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skysurfer
member
Reged: 10/05/09
Posts: 31
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There are now special SQM's available from some shops which display the darkness in magnitudes per square arc seconds, wherr 18 is a heavily light polluted sky and 21-22 is a dark sky.
http://www.unihedron.com/projects/sqm-l/
The advantage is that the value is not dependant on somebody's eyes which vary from person to person.
The problem is a relatively high price ($100) and also that a dark sky with hazyness of thin cirrus cloud will issue a high (> 20) SQM value but not the LM (limiting magnitude) which belongs to it.
When one takes a picture with any camera (can also be a compact one) as long the exposure time (8-15 sec) , aperture, field size (zoom), white balance (preferably daylight) and ISO settings are the same. And in case of an SLR the lens used.
The LM of these photos will tell the 'real' sky quality, as with a dark hazy sky a lower LM will be shown which an SQM does not.
Moreover is is far cheaper (assumed you have a camera).
What are your ideas on this ?
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Two eyes (5-7mm in full darkness)
TS 15x70mm binos
TeleVue Genesis 100mm f=500mm
Orion XT-10i 250mm f=1200mm
Canon EOS 40 w/85mm f/1.8, 50mm macro f/2,5
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Tom Polakis
scholastic sledgehammer
Reged: 12/20/04
Posts: 816
Loc: Tempe, Arizona
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I think you will run into the same problems that we currently have with the non-uniformity of SQM readings. Even if you give explicit instructions on where to point the camera and the settings to use, there will be user errors and instrumental errors even among the same camera model. It would be difficult to have the set of images from different folks at different sites tell a consistent story, but it might be worth a try.
Having said that, I have often thought it would be useful to point my fisheye lens straight up, and take comparative images to show the sky glow from each of our commonly used sites.
Tom
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Tom Polakis
Tempe, AZ
Visual observing, DSLR photography, lunar & planetary imaging
http://www.pbase.com/polakis/
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Tony Flanders
Post Laureate
Reged: 05/18/06
Posts: 3934
Loc: Cambridge, MA, USA
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Quote:
When one takes a picture with any camera (can also be a compact one) as long the exposure time (8-15 sec) , aperture, field size (zoom), white balance (preferably daylight) and ISO settings are the same. And in case of an SLR the lens used.
The LM of these photos will tell the 'real' sky quality, as with a dark hazy sky a lower LM will be shown which an SQM does not.
I have made extensive sky-brightness measurements with digital cameras. I started before the SQM was available, and have continued since as a cross-check on the SQM, and for use in situations where an SQM won't work.
I think that going by stellar limiting magnitude would be exceedingly problematic, for multiple reasons. First, even casual photos pick up surprisingly faint stars, and determining all their magnitudes and finding out which was faintest would be a huge job. Second, how do you decide when a star is visible and when not? It's not exactly an either-or thing. Third, this is going to be critically sensitive to how well the lens is focused, which is almost impossible to reproduce from one session to another. Fourth, a multi-second exposure is going to be affected by poor seeing much more than naked-eye viewing is. I can think of numerous other problems, but I'll stop there.
What *is* easy is what I was doing -- and occasionally still do -- namely using it to measure mean sky brightness, just like an SQM. Remove the stars from the image, then measure the mean pixel levels in the center of the image. This correlates extremely well with SQM readings.
Advantages of SQM:
Instant results, requires no auxiliary equipment.
Disadvantages of SQM:
Very poor resolution, extremely sensitive to stray light, requires unobstructed view of sky.
Advantages of camera:
Superb spatial resolution.
Automatically measures in multiple wavelengths.
Automatically time-stamps each measurement.
Precise direction can be determined from star field.
Disadvantages of camera:
Require post-processing at home to obtain results.
Results not standardized, vary from camera to camera.
Oh, by the way, regular point-and-shoot cameras do fine in cities and bright suburbs, but aren't sensitive enough to measure semi-dark or truly dark skies.
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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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DblVision
professor emeritus
Reged: 10/11/06
Posts: 634
Loc: 29.99N 92.15W
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My then eighth-grader did a project last year where she tried to see if a local security light would cause a decrease in NELM. We (the whole family) made some obs with the light on and off, and did notice a difference. She also tried to see if the same effect could be observed with short exposures from a film camera, first scanning the negatives, then inverting them, printing onto clear film (Real high tech...her mentor is old!) and having a go at them on a light table. The “light on” & “light off” photos went to about the same LM (8.5-9.0)…it was just that there was more “noise” on the shots with the light on. Conclusion was that eyes were more affected due to being blasted from local reflected light, which the camera never saw.
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Neal
G.O. Sig 10.5x70
Swift 761 8X42
60mm Spotter
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