Tuesday 7 May 2019 — ASTROPHOTOGRAPHY
It’s obvious why this cloud of gas in the constellation of Cygnus was given its name when first photographed in the 1890s.
The nebula covers quite a large area of the sky, more than four times bigger than a full moon, but the surface brightness of the glowing gas is low. It can be glimpsed by the naked eye under exceptional conditions, appears as a fuzzy or misty area in binoculars or small telescope, provided you have a genuinely dark sky, but emerges in glorious detail only through photography, which can collect photons over time to build an image.
The resemblance to the map of N America is coincidental. Most of the detail is actually formed by dust between us and the nebula absorbing light, to create dark areas and shapes. The nebula is in a rich part of the Milky Way (looking across the arm of our own galaxy where there are very many faint and brighter stars).
Scientists think the gas is glowing because it is energised by radiation from a hot nearby star, but which star, and therefore how far away the nebula is, is uncertain. There are probably many areas of mostly hydrogen gas and dust but we only really see the ones where there is sufficeint radiation to make the gas glow as a plasma (like the gas in a neon tube)
There were a few problems with the data I used to create this image, most visible as strange multicolour hales around the bright stars, and how the stars in front of the nebula seem embossed or “lumpy”. These images are made by combining many exposures, in different colours. While the data is combined using averages and other techniques to “even out” any noise or glitches in individual images, there is a point where misalignment (e.g because the telescope was shaken by the wind or a filter was not perfectly fitted) will show. Processing the image to enhance the faint gas can also easily cause artefacts like these. I will return to re-process this image, but I am pleased to photograph something that I remember filling me with awe when I saw a (much less detailed) photograph in a glossy astronomy book, when I was a child.
This is a 106mm refractor sited in new Mexico, USA, and fitted with a specialist camera for deep sky images. The colour image was made from 15 one-minute exposures each through clear (for luminance), and five each of one minute exposures for red, green and blue filters. I processed it in PixInsight software integrating the separate images before bringng out the detail, colour and contrast in startools software.