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Astrophotography - The Amateur Connection, The Roles of Photography in Professional Astronomy, Challenges and Changes

photographic images digital telescopes

DAVID MALIN
Anglo-Australian Observatory, RMIT University

DENNIS DI CICCO
Sky Publishing Corporation

Astronomy is the study of the sun, moon, planets, stars, galaxies, and other celestial objects. Historically, it was primarily an observational science, so the public unveiling of photography in 1839 was immediately recognized as important. Although Daguerre’s discovery was championed by astronomers Francois Arago and John Herschel (who was the first to use the word photography in English), the early photographic processes were too insensitive to record anything but the brightest objects in the sky. Nevertheless, by 1851 professional astronomers had succeeded in making daguerreotypes of the sun, moon, Jupiter, and the brilliant star Vega, setting the stage for future advances.

It was not until the early 1880s, after the introduction of the dry gelatin process, that long exposures of the night sky became practical. And these exposures revealed objects that were too faint to be seen by the eye even with the largest telescopes. The dramatic transformation of photography from a recorder of the visible to a detector of the unseen, opened a window onto a universe that was much bigger and more mysterious that anyone had imagined.

The introduction of photography into astronomy also led to a revolution in telescope construction, with refractors giving way to ever-larger reflectors, most of which were designed from the outset as huge cameras. Photography also changed the way astronomy was done. While astronomers still spent many hours at the eyepiece, they were guiding the telescope as it focused light on to a photographic plate, and the image became the data that were later analyzed.

Conventional (silver-based) photography was still used in professional astronomy until the 1990s when its gradual replacement by a variety of electronic detectors was almost complete. It is a remarkable achievement that such a seemingly simple technology as emulsion-based photography served as the workhorse of astronomy for about 100 years.

The Amateur Connection

While it was often a blurry line that separated professional and amateur astronomers during the 19th century, “gentleman scientists” in Europe and America achieved many of the milestones that transformed photography from astronomy’s interesting curiosity to one of its greatest tools. During the 1850s Warren De la Rue advanced lunar photography using wetcollodion plates with his homemade 13-inch reflector at Kew Gardens near London. He also initiated the first photographic patrol, making daily photographs of the sun between 1858 and 1872, and, in July 1860, he obtained the most significant results during the first total solar eclipse to be successfully photographed.

In the United States, New York “private scientist” Lewis Morris Rutherfurd made great strides in photographing star clusters and other celestial objects in the 1860s using telescope objectives of his own design that were optimized for photography. Fellow New York amateur astronomer Henry Draper obtained the first photographic record of a star’s spectrum in 1872 and the first photograph of the Orion Nebula in 1880, two years before his untimely death at age 45.

It was, however, another amateur’s photograph of the Orion Nebula that proved pivotal in the history of astrophotography. A 37-minute exposure of the nebula by English engineer Andrew Ainslie Common in January 1883 showed stars fainter than those seen with the world’s largest telescopes—the photographic plate had proved that it could look deeper into the universe than the human eye.

The juxtaposition of astrophotography’s escalating successes and the culmination of several extensive, but disappointing, projects done by traditional visual astronomy, almost certainly was the impetus for many professional astronomers to embrace photography during the latter half of the 1880s. By the beginning the 1890s there was no looking back—the photographic plate had become astronomy’s detector of choice. And it was now professionals who were spearheading the advancement of photography in astronomy.

Throughout most of the 20th century there was a distinct division between astrophotography done by professional and amateur astronomers. Although there were noteworthy exceptions on both sides of the divide, professionals used photography for science while amateurs used it to pursue aesthetic goals. Amateurs, however, were quick to exploit the latest photographic technology for their hobby, and this was especially true following the introduction of color emulsions, which were largely ignored by professionals. At the close of the 20th century many amateurs followed their professional counterparts in switching from conventional silver-based photography to digital imaging, and this also renewed interest among amateurs in using imaging for science.

The Roles of Photography in Professional Astronomy

Photography had three primary roles that changed in importance over time. The main one was direct imaging, especially the making of sky surveys and the morphological study of galaxies, star-forming regions, etc. The first survey (begun in 1887) was concerned with astrometry, the accurate measurement of star positions. The last all-sky photographic surveys were completed in the late 1990s and are still used for a wide range of astronomical research; their digitized data is publicly available on the Internet.

Photography was often used quantitatively in photometry, the measurement of the brightness of individual stars and non-stellar (extended) objects. Comparison of images taken through various color filters allowed the temperatures of stars to be determined for the first time. If the distance and temperature were known, the energy output of a star could be calculated. However, photography is a difficult and quirky tool for photometry, especially for point sources such as stars, and it was gradually replaced by photoelectric photometry beginning in the 1920s and more recently by charge-coupled devices (CCDs).

Perhaps the most important application of photography in astronomy was its most challenging and least visually spectacular; the recording of spectra. From spectra, astronomers can learn the chemical composition, rate of rotation, age, velocity of recession, and more about stars and galaxies. By looking at the photographic spectra of galaxies, the expansion of the universe was discovered early in the 20th century, and in the 1960s the spectra of some star-like objects showed them to be quasars, ultra-luminous galaxies at enormous distances, shining in the early universe. Various kinds of electronic detectors displaced photography from this role from the mid-1970s onward.

