The Diverse Universe of Astrophotography
“The fact that we live at the bottom of a deep gravity well, on the surface of a gas-covered planet going around a nuclear fireball 90 million miles away and think this to be normal is obviously some indication of how skewed our perspective tends to be.” — Douglas Adams
Amateur astrophotography can be an end in itself or a means of scientific research and in some cases, a bit of both. It might be a surprise for some, but amateur astronomers, with differing degrees of patronage, have significantly contributed to our understanding of the universe, in addition to that from the scientific institutions. As an example, Tom Boles in Suffolk, England has identified over 124 supernova with his private observatory; these brief stellar explosions are of scientific importance and their spectra help determine the size and expansion of the universe. The professional large observatories cannot cover the entire sky at any one time and so the contribution from thousands of amateurs is invaluable, especially when it comes to identifying transient events. I might chance upon something in my life-time but I have less lofty goals in mind as I stand shivering under a mantle of stars.
Astrophotography is not one hobby but many: There are many specialties and individual circumstances, as well as purpose. Depending on viewing conditions, equipment, budget and available time, amateur astronomers can vary from occasional imagers using a portable setup, to those with a permanent installation capable of remote control and operational at a moment’s notice. The subjects too are just as numerous; from high magnification planetary and deep space imaging, though medium and wide-field imaging in broad or selective wavelengths. Then there is lunar and solar photography as well as environmental astrophotography, which creates wonderful starry vistas. As with any hobby, there is a law of diminishing returns and once the fundamentals are in place, further enhancements often have more to do with convenience and reliability than raw performance. My own setup is fit for purpose and ultimately its limiting factor is my location. Any further purchase would do little to increase my enjoyment. Well, that is the official line I told my better half!
A Public Health Warning
The following touch on some of the more common forms of astrophotography and the likely setups. Unlike digital photography, one-upmanship between astrophotographers is rare but even so, once you are hooked, it is tempting to pursue an obsessive frenzy of upgrades and continual tuning. It is important to realize that there is a weak link in the imaging chain and that is often your location; light pollution, weather, stable atmosphere, obscuration and family commitments. Suffice to say, I did warn you!
The Moon is the most obvious feature of the night sky and easily passed over for more sexy objects. Several astronomers, including the late Sir Patrick Moore, specialized in lunar observation and photography. Being a large and bright object, it does not mandate extreme magnifications or an expensive cooled CCD camera. Many successful lunar photographs use a modest refractor telescope and a consumer CCD-based webcam adapted to fit into the eyepiece holder.
The resultant video image jumps around the screen and many frames are blurred. The video is only the starting point; subsequent processing discards the blurred frames and the remainder are aligned and combined to make a detailed image. Increasingly, digital SLRs are used for lunar photography, either in the increasingly popular video modes or as individual still frames at high shutter speeds. The unique aspect of the Moon, and to some extent some planets too, is that their appearance changes from night to night. As the Moon waxes and wanes, the interesting boundary between light and shade, the terminator, moves and reveals the details of a different strip of the lunar surface. No two nights are precisely the same.
The larger and brighter planets, Jupiter, Saturn, Venus and to a lesser extent Mars, have very similar challenges to that of lunar imaging. These bright objects require short exposures but with more magnification, often achieved with the telescope equivalent of a tele-converter lens. A converted or dedicated webcam is often the camera of choice in these situations since its small chip size is ideally matched to the image size. Some use digital SLRs but the larger sensors do create large video files and only at standard video frame rates between 24 frames per second (fps) and 60 fps. I have made pleasing images of Jupiter and Mars using a refractor with a focal length of just over 900 mm combined with a high-quality 5x tele-converter and an adapted webcam.
The smaller and more distant planets are more challenging still, since not only are they are more difficult to locate but amateur equipment typically will render them as a simple blob. These require more magnification, and as the magnification increases, so does the effect of vibration, tracking errors, focus errors and most significantly, atmospheric effects. The work of Damian Peach sets the standard for amateur imaging. His astonishing images are the result of painstaking preparation and commitment and his website (www. damianpeach.com) is well worth a look.
