Portrait of a stellar nursery
Capture the Orion Nebula’s ethereal pink swirls of gas and dust that are giving birth to new stars
EXPERIENCE LEVEL Intermediate to advanced WHAT YOU’LL NEED A small refractor or Newtonian telescope carried on a motorised tracking mount, plus a monochrome CCD camera (and a computer to control it) with a set of LRGB imaging filters and a filter wheel. For exposures of more than a few minutes it’s also a good idea to use an autoguiding system alongside the above, though this is not absolutely necessary.
There are few greater tests of a deep-sky astrophotographer’s skills than the magnificent Orion Nebula, M42. Among the many challenges it provides are the faint outer regions of the nebula that can be lost in processing, or simply not picked up at all during the imaging process, and its dazzlingly bright core that requires careful planning to capture. In the step-by-step guide below we’ve described the basic process of how to go about shooting M42 with the kind of setup you might typically have if you’re starting out in CCD imaging – that is a monochrome CCD camera and a set of LRGB filters (luminance, red, green and blue) with which to make a full-colour image.
STEP 1 Set up and polar align accurately
Once you’ve got your equipment set up, spend some time finessing the polar alignment of your mount. This is so you’ll be able to get the longest unguided exposures your mount is capable of before the stars drift out of position – this is especially important if you’re not using autoguiding equipment.
STEP 2 Capture different length luminance exposures
Use short, ‘binned’, test exposures to compose the image. Then take three groups of exposures through a clear luminance filter: short ones for M42’s bright core, longer ones for the main body and, for the faint outer regions, as long as your unguided mount can manage without the stars ‘wandering’ (usually several minutes).
STEP 3 Get the RGB colour
When you’ve got around 10-15 sub-exposures for each of the three groups of luminance data, you can move on to capturing the colour data through red, green and blue filters. Capture at least 10-15 images per colour channel – aim for an exposure length similar to your shots of the main body of M42 with the luminance filter.
STEP 4 Take dark IUDPHV DQG DW HOGV
After capturing each ‘LRGB’ channel, carefully stretch a clean white pillowcase or t-shirt over the scope aperture (without touching the lens) and illuminate it with a torch before taking an image. This is a flat field, which records image artefacts such as vignetting and dust on the optics. Also take a set of dark frames if the data from your CCD needs them.
STEP 5 Stack and calibrate the data
You should now have six sets of sub-exposures: three luminance groups of varying exposure length and one for each of the RGB channels. Load them into your preferred astronomical imageprocessing software (for example, DeepSkyStacker) and use the flat fields and dark frames to calibrate them before stacking those calibrated sets into six images.
STEP 6 Combine the three luminance images
Bring the three luminance images into layers-based image processing software, such as Photoshop or GIMP. With each image in a separate layer, erase the overexposed portion of the long-exposure image so that the ‘main-body’ exposure shows through – do the same for the main body layer so the core shows clearly. Merge the layers.
STEP 7 Add the colour DQG PDNH QDO processing adjustments
Next, place your red, green and blue filtered images in their respective colour ‘channel’ in a new image file. Copy the resulting full-colour image, as a separate layer, into the luminance file created in Step 6 and turn its blending mode to ‘Colour’. Lastly make any final image tweaks to your taste.