Test your skills by drawing out the colour, detail and objects in and around Orion’s Sword
The Orion Nebula, M42, is a bright emission nebula in the centre of Orion’s Sword. It is a popular starting point for astrophotography because it reveals how the visual greyness of most deep-sky objects becomes a wonderful colour palette when a camera is used In this article we will show you how to explore and capture the sights in and around the nebula.
M42 has a small cluster at its heart known as the Trapezium, named after the shape formed by its four brightest stars. If you use a camera’s ‘Live View’ mode with sufficient magnification, you will find that separating and focusing the Trapezium stars is quite easy to do.
The Orion Nebula’s core, known as the ‘Thrust’, is kidney-shaped and brighter than its surroundings. The Trapezium lies within the Thrust. Exposing properly for the Thrust will show its mottled texture and the Trapezium stars, but the outer ‘wings’ (the ‘Sail’ and ‘Sword’) may look underexposed. The faint wisps of nebula that extend beyond the Sail and Sword probably won’t show at all in an exposure designed to capture the Thrust. Conversely, an increase in exposure will reveal the fainter parts of the nebula but will overexpose the Thrust, losing the stars of the Trapezium in the process.
The solution is to use two or more exposures and combine the results to produce a higher dynamicrange shot than your camera can deliver in just one. Doing this in layer-based image editing software is easy, especially if it supports layer masks. The skill is combining these to look natural, disguising the fact that the result has been created from several images.
Expanding the search
After exploring the core, you can expand your sights to the surrounding area, as this is a great way to build your deep-sky imaging skills. Next to the Thrust is comma-shaped M43 and, as you head south, you will find the open cluster NGC 1980, which forms the southern end of Orion’s Sword with Iota (ι) Orionis.
Head north along the Sword to the faint emission and reflection nebula NGC 1977. Long exposures reveal darker lanes passing across the glowing cloud, which resemble the outline of a running figure. This is known as the Running Man Nebula. The northern end of the Sword is truncated by open cluster NGC 1981.
Wide-field shots of the Sword can be taken using a general camera lens. The low image scale used here will alleviate the need to have ultra-precise tracking. Close-ups of the Orion Nebula are best suited to camera-telescope combinations, where more tracking precision is required to avoid blurring.
Recommended equipment: a DSLR camera or equivalent, a general photographic lens or telescope, and a polar-aligned tracking mount
Step by step
STEP 1
Decide on the type of image you want to take. A general photographic lens with a focal length of 500mm on an APS-C sensor (equivalent to an 800mm lens on a full-format camera) will capture the entire Sword region. A 1,000mm lens (APS-C) or 1,600mm lens (full-format) will get you a lovely close-up of the main nebula.
STEP 2
For a photographic lens, set the focus to manual and the aperture to wide open, closing by a couple of stops if distortion occurs at the edges. Set the ISO to a low/mid value (lower is better for nebulae close-ups with an accurate tracking mount). Using a shutter-release cable is also recommended.
STEP 3
For the best quality, use the camera’s RAW format to view, open and edit images. Focus accurately – use ‘Live View’, if possible, on the brighter field stars. For a close-up of M42, use the Trapezium Cluster stars as targets. Take a test shot and review, adjusting exposure for the bright central Thrust region.
STEP 4
Increase the exposure time, maybe the ISO too, to capture the nebula’s outer regions, while overexposing the Thrust. If you can align and stack images, take shots of each type for this purpose. Take similar shots with the lens cap for darks. These should be subtracted from the corresponding images to remove hot pixels.
STEP 5
Open the correctly exposed Thrust image. Next, cut and paste the outer region shot into the Thrust image as a separate layer. Align the layers accurately. Draw a selection around the over-exposed Thrust in the upper layer; copy and use it to create a layer mask (Alt + layer mask button) for the upper image.
STEP 6
The correctly exposed Thrust image should show through the correctly exposed outer-region image with sharp edges. Select the layer mask, applying a ‘Gaussian Blur’ to it to soften the edges. Tweak the ‘Curves’ for both layers to achieve a smooth transition. When you’re done, copy the layers, merge and save.
Send your images to: gallery@skyatnightmagazine.com
Pete Lawrence is an expert astro imager and a presenter on The Sky at Night