Last weekend’s star party was relatively relaxing, largely because the target I was after could be photographed for only a few hours each night, so I got to bed by 2 or 2:30 each night. The target was 3 nebulae in the constellation Sagittarius, the Lagoon (M8), Trifid (M20), and Cat’s Paw (NGC6334). All of these are primarily emission nebulae, but are often photographed in broadband color (RGB) to include some reflection components, especially the blue region on the north side of M20. North is down in this photo, and this blue region shows only very faintly as a light grey patch (Trifid is the smaller nebula in the lower left):
But perhaps you are just wondering why most of the nebulosity is shown in purple – not a color we often see in astro-photos. The color assignment is largely arbitrary, but I try to maximize the contrast and texture details when shifting the color. And there is something else different about the processing of this image, the use of the Sulfur-II channel as luminance (it defines the brightness, while the other channels determine the color). I often use the Hydrogen-alpha channel this way because it usually covers everything else, but in this case there is a strong sulfur signal everywhere, and it is more “textured” in the core of the large Lagoon Nebula.
The Lagoon is one of the few nebulae that is bright enough to be seen naked eye. And with just good binoculars it is quite a magnificent sight. The Trifid can also be detected with binoculars, but higher magnification is needed to appreciate its structure. The Cat’s Paw is much dimmer and difficult to see even in a large telescope. This is the best time of year to see them from here in the Pacific Northwest, as they are even lower in the southern sky the rest of the year.