Telescopes are primarily designed to collect light and focus it onto detectors. This is also a definition of a photographic lens, but an astronomical telescope takes this process to extremes. Most modern telescopes are reflectors, and use huge concave mirrors many meters in diameter to capture and focus light. The focal length and focal ratio varies between instruments, but the largest have mirrors 8 to 10 meters in diameter with focal lengths of 17 to 20 meters, giving them focal ratios of around f/2. The prime-focus fields of these very fast telescopes are quite small, so images are usually made at the Cassegrain focus. This involves adding a smaller secondary mirror, usually a hyperboloid, which folds the optical system and produces a much larger field of view, but at a longer focal length and slower focal ratio. The focal surface can be much larger than any currently available solid-state detector. For example, the generation of 4-meters telescopes that were built in the 1970s and designed for traditional photography had fields of one degree (the full moon is a about half a degree), which were recorded on photographic plates 250 millimeters (10 inches) square. To use these instruments effectively with digital detectors, images are made by scanning a strip of sky with a linear array of CCDs, or as a series of small tiles that are “stitched” with software into wide-field images. Even the largest astronomical Schmidt cameras, which were designed for photographic surveys of the sky using plates up to 356 millimeters (14 inches) square covering 6.5 degrees on a side, have been re-fitted with scanning or tiled CCD systems.

Challenges and Changes

This variety of configurations and research applications presented many challenges to the makers of photographic emulsions, and for more than 60 years the Eastman Kodak Company manufactured a range of “spectroscopic” plates specifically for astronomy. Initially these were designed to be extremely sensitive to faint light by having high efficiency at low photon-arrival rates (plates having little low-intensity reciprocity failure, LIRF). But they also had high granularity and low resolution. In the late 1960s it was realized that for many astronomical applications a high signal-to-noise ratio was of primary importance, so fine-grain materials were designed that offered high resolution and contrast. These had lower sensitivity than earlier astronomical emulsions, but they were able to reveal faint objects against the uniform glow of the natural night sky and were ideally suited to Schmidt cameras.

They also responded very well to several hypersensitizing techniques that astronomers developed, especially the technique of “soaking” the emulsion in hydrogen gas (or a less-dangerous mixture on hydrogen and nitrogen known as forming gas) before exposure. These “gas-hypering” techniques effectively eliminated LIRF, so plates retained as much sensitivity during a 60-minute exposure as they had for a 0.01-second exposure. The penalty paid for this hypersensitizing process was a very short shelf life (hours instead of years), high chemical-fog levels, and, for optimum results, the necessity to expose the treated plates in an atmosphere of dry nitrogen.

This became a very specialized business, and as digital imaging advanced in the 1980s most observatories were relieved to exchange photographic specialists for a new generation of electronics experts who could provide small-area, instant-readout detectors that captured 80 percent and more of the incident photons instead of large-area, chemically developed detectors that had less than 10 percent efficiency.

Today professional telescopes all use solid-state, pixellated arrays for imaging and spectroscopy. These are often expensive and custom-made; however, they cover a much wider spectral range than photography ever could, and their construction, operation, and management require many more in-house specialists than did photography. But for many astronomical applications their benefit is equally substantial. In addition to much higher efficiency at recording photons, most digital detectors respond linearly, meaning that they consistently produce twice the signal for double the exposure, unlike photographic plates. This makes digital detectors ideal for measuring the brightness of astronomical objects.

Of course, not all professional telescopes are on the ground, and some of today’s finest astronomical images are made from space, where digital imaging is essential. The most conspicuous success is the Hubble Space Telescope, launched in 1991. Although initially hobbled with flawed optics, it was soon repaired and has provided a stream of astonishing images and groundbreaking science ever since. Several upgrades have given this rather small (2.3 meters) telescope a series of ever more sophisticated detectors, with capabilities far outstripping its original expectations.

Other earth-orbiting telescopes are making images at X-ray, gamma-ray, and ultraviolet wavelengths that cannot be recorded at ground-based observatories because they are absorbed by the earth’s atmosphere. At longer wavelengths, other kinds of solid-state arrays sensitive to infrared radiation both reveal and penetrate dusty regions where stars are forming in the arms of spiral galaxies, and outline the otherwise invisible arms themselves.

From both the ground and in space, the pixel arrays of digital detectors also offer accurate spatial information without resorting to the intermediate step using a measuring engine to derive positions on a negative. Precision astrometry has become as simple as clicking a computer mouse in the digital age. Indeed, once solely the domain of professional astronomers, astrometric measurements of asteroids and comets are now routinely done by amateur astronomers imaging with CCD cameras.

Amateur Astrophotography Although astronomy is filled with transient phenomena that offer unique photographic opportunities—solar and lunar eclipses, meteor showers, auroral displays, comets, etc.—outside of our solar system stars, star clusters, nebulae, and galaxies change relatively little with time. This makes astrophotography an endeavor driven as least as much by technique as opportunity. And considering the vastness and diversity of the universe, it is not surprising that astrophotography constantly adapts to the latest developments across the length and breath of modern photography.

