SHOOTING THE MILKY WAY’S WIDE-FIELD WONDERS
How to image our Galaxy’s treasures with a tracking mount, DSLR and long lens.
Astrophotography can offer an interesting opportunity to look at the bigger picture. This is especially true of deep-sky imaging where wide-field photographs can show star clusters, nebulae and galaxies in context amid their broader surroundings.
In this article we’re going to explore capturing wide-field deep-sky images using a DSLR camera, motorised tracking mount and a – relatively – long camera lens. While that may seem like a fairly simple kit list compared to modern ‘close-up’ deep-sky imaging rigs, such a setup can nonetheless produce spectacular results.
What’s more, with the last of the summer Milky Way low in the southwest and the magnificent disc of the Galaxy running high overhead this month, there’s a tremendous array of potential targets for this kind of astrophotography in October’s night skies. So whether your subject is the star fields of Scutum and Aquila or the regions of nebulosity in Cygnus, there’s never been a better time to start photographing the Milky Way’s wide-field wonders.
One of the most attractive aspects of imaging deep-sky targets with a DSLR and motorised mount is that it is much less demanding on the mount’s tracking capabilities than a traditional deep-sky setup. With the latter, the view of a celestial object is highly magnified on the camera chip, meaning that you’d usually have to use another scope, camera and computer in conjunction with the main imaging telescope to ‘autoguide’ out any small imperfections in the mount’s tracking. With a wide field of view you shouldn’t need any of this kit. As long as your mount is reasonably well polar aligned and you can get exposures of a few minutes in length from the setup without the stars blurring, you should be fine. The trade-off, of course, is that the view of a deepsky object or celestial scene is much less magnified and set within a larger field of view – but as we’ve already discussed, that’s not always a bad thing.
The fact that only a basic, but sturdy, driven mount is needed makes this perfect for deep-sky imagers who need, or want to travel further afield for their photography. Both small equatorial mounts with a single, battery-powered motor or portable tracking mounts lend themselves well to astrophotography of this kind.
As well as the mount and the camera itself, the other key component is the lens you use. In our opinion, suitable lens focal lengths for this type of
wide-field imaging work sit roughly between 90200mm. Having the right equipment will only get you so far, though. The real secret to a successful deep-sky, wide-field shot is framing and composition.
Software solutions
Planetarium apps and software like SkySafari and Stellarium can help with this. Stellarium’s ‘Oculars’ plug-in, which overlays a given astrophotography setup’s field of view on the sky, is particularly useful for scoping out potential compositions; it really comes into its own, however, when combined with Stellarium’s ability to project various astronomical survey images onto the simulated sky using the recently added ‘Surveys’ feature.
For example, if you go into the ‘Surveys’ tab via the viewing options window (F4) you can select from the menu the option to show ‘DSS colored’, which displays deep-sky imagery from the Digitized Sky Survey (DSS). This is really helpful for planning wide-field, deep-sky images where you’ve got a lot of dense Milky Way star fields or bright emission nebulae. That would certainly be the case this month if you wanted to image, say, the mixture of nebulae, dust clouds and star fields that sit on the borders between Scutum, Ophiuchus and Sagittarius.
With the DSS image data displayed you can see these regions in far more detail than is generally shown in the main Stellarium simulation. And with the field-of-view box enabled from the ‘Oculars’ plugin you can then play around with different lens focal lengths and camera orientations to create the most aesthetically pleasing framing. Once you’ve settled on a composition that you like, note the positions of the brighter stars in the view, as this will help when you come to initially frame up your shot in the field.
When you have your equipment set up under the stars and your subject framed and focused, it’s worth
doing some initial exposure tests for light pollution, as this is one of the main things that impacts imaging of this kind. You want to find an exposure length and ISO-setting combination that gives you detail in any faint star fields or nebulae in shot, but where the background skyglow doesn’t ‘fog’ out the whole frame. If in doubt, it’s better to err on the side of a darker exposure setting. (For more on dealing with skyglow, see Image Processing on page 83.)
If you’ve decided to image a patch of the Milky Way dominated by dark dust clouds and dark nebulae set against dense star fields – like the region to the west of the bright star Altair, for example – try to keep the ISO setting on your DSLR low (in the region of 200-400) if you can; that’s because camera noise can be very noticeable in darker regions of an astro image, so shooting with a lower ISO will result in those areas appearing smoother and less noisy. Similarly, experiment with aperture setting before imaging. ‘Stopping down’ or reducing the aperture by a few stops can often reduce vignetting – a darkening effect around the corners and edges of a photograph – and improve the sharpness of stars.
Flat and dark frames
Once you’re happy with your settings, capture at least 20 to 30 minutes’ worth of exposures, discarding any that contain signs of thin cloud or airglow. Also take a set of ‘dark frames’; images captured under the same conditions and settings as your images of a celestial target, except with the lens cap on. They will help reduce the effects of camera noise during processing.
Before packing up, it’s also important to capture a set of ‘flat frames’, another form of calibration image used to remove the effects of vignetting and other optical artefacts. Take them by setting your DSLR to auto-exposure then stretching some plain, white fabric over the end of the lens hood; illuminate the fabric with a torch or headlamp and fire the camera shutter, say, 15-20 times to capture a collection of ‘flats’. These can later be combined into one ‘master’ flat frame by your stacking software.
By the end of your imaging session you should have your raw images showing your chosen swathe of the Milky Way as well as a set of darks and a set of flat frames. All these can then be loaded into astronomical stacking software, which will calibrate and combine them to create a single, stacked image that can be further processed and enhanced in software like Photoshop or GIMP.
And don’t worry if by the end of October you’re only just getting the hang of wide-field Milky Way imaging. With winter looming, there will soon be a new array of targets high in the sky at a sociable hour for you to test your newfound skills on.