.
Lee, probably the best way I can think of to gauge the degree of LP using somewhat calibrated measurements would be by using a Sky Quality Meter, which measures background sky brightness in MPSAS (magnitude per square arc-second). This is a nifty little gadget. I bought one when Agena Astro had it on sale this past winter, but admit that I haven't used it a whole lot yet, other than to measure the sky brightness at my home location.
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derangedhermit
sage
Reged: 10/07/09
Loc: D/FW, TX
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Re: Estimating light pollution at a new site
[Re: Scott in NC]
#5372256 - 08/16/12 10:25 PM
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Thanks, Scott. I'm aware of the SQM. Since the forecast for the next 2-3 days is mostly cloudy to overcast for the area, I have some time to ponder the problem. I don't see myself using an SQM a lot, though, so I probably won't buy one.
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RobbW
sage
Reged: 02/09/10
Loc: Elgin, IL
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5372321 - 08/16/12 11:22 PM
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Lee, I don't know if it will help you or not, but I can give you the exposure settings I use when taking night shots for comparison purposes. I put my Canon 40D with 50mm f1.8 lens on a tripod in front of my home one night. I photographed the front of my home with its exterior lights on. I kept adjusting all the exposure settings until I came up with a combination that reproduced the scene as it appeared to my naked eyes.
I take a lot of night-time photographs of different exterior lights to provide comparisons for light pollution research. I needed one exposure setting that I could use in all night-time situations in order to provide a uniform comparison.
The exposure settings I use on my 40D are ISO800 f/2.8 @ 1/8 second. This is with a 50mm f/1.8 lens on a tripod using a remote shutter release cable. YMMV!
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Joe F Gafford
Carpal Tunnel
Reged: 12/15/06
Loc: Denver, Colorado, US
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Re: Estimating light pollution at a new site
[Re: RobbW]
#5372460 - 08/17/12 01:13 AM
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My club has an SQM. We use it to check the sky quality at the observatory in town and out at the DSS. We had requests to check other sites for the IDA. This is one of the few items NOT on our loaner program. We use it so much for club business.
Joe.
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Tony Flanders
Postmaster
Reged: 05/18/06
Loc: Cambridge, MA, USA
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5372632 - 08/17/12 06:47 AM
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The next two nights I will try to estimate the light pollution at a potential observing site. I don't have the experience to just look up and say something meaningful.
The location is near Ranger, TX.
The Dark Sky Finder puts the center of Ranger in the yellow zone and the surrounding area in the green, and strongly suggests that the dominant light sources are local, from Eastland and Ranger itself, rather than from DFW or Abilene.
If you take identical exposures of the horizon and the zenith, you should get some sense by comparing the brightness of the light domes against the brightness of the Cygnus Star Cloud. Also see how well the Sagittarius Milky Way stands out against the skyglow. Pretty well, I would think.
Also, within a factor of two or so, I can give you a numerical estimate of magnitude per square arcsecond if you tell me the f/ratio, exposure time, ISO setting, and average pixel value near the center.
Edited by Tony Flanders (08/17/12 06:48 AM)
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csa/montana
Den Mama
Reged: 05/14/05
Loc: montana
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5372774 - 08/17/12 09:10 AM
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Thanks, Scott. I'm aware of the SQM. Since the forecast for the next 2-3 days is mostly cloudy to overcast for the area, I have some time to ponder the problem. I don't see myself using an SQM a lot, though, so I probably won't buy one.
This is just my experience; however, I use my SQM each time I observe in my observatory. Noting the readings makes interesting information for me to compare throughout the years.
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derangedhermit
sage
Reged: 10/07/09
Loc: D/FW, TX
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Re: Estimating light pollution at a new site
[Re: RobbW]
#5373458 - 08/17/12 04:14 PM
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Thanks, Robb, I will try that. I'm glad you didn't choose faster than f/2.8, I can't go there:)
Since we are both using a 40D, the results should be similar.
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derangedhermit
sage
Reged: 10/07/09
Loc: D/FW, TX
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Re: Estimating light pollution at a new site
[Re: csa/montana]
#5373476 - 08/17/12 04:23 PM
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Carol, I broke down and ordered an SQM-L. I was thinking they were $200 US, but the L with no data logging or interface is cheaper. I'll combine its use and the DSLR and compare results. I think the DSLR will be good for sharing the light pollution in a way the SQM numbers cannot.
