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Image ID: m31-500mm-8780-8825-av46_RJ.v5-600.jpg
TITLE=The Andromeda galaxy, Messier 31, from Denver
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The Andromeda Galaxy, M31, is a spectacular large galaxy. However, its surface brightness is very low and long exposures are needed to record the detail. It usually is not imaged from deep within the light pollution in large cities such as Denver, but this image was done in bright light pollution.
Camera: Canon 10D 6-MPixel digital camera, set at ISO 400. Lens was a 5-inch aperture, 500 mm focal length F/4 (Canon's 500 mm f/4 IS L telephoto lens). Data were recorded in raw mode, 1 minute exposures, and 46 exposures were added in ImagesPlus. Note: this was an experiment using jpeg 8-bit files extracted from the raw files. No dark subtraction! Processing included light fall-off correction in photoshop using lighting effects to generate a light fall-off profile and reducing the falloff using a color burn layer. Contrast stretch in ImagesPlus using digital development and in photoshop using LAB mode luminance channel only. Unsharp mask on luminance channel and Levels adjustments in photoshop. The light fall-off correction was significant because of the strong stretch needed to separate the galaxy from the bright sky background. The sky was several times brighter than most of the galaxy, resulting in extremely low contrast.
Image guided with an ST4 autoguider on a 3-inch f/8 refractor and a Losmandy G11 mount.
Compare the above final image to a single frame unprocessed image below.
The sky brightness in the Canon red, green, and blue pixels was: red = 17.1, green = 17.9 and blue = 18.6 magnitudes per square arc-second. Dark country skies are typically fainter by about a factor of 100 in red and green magnitudes about 22 to 23). The sky was brightest in the red channel because of the sodium street lights.
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Keywords to this image = astrophoto-1 galaxy Messier digital_astro
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Last updated August 23, 2014