Solar imaging is another rewarding activity, especially during the summer months, and provided it is practiced with extreme care, conventional telescopes can be employed using a purpose-designed solar filter fitted to the main and guide scope. Specialist solar scopes are also available which feature fine-tuned filters to maximize the contrast of the Sun’s surface features and prominences. The resulting bright image can be photographed with a high-speed video camera or a still camera.
Large Deep Space Objects
One of the biggest surprises I had when I first started imaging was the enormous size of some of the galaxies and nebulae; I thought the Moon was the biggest object in the night sky. With a dark sky one can just make out the center of the Andromeda Galaxy with the naked eye but the entire object spans six times the width of our Moon. It is interesting to ponder what ancient civilizations would have made of it had they perceived its full extent. These objects are within the grasp of an affordable short focal-length lens in the range 350-500 mm. The lower image magnification makes accurate star tracking less critical and even in light polluted areas, it is possible to use special filters and reduce the effect of the ever-present sodium street light. Successful imagers use dedicated CCD cameras or digital SLRs, either coupled to the back of a short telescope or with a camera telephoto lens. Typically, the camera system fits to a motorized equatorial mount and individual exposures range from a few 10s of seconds to 20 minutes. Short focal length telescopes by their nature have short lengths and smaller diameters with correspondingly lightweight focus tubes. The technical challenges associated with this type of photography include achieving fore-aft balancing and the performance of the focus mechanism and tube as a result of a heavy camera hanging off its end. If you live under a regular flight path, the wide field brings with it the increased chance of plane trails across your images.
Small Deep Space Objects
The smaller objects in the night sky require a longer focal length to make meaningful images, starting at around 800 mm. As the magnification increases, the image brightness reduces, unless the aperture increases at the same rate. This quickly becomes a lesson in practicality and economics. Affordable refractor telescopes at the time of writing have typically a 5-inch or smaller aperture and at the same time, reflector telescopes have between 6- and 10-inch apertures. Larger models do exist, to 16 inches and beyond, but come with the inherent risk of an overdraft and a hernia. The longer exposures required for these highly magnified objects benefit from patience, good tracking and a cooled CCD camera. At higher magnifications, the effects of atmospheric turbulence are noticeable and is often the weakest link in the imaging chain.
I have coined this phrase for those shots that are astronomy-related but typically involve the surrounding landscape. Examples include images of the Northern Lights or a wide-field shot of the Milky Way overhead. Long exposures on a stationary tripod show the customary star trails, but shorter exposures with a wide-angle lens can render foreground and stars sharply at the same time. Digital SLRs and those compacts with larger sensors make ideal cameras for these applications and a great place to start with no additional cost. At a dark field site, a panorama of the Milky Way makes a fantastic image.
Spectroscopic analysis, supernova hunting, asteroid, minor planet, comet and satellite tracking are further specializations for some astrophotographers. Supernova hunting requires a computer-controlled mount directing a telescope to briefly image many galaxies each night, normally using a programmed sequence. Each image in turn is compared with prior images of the same object. The prize is not a pretty image but the identification of an exploding star.
Each of these specialties have interesting technical challenges associated with object location, tracking and imaging. For instance, on Atlantis’ last flight it docked with the International Space Station. Thierry Legault imaged it with a mobile telescope as it transited the Sun. The transit time was less than a second and he used a digital SLR, operating at its top shutter speed and frame rate to capture a sequence of incredible images, paparazzi-style. The incredible images can be seen at www.astrophoto.fr
Excerpt from The Astrophotography Manual: A Practical and Scientific Approach to Deep Space Imaging by Chris Woodhouse © 2016 Taylor & Francis Group. All Rights Reserved.
Save 20% when ordering from www.focalpress.com. Use discount code FOC20 at checkout.