INTRODUCTION

The first known photograph of an astronomical body is a daguerrotype of the moon—one of the earliest photographs ever—taken by Daguerre himself in 1838. The Sun was first photographed in 1842, and the first picture of a star—Vega—was obtained in 1850.

Photography makes use of film, which is a sheet of plastic—either polyester or celluloid—over which a layer of gelatin which contains light-sensitive compounds—such as silver nitrate or silver bromide—has been applied. The gelatin plus the silver crystals is known as a film's emulsion.

A frame of film may come in many sizes, but the most popular in amateur astronomy is the 135 format, which consists of rectangular frames measuring 24 × 36 mm. Frames used until recently in professional astronomy were usually larger—measuring about 350 mm on a side—and the emulsion was deposited into a glass blank, rather than one made of plastic. Such medium is known as a photographic plate—its use in mainstream photography was widespread only from the late 19th century to the early 20th century, yet, because of its physical rigidity, it remained the format of choice for observatories until the 1980's.

A photographic emulsion in a glass plate is able to record about 2% of incident light [Freedman, Kaufmann, 2002], while emulsions used by amateur astronomers—some of which contain larger silver crystal, or grains—may register in the range of 10%. A process known as hypersensitizing—which involves a variety of methods, such as heating the film or exposing it to chemicals— is frequently used in amateur astronomy to gain additional sensitivity.

Film originally tended to be most sensitive in the blue segment of the visible spectrum, peaking around a wavelength of 450 nm and extending only to the green—this pattern actually corresponds to the spectral response of the silver salts—but nowadays treating the emulsion with color dyes enables it to respond well into the red. Monochromatic—or black and white—film is naturally responsive to the near ultraviolet, down to about 250 nm, while and some infrared-sensitive emulsions have been made, some reaching to wavelengths as high as 1,200 nanometers [Malin, 2000].

Film suffers from a limited dynamic range, which may be defined as the ability to respond to a wide range of luminosity levels. The dynamic range of an emulsion is usually less than 1,000 (corresponding to an interval of less than 7.5 magnitudes) [Kitchin, 1998], although this can be considerably extended by computerized image stacking and processing. Film also tends to gradually lose sensitivity with long exposures, with the last 15 minutes of a 30-minute exposure—to give an example—recording a significantly lower amount of light as the initial 15 minutes. Known as reciprocity failure, this limitation may be minimized by hypersensitization of the film.

The first known use of CCDs in astronomy was an observation of Uranus in the methane band by Gerald Smith, Frederick Landauer, and James Janesick, made in 1975. At first—according to Janesick—professional astronomers were reluctant to abandon photography, yet, the new technology soon caught on. By the late 1980's, most important observatories—and even a few pioneering amateurs—were routinely using CCDs.

A CCD camera uses semiconductor chips—which are usually made of metalloids such as silicon or germanium—to produce an electrical charge, which is then read to computer system, and recorded as digital image file.

Regular CCD systems such as those used by observatories are capable of responding to about 70% of incident light [Freedman, Kaufmann, 2002], while those used by amateurs may surpass 50%. Thus, it may be said that a CCD's quantum efficiency (speed, or more exactly, sensitivity of detection) is about 35 times better than that of ordinary, untreated film.

CCD cameras are most sensitive in the red part of the visible spectrum, peaking around a wavelength of 750 nm. This translates into a good sensitivity well beyond the visible range, into the near-infrared segment of the spectrum. The resulting poor response in blue light is still quantitatively better than that of most films, but can still be enhanced by applying a phosphorescent compound to the CCD semiconductor chips [Kitchin, 1998].

CCDs have a linear response to light—that is, the response to incoming light does not weaken with long exposure times—thus CCDs are not affected by reciprocity failure. They also feature much greater dynamic ranges, in the order of about 100,000 to 500,000 (corresponding to an extent of 14.5 magnitudes).

An undesirable feature of CCDs is that of noise, which is caused primarily by cosmic rays hitting the silicon layers of the camera. It is possible, however, to eliminate this problem by digitally comparing several frames taken of the same target object. Another issue with CCDs is blooming, which refers to vertical streaks visible in a digital image, caused when an excessively bright pixel "saturates" and strays into adjacent pixels. Anti-blooming gates are now being built-in some CCD cameras, but their use results in decreased sensitivity and resolution.

Better sensitivity—
Higher quantum efficiency (speed), linear response and better dynamic range. The spectral range of a CCD is about 400 nm to 1,100 nm, wider than that of ordinary film. This covers essentially the entire visible spectrum as well as part of the near infrared.

Dimensionally stable—
The light sensing medium in a CCD—a flat semiconductor layer—is physically rigid. Positional measurements benefit greatly from this stability, as against those made from a flexible, photographic film.

Less processing delay—
Captured images go directly into a computer memory, and are ready for viewing and analyzing, while traditional photography requires film processing and other time-consuming preparations such as hypersensitization.

Loss-less medium—
Since the images are directly recorded into a digital file in a non-compressed format (FITS or TIFF), all data is preserved, while the process of scanning a film negative or slide may lead to significant data losses.

Film-based imaging, however, still enjoys a few advantages:

Cost—
This is, right now, the main advantage of traditional photography. CCDs are not very cost-effective, at least regarding resolving power. A photographic emulsion gives much better resolution than CCDs for the same price [Covington, 2000].

Larger field of view—
This is another big advantage of film. CCDs are physically small, ranging from about 10 to 75 mm across [Davenhall, Privett, Taylor, 2001], with those used in amateur astronomy still on the 10 mm range. A frame of ordinary 135 film measures 36 mm wide, with the larger formats measuring well over 100 mm, meaning that its light-collecting area—an important factor in determining the actual field of view—will be considerably larger, in a scale of about 3 times. This limitation of CCDs may be overcome by creating digital mosaic images, but this is a very time-consuming process.

Easier color imaging—
There is a variety of color slide films available—such as Kodachrome 64 and 200—capable of producing fine astronomical images just out of the box, while most CCD cameras are monochromatic. True color in digital imaging may be achieved by combining three sets of images—taken each with blue, green and red filters—but this could be burdensome for an amateur who may be working on a remote location, or trying to shoot an event occurring within a short frame of time.

If I, for example, were asked which method—film or CCD—is preferable for amateur astronomy, I would say that it depends on the type of work to be done. Any application involving photometry or astrometry would greatly benefit from digital imaging, while wide-field imaging—which is a favorite subject of amateurs—would still be better done with traditional photography. Yet, because of budget considerations, film-based imaging will still be appealing to the non-professional for a couple of decades.

In any case, it will be the continued demand for film products in the next few years, which will force the manufacturers to reconsider these somewhat premature decisions. In the words of an amateur astronomer, "Though CCD has revolutionized astronomical photography and Kodak is at the forefront of this research, many imagers firmly believe that film has many more useful years" [Keller, 2002].