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Thread: Stars and skyglow simulation.

  1. #1
    Join Date
    Mar 2007
    Falls Church, VA (near Washington, DC)

    Stars and skyglow simulation.

    I have made some test images in my Corel Paintshop Pro software that I can display on a 15-year-old piece of legacy equipment, an excellent NEC MultiSync CRT monitor which will go really black, unlike current flat screen monitors which show light gray in a dark location. I made samples of backgrounds ranging from white to black. To calibrate them I used a professional grade exposure meter as far down as it would read, and then extended the dark range by visually comparing swatches by looking at the brighter swatch through a neutral density filter. From these measurements I made a graph of the luminance as a function of the computer settings which go from 0 (black) to 255 (white). Then I made 1-pixel spots to simulate stars at 1-magnitude intervals. I put the monitor by a window so I could see Aldebaran over the top of the monitor, and found a pair of spots that bracketed it, thus giving me the actual stellar magnitude of each pixel. At the viewing distance a pixel subtended 2 arcminutes, so some number crunching gave me the background surface brightness in magnitude per square arcminute. I finished up by making a test pattern of 16 swatches simulating skyglow starting at a published value for natural airglow and going up at roughly 1/3 magnitude intervals.

    My first test was my limiting magnitude for stars in a dark sky. For the skyglow I used simulated airglow set to a published typical value of about magnitude 13 per arcminute, which would be the darkest sky possible from anywhere on the ground. It was a joy to be able to see a 6th magnitude star, for which I give a thumbs up to the recently retired ophthalmic surgeon who did the cataract surgery on these 69-year-old eyes.

    When I reset the background to the darkest the monitor would go, which is about 1/8 of the previous setting, I could see a 7th magnitude star with averted vision. Today I made a totally black background by drilling holes in a sheet of aluminum flashing and putting it in front of the monitor, with swatches of the right surface brightness behind the holes. With about 30 minutes of dark adaptation I could see the 7th magnitude star steadily in my central vision and one of 8th magnitude with averted vision. The sky can be darkened close to this with plenty of magnification in a telescope. Now I am satisfied that Scotty Houston, in one of his Deep Sky Wonders articles in Sky and Telescope, wasn't kidding when he said he could see 14th magnitude stars in his 4-inch Clark refractor under favorable conditions.

    This computer system will run on my DieHard inverter, so I plan to haul it out to our club observing sites to measure the skyglow. I think I can do that by using a dental mirror to superimpose samples of the sky on the skyglow test pattern.

  2. #2
    Join Date
    Feb 2005
    Nice work!

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