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Capturing the Hunter
The familiar winter constellation of Orion holds many surprises for imagers who want to delve a little deeper, says Will Gater.
There are few constellations that grab the attention quite like that icon of the winter heavens, Orion. The glittering bright stars, the instantly recognizable 'belt' and the many glowing nebulae scattered within the Hunter's boundaries all make Orion a wonder to behold on a frosty, dark night. But the constellation is also a rich hunting ground for astrophotographers seeking captivating targets of many kinds. In this article we're going to explore some of the different ways Orion's splendors can be captured on camera, from a simple nightscape that conveys the naked-eye view to advanced CCD imaging techniques that can reveal the constellation's extraordinary deep-sky features. And hopefully, by the end of this piece, you'll agree with us that no matter how many times you catch sight of the Hunter, you'll always find something new to inspire you and test your astrophotography skills.
The Hunter in his element
Experience Level: Beginner to Intermediate
What You'll Need: A DSLR or bridge camera and a sturdy photographic tripod. A wide kit lens (of the kind that comes with most DSLRs) will be perfectly sufficient. More experienced astrophotographers may also want to use a portable tracking mount to capture longer exposures.
There's something tremendously evocative about glimpsing the bright stars of Orion over a wintery landscape — or towards the end of the autumn months just as the nights start to get longer and colder — so in this project we're going to look at how to shoot a 'nightscape' that attempts to capture some of that magic.
STEP 1: Make a conceptual plan
Thinking about the emotions you want to convey or elicit with your shot can help you to plan a powerful picture, and it'll inform every stage of the photographic process. For example, if you wanted to evoke the harsh iciness of winter observing you might shoot Orion over an isolated, leafless tree in a barren landscape, and process in such a way as to create a hard contrast between land and sky.
STEP 2: Select your focal length or a prime lens
Once you've thought about what atmosphere you want to capture with your image, you can select the focal length you'll be shooting at. A typical kit lens set to around 24mm, or an equivalent prime lens, provides a wide field of view for Orion on a camera's sensor, allowing you to fit in the brighter central stars and the Hunter's fainter outlying 'arms'.
STEP 3: Focus the view
Next focus the view. Some cameras have a live preview function that can be zoomed onto a suitable star, giving you instant feedback as you make slight focusing adjustments. With Orion there's no shortage of bright stars that can be used for this. Repeat the process a few times — checking that the star is a small as possible — so you're certain the image is as sharp as it can be.
STEP 4: Compose with the landscape and sky conditions
To compose your nightscape you can take short, very high ISO test exposures to show you the balance and positioning of foreground and sky, and any structures or landscape features in frame. Try to use the foreground — trees, buildings, etc. — to lead the viewer's eye toward Orion. Sometimes clouds can be used as a framing device too, and thin cloud can even 'bloat' and enhance the colors of bright stars.
STEP 5: Set the exposure length, aperture and ISO
When shooting, keep the lens aperture wide open (lowest f-stop), though some lenses will perform better when reduced a few stops. Experiment with the ISO and exposure length until you're happy with the look. You may need to use an exposure that very slightly trails the stars in order to define the foreground.
STEP 6: Process your image
When processing nightscapes, reducing the noise in the image and bringing out foreground detail are the main challenges; as long as you shoot in RAW format, modern image-processing software is well-equipped to handle these tasks. In Photoshop or GIMP you can correct the color balance, and use the 'Curves' tool to bring out star fields and improve overall contrast and definition.
Far and Wide
Reveal the hidden delights lurking within Orion with the help of long-exposure, wide-field imaging.
Experience Level: Intermediate
What You'll Need: A DSLR, a tracking mount and either a relatively long focal length camera lens (between 100 and 300mm focal length on a full-format DSLR) or a short focal length refractor. You could use a CCD camera, but the field of view produced by your setup will need to be at least 5° across or you'll need to mosaic.
One of the things that makes Orion so attractive for astrophotography is the diversity of deep-sky objects within its borders, from pinkish-red star forming regions to blue-tinted reflection nebulae.
The proximity of these targets to one another means that long-exposure wide-field imaging of Orion can produce some spectacular compositions. Not only do such wide-field images show the positions of objects such as the Orion and Horsehead Nebulae in relation to one another, but they can also reveal the rarely seen fainter surroundings of objects that are usually given the 'close-up' treatment, such as the aforementioned nebulae.
A DSLR with a long focal length lens and mounted on some form of equatorial tracking mount is probably the simplest setup with which to get started in wide-field imaging. Unlike most deep-sky imaging, wide-field deep-sky astrophotography generally doesn't require auto guiding, as it's possible to capture good data with unguided sub-exposures of just a minute or two.
With fast prime lenses and those relatively short exposure lengths, you may be surprised at how easily you can pick up some of Orion's most recognizable deep sky objects. For the best results capture multiple sub-exposures (as well as dark frames and flat fields) and then calibrate and stack them, using software such as the free DeepSkyStacker, before final enhancements in your preferred image processing software.
With the right setup you can show Orion is more than just white stars against a black background.
Experience Level: Beginner
What You'll Need: A basic DSLR or bridge camera fitted with a lens that allows manual focusing (some compact digital cameras will also work depending on the lens/focusing mechanism they use). You'll also need a photographic tripod and your camera will need to be able to take exposures of a few seconds.
The color variation of Orion's bright stars is one of the most captivating things about the constellation, yet it can be tricky to capture these wonderful hues as the chromatic aberration in some camera lenses overwhelms the true star color. One method for showing the tints of stars such as Betelgeuse, Rigel and W Orionis is to manually defocus the image. It's a technique that was made famous by the renowned astrophotographer David Malin many years ago. You can use this method with a wide lens (or a fast long lens) on a static tripod, as long as you use short exposures — a second or so in the case of a longer lens. All you do is frame the star (or constellation), defocus the lens a little by hand and capture an exposure, usually at a mid-to-high level ISO setting. In the two composite images below we focused on Betelgeuse and Rigel. We captured a number of exposures and in between each one we defocused the lens a bit more. Then we combined them into one frame using processing software. It's a very artificial composition, but it does give a flavor of one of the things that makes observing and imaging Orion special.
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 motorized 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-color 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 color
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 color data through red, green and blue filters. Capture at least 10-15 images per color channel — aim for an exposure length similar to your shots of the main body of M42 with the luminance filter.
STEP 4: Take dark frames and flat fields
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 image processing 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 color and make final processing adjustments
Next, place your red, green and blue filtered images in their respective color 'channel' in a new image file. Copy the resulting full-color image, as a separate layer, into the luminance file created in Step 6 and turn its blending mode to 'Color'. Lastly make any final image tweaks to your taste.
ABOUT THE WRITER
Will Gater is an astronomy journalist, author and presenter. Follow him on Twitter at @willgater or visit willgater.com.
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