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derangedhermit
sage
Reged: 10/07/09
Loc: D/FW, TX
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Re: Estimating light pollution at a new site
[Re: Tony Flanders]
#5373492 - 08/17/12 04:30 PM
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Tony, it's for the planned new Texas State Park, provisionally named Palo Pinto Mountains State Park. It's between Strawn, TX, and Ranger.
I will take you up on your offer of estimating the sky brightness, or if you will share how you do it, I'll run the numbers myself.
Lee
Edited by derangedhermit (08/17/12 11:28 PM)
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AlBoning
sage
Reged: 03/06/11
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5373634 - 08/17/12 06:16 PM
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Visual Magnitude Estimation Charts such as to be found here
http://www.astronomylogs.com/pages/resource.html
might prove useful.
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csa/montana
Den Mama
Reged: 05/14/05
Loc: montana
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5374384 - 08/18/12 09:28 AM
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Carol, I broke down and ordered an SQM-L. I was thinking they were $200 US, but the L with no data logging or interface is cheaper. I'll combine its use and the DSLR and compare results. I think the DSLR will be good for sharing the light pollution in a way the SQM numbers cannot.
 I think you will get great enjoyment & benefits from the SQM!
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Scott in NC
Mad Hatter
Reged: 03/05/05
Loc: NC
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Re: Estimating light pollution at a new site
[Re: AlBoning]
#5374643 - 08/18/12 12:47 PM
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Visual Magnitude Estimation Charts such as to be found here
http://www.astronomylogs.com/pages/resource.html
might prove useful.
Great link--thanks for sharing that!
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Scott in NC
Mad Hatter
Reged: 03/05/05
Loc: NC
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Re: Estimating light pollution at a new site
[Re: csa/montana]
#5374644 - 08/18/12 12:47 PM
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I think you will get great enjoyment & benefits from the SQM!
+1 
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: Estimating light pollution at a new site
[Re: Scott in NC]
#5395318 - 08/30/12 05:26 PM
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A late addition, but it bears pointing out that 1/8 second exposure with a DSLR, even at f/1.8, is probably much too brief for assessing sky glow. This would tend to keep the sky pretty near black, where the idea is to well enough record sky glow to raise it well above the noise level. This will nicely bring out the milky way, which is itself a good datum, in that its contrast against the sky is a pretty sensitive indicator of sky brightness.
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csrlice12
Post Laureate
Reged: 05/22/12
Loc: Denver, CO
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Re: Estimating light pollution at a new site
[Re: GlennLeDrew]
#5396277 - 08/31/12 09:11 AM
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A late addition, but it bears pointing out that 1/8 second exposure with a DSLR, even at f/1.8, is probably much too brief for assessing sky glow. This would tend to keep the sky pretty near black, where the idea is to well enough record sky glow to raise it well above the noise level. This will nicely bring out the milky way, which is itself a good datum, in that its contrast against the sky is a pretty sensitive indicator of sky brightness.
I believe the OP said he was trying to "recreate" digitally, the same brightness his eyes saw. To a camera, that would be pretty quick. Granted, for AP, that won't help much, you would need longer exposures.
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john smith
member
Reged: 01/01/07
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Re: Estimating light pollution at a new site
[Re: csrlice12]
#5398742 - 09/01/12 09:16 PM
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The SQM is a sensitive instrument so make sure the sky conditions are the same when comparing measurements with other locations. Such things as position of the Milky Way. I even like to get close to the same time of evening.
John S.
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derangedhermit
sage
Reged: 10/07/09
Loc: D/FW, TX
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Re: Estimating light pollution at a new site
[Re: csrlice12]
#5399002 - 09/02/12 12:59 AM
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I believe the OP said he was trying to "recreate" digitally, the same brightness his eyes saw. To a camera, that would be pretty quick. Granted, for AP, that won't help much, you would need longer exposures.
Being unsure exactly what I do want, it's easy enough to do exposures of 1/8s, 1s, and 8s.
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GlennLeDrew
Postmaster
Reged: 06/18/08
Loc: Ottawa, Ontario, Canada
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Re: Estimating light pollution at a new site
[Re: derangedhermit]
#5403552 - 09/04/12 06:04 PM
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Good plan, Hermit! Even longer wouldn't hurt, at least for darker conditions or lower ISO settings; it's not like you're paying film development costs.  And having a wider range allows you to assess light pollution in potentially different ways. At any rate, a surplus of data is always better than a deficit.
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