Advances in Astronomical Photography at Low Light Levels

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  • Ann. Rev. Astron. Astrophys. 1979. 17: 43-71 Copyright 1979 by Annual Reviews Inc. All rights reserved

    ADVANCES IN x2145 ASTRONOMICAL PHOTOGRAPHY AT LOW LIGHT LEVELS Alex G. Smith Department of Physics and Astronomy, University of Florida, Gainesville, Florida 32611

    A. A. Haag Lowell Observatory, Flagstaff, Arizona 86002

    1 INTRODUCTION

    Upon seeing his obituary in a newspaper, Mark Twain commented that the rumors of his death had been greatly exaggerated. So it is with astronomical photography. Periodically we hear that photography is being replaced by electronic detectors, and that analog recording is giving way to digital techniques. In the face of this adversity astrophotography perversely continues to flourish, with a new generation of large widefield telescopes recently coming on line. What is the reason for this stubborn refusal to die?

    Certainly, simplicity and low cost are among the virtues of direct photography. Many observatories cannot afford the initial costs of electronic imaging detectors, to say nothing of the expenses of maintenance. Related to the notion of simplicity is the fact that photography displays information in a direct visual format, the perceptual mode to which nearly all workers relate most easily; it is interesting that many types of numerical data are finally presented in pseudo-photographic form to aid comprehension.

    Even more fundamental is the fact that the photographic emulsion is a compact solid-state information storage device of enormous capacity easily adapted to large formats. Whereas it has proven difficult to produce electronic detectors with linear dimensions exceeding about 50 mm, photographic plates with areas two orders of magnitude greater are in regular use. Since a typical fine-grain emulsion has a storage capacity

    43 0066-4146/79/0915-0043$01.00

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  • 44 SMITH & HOAG

    of 106 bits cm - 2 (Kowaliski 1972), such a plate can store several billion bits of information. And if the plate is properly processed and preserved, such storage is essentially permanent. The serendipitous possibilities of wide-field, high-density, long-term information storage have been repeatedly illustrated by the seemingly endless applications of the Palomar Sky Survey to problems undreamed of a quarter-century ago when the plates were taken.

    Not the least of the reasons for photography's present good health is the introduction of new high-performance emulsions and the discovery of techniques whereby the user can hypersensitize these emulsions to practical speeds. Discussion of these techniques is the primary concern of the present article. Condensed reviews of this topic have been given recently by Barlow (1978), Sim (1978), and A. G. Smith (1977).

    1.1 The Latent Image and Reciprocity Failure A discussion of methods of increasing the sensitivities of photographic materials is aided by a minimal understanding of the photographic process itself. Curiously, we know of only one group of chemical compounds, the silver halides AgCl, AgBr, and AgI, that display enough sensitivity to be useful for most photographic purposes. These substances have been described as solid-state amplifiers with a gain of a billion, which is to say that when light releases an atom of silver it can result in the production of as many as 109 silver atoms in the developed image (Dainty & Shaw 1974). How is this tremendous gain achieved?

    1.1.1 FORMATION OF A LATENT IMAGE According to the mechanism proposed by Gurney & Mott (1938), absorption of a photon by a silver halide grain excites an electron into the conduction band, leaving behind a positively charged hole. The mobile photoelectron is trapped at a defect in the crystal lattice, where its negative charge attracts an Ag+ ion. The electron and the ion combine to deposit an atom of neutral silver Ago. Because a single Ag atom is unstable, the process must be repeated several times at the same site to build up a stable latent image cluster of at least three or four silver atoms, which renders the grain developable. A smaller cluster, of perhaps two atoms, may be stable but too small to trigger development ; such subdevelopable latent image is the target of efforts at latensification (see Section 3.7). The positive hole is important in a detrimental sense because it may compete with the foregoing process by destroying the photoelectron through recombination, or by oxidizing latent image silver back to Ag+ . Certain sensitizing and hypersensitizing procedures are successful precisely because they inhibit the destructive tendencies of the holes.

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  • ASTRONOMICAL PHOTOGRAPHY 45

    At least three methods are used to create the vital lattice defects or electron traps in the grains (cf Duffin 1966). In the oldest method a liquid emulsion containing sulfur is "ripened" by heating, the result being the formation of "sensitivity specks" of silver sulfide on the grains. In reduction sensitization, sensitivity centers composed of atoms of pure silver are created by heating liquid emulsion in the presence of a reducing agent such as sodium sulfite. Finally, gold sensitization is achieved by adding a gold compound during the latter stages of emulsion-making; it is not clear whether the active agent is gold or gold sulfide. It appears that the fastest modern emulsions an, prepared by a synergistic addition of gold and sulfur sensitizers.

    1.1.2 DEVELOPMENT Development of an exposed emulsion is a process of selective chemical reduction, in which grains containing latent image silver are reduced entirely to metallic silver to form the final image; unreduced silver halide is later removed by a solvent during "fixation." How the latent image "tags" a grain to render it developable is more controversial than the Gurney-Mott theory itself. According to one hypothesis, development is an electrochemical process in which the silver latent image speck acts as an electrode to initiate the reaction (cf James 1977). Unfortunately, real emulsions contain "fog" grains that develop whether or not they have been exposed, introducing noise into the photographic system. Almost any treatment to which a finished emulsion is subjected, including merely letting it age, increases the number of fog grains.

    1.1.3 RECIPROCITY FAILURE The Gurney-Mott theory explains a phenomenon that is the bete noire of astronomical photography. In 1862 Bunsen & Roscoe propounded a "reciprocity law" for photochemical reactions which states that the total product of such a reaction depends only on the total energy absorbed, i.e., on the product It of the intensity of illumination I and the time of exposure t, and not on either factor alone. In everyday photography we invoke the reciprocity law when we make compensating adjustments of the shutter speed and lens aperture of a camera. When t becomes large, as in astronomical exposures, we encounter "failure of the reciprocity law," and a given It product yields less image silver than is the case for exposures in the usual snapshot range.

    This loss in sensitivity at long exposures resides in the instability of latent images consisting of single Ag atoms. If photoelectrons are created in a grain at a low rate, thermal diffusion may cause the resulting Ag atoms to wander away from the sensitivity specks before they can be joined by additional silver atoms, so that formation of a stable latent

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  • 46 SMITH & HOAG

    image cluster is delayed or even prevented. That this effect is more than academic is illustrated by the curve for the 11-0 emulsion in Figure 1. For exposures of an hour, this typical emulsion requires about 20 times as much energy to produce a standard density as it