A Public Health Warning

The next few pages 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!

fig. 2 A full moon has a serene beauty but the reflected illumination adds considerably to any light pollution. This is likely to restrict any other imaging to bright planets or clusters. I have a theory that full moons only occur on clear nights.

Lunar Imaging

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 with 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 resulting video is a 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 take individual stills 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.

fig. 3 The Rosette Nebula appears as a small cluster of stars when observed through a short telescope. The nebula is almost invisible, even in a dark sky. Our eyes are the limiting factor; at low intensities, we have monochromatic vision and in particular, our eyes are less sensitive to deep red wavelengths, which is the dominant color for many nebulae.

Planetary Imaging

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 just a refractor with a focal length of just over 900 mm combined with a high-quality 5x tele-converter and an adapted webcam.

These and the smaller planets pose unique challenges though and are not the primary focus of this book. Not only are they are more tricky to locate with portable setups but to show sufficient surface detail requires high magnification. At high magnification, every imperfection from vibration, tracking errors, focus errors and most significantly, atmospheric seeing is obvious. 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.

fig. 4 By way of comparison, if a digital camera is substituted for the human eye, we are able to record faint details and in color too. The above image has been mildly processed with a boost in shadow detail to show the detailed deep red gas clouds in the nebula. This is a large object, approximately 3x wider than the Moon.

Solar Imaging

Solar imaging is another rewarding activity, especially during the summer months, and provided it is practised 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 Sky 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 once thought the Moon was the biggest object in the night sky. Under a dark sky one may just discern the center of the Andromeda Galaxy with the naked eye but the entire object span is 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. At lower image magnifications accurate star tracking is 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 mechanical 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, a wide field brings with it the increased chance of aircraft trails across your images.

Small Deep Sky 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 effect of atmospheric turbulence is noticeable and it is usually the weakest link in the imaging chain.

fig. 5 A few months after I started using a CCD camera for astrophotography, a supernova was announced in the galaxy M95. I recorded an image of the dim galaxy (top) and used the Internet to identify the supernova position. The color image below was taken a few years later by which time the supernova has disappeared. I now have software that allows one to compare two images taken of the same object from different nights. This automatically identifies any “new” stars or, as in the case of a supernova in our own galaxy, a star that just becomes suddenly very much brighter. Galaxies are the favorite location for likely supernova, as they contain the most stars. A friend was imaging a galaxy as a supernova exploded. His series of unprocessed images proved useful to NASA since they showed the event unfolding between the separate image captures.

Environmental Imaging

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 (or slow tracking) 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.

Other Activities

Spectroscopic analysis, supernova hunting, asteroid, minor planet, exoplanet, comet and satellite tracking are further specializations for some astrophotographers. Supernova hunting requires a computer-controlled mount directing a telescope to briefly image multiple galaxies each night, following 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 specialities 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. His amazing images can be seen at www­.as­tro­pho­to.­fr.