From wide-angle photographs of the Milky Way captured with fisheye lenses having effective apertures of a few millimeters to pictures of planets and small nebulae made large-aperture telescopes having focal lengths of 10,000mm and more, there are astrophotography applications requiring virtually every lens and telescope made today. The range of exposures is equally diverse, spanning from 0.001 second for the sun (even after 99.999 percent of its light is blocked by appropriate solar filters) to hours for the faintest nebulae.

While many astronomical sights visible to the unaided eye, such as constellations, planetary groupings in a twilight
sky, meteor showers, and auroral displays can be recorded with cameras mounted on fixed tripods, most astrophotography relies on having a camera track the sky’s diurnal motion as objects rise in the east and set in the west. For work with wide-angle and normal lenses, the tracking tolerances are relatively liberal, and quality results have been obtained with hand-driven mounts made from scrap materials (examples can readily be found on the Internet by entering “barn-door mount” into any search engine).

The tracking requirements for longer focal lengths are more demanding, and today there are numerous camera mounts and telescopes with motorized drives specifically made for astrophotography and marketed to amateur astronomers. The once-tedious task of manually guiding a telescope to keep it precisely aimed at its target as it moved across the sky can now be relegated to electronic “autoguiders.”

Although the switch from photographic emulsions to digital detectors has benefited amateur astrophotography as much as it has professional research, the real “digital revolution” involves what happens after the exposures are made. Today the conventional darkroom is almost extinct in the world of astrophotography, replaced by myriad computer programs that perform once laborious darkroom manipulations with the blink of the eye. And these programs can accomplish tasks that were all but impossible even for the most skilled darkroom technicians.

The most significant of these manipulations involves combining, or stacking, multiple exposures to produce an image with a higher signal-to-noise ratio than found in any of the individual exposures used for the “stack.” In principle the process is similar to the elaborate darkroom schemes developed in the early 1960s to combine multiple negatives from aerial reconnaissance missions to produce photographs showing finer detail than recorded by any one negative. In astronomy a variant of this technique was used to create deep images that showed extremely faint features of galaxies and other “extended” (i.e., non-stellar) objects. It has also been used to create detailed images of the planets, but the complexity of the darkroom work limited its widespread adoption. Digital imaging, however, makes the combining process so simple that today it is rare to find any astronomical photograph made from a single exposure.

The ready ability to combine multiple images in register makes it the method of choice for color astrophotography. Commercial color films were never designed for long exposures. Though pleasing results were obtained using color film to photograph the brighter galaxies and nebulae, exposures were very long, and sometimes elaborate and inconvenient techniques such as chilling the film with dry ice were required. Professional astronomers, however, had long been making monochrome images on blue- and green-sensitive plates, primarily to measure the colors (and thus the temperatures) of the stars. It was but a small step to make a corresponding red-light plate. From this trio of exposures using conventional darkroom equipment, separate positive copies that could be combined onto color film to produce a true-color picture were made.

This additive, color-separation process mimicked that used to create the world’s first color picture made by the physicist James Clerk Maxwell in 1861. It also allowed processes such as unsharp masking and enhancement to be used before the color separations were combined into a color picture. This allowed complete control over dynamic range, hue, and contrast, and could reveal detail in heavily exposed regions. These controls are mimicked in modern digital image editing software.

Another aspect of astronomical photography to reap tremendous benefits from the ability to stack images is planetary imaging. To that end, today’s cameras of choice for planetary photography are simple computer webcams that record digital movies with frame rates between 5 and 30 frames per second. Software (some of it available for free on the Internet) then sifts through these frames selecting the sharpest ones recorded during moments of low atmospheric turbulence and automatically stacks them. It is not uncommon to see planetary pictures made from stacks of hundreds of frames culled from brief movie clips containing thousands of frames. Amateur planetary images made with modest backyard telescopes often rival those obtained with the Hubble Space Telescope, and they are getting better with time.

From the earliest days of wide-field astrophotography it has been known that dark skies away from urban lighting are necessary for the best results. This remains true today even though digital image processing can extract noteworthy pictures from exposures made under heavily light-polluted conditions. While many astrophotographers pursue their hobby by transporting equipment to rural locations for a few nights each month around the time of the new moon, a growing number are observing under dark skies via their computers, controlling remote telescopes and cameras and downloading images over the Internet. Some are using commercial facilities that rent observing time on an hourly basis, while others have established private “robotic” observatories built with entirely off-the-shelf hardware and software.

These developments reflect the enduring interest in the sky that has sustained amateur astroimaging for many generations. Making images of the fleeting stars and distant planets is not easy, and it attracts people who enjoy a challenge. Some even make their own telescopes, though this is not as common as it once was. Others are interested in photography or digital imaging and wish to stretch their skills. Yet others see the CCD as the silicon eye of the computer and are interested in extracting all the information that they can from the detector. Some endure the layers of technology because they find imaging the sky inspirational and the images beautiful. The most successful manage to combine all or most of these diverse interests.

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