M17 - Omega Nebula

M17 - Omega Nebula: The Omega (or Swan) nebula in the constellation Sagittarius is one of several bright emission nebulae in the direction towards the Milky Way's center. As the gas cloud collapses, the first stars to form are the very luminous, but short-lived, blue stars that light up the remaining gas with intense ultraviolet light, causing the gas to re-radiate at longer wavelengths, mostly in the H-alpha (red) color.

Exposure: L = 12x1 minute (1x1bin) + 3x5 minute (2x2bin) subframes
R:V:B = 12:12:10 in 2 minute subframes.
The camera was at -30°C.Processing Notes: Data acquisition with CCDSoft. Reduced and aligned in CCDStack. Subs combined in Sigma Beta. RVB combined in AstroArt with a ratio of 1:2:4 (from calculations taking into account camera efficiency filter transmission and atmospheric extinction), and saved as a TIFF for import into Photoshop. Adjusted curves and levels, and added a Gaussian blur on the color image. Luminosity was imported directly into photoshop with a log stretch. Adjusted levels and curves. Combined the blurred dim areas from the 3x5 minute L image. Sharpening on non-star bright areas. Slight, faded, minimum filter to reduce star sizes. Combined with color in three layers: Luminosity on the bottom, Color on the top, and the RVB data as a "multiply" layer at 35% in the middle.Scale: High resolution image ~0.5"/pixel

The R filter in the photometric set is significantly sensitive in IR, so many of the cooler stars show up in this image as very red stars, where they might not be seen at all with a "photographic" red filter. The other significant difference in color balance is that [OIII] is detected more in the V ("visible", or green) filter than the B, or "blue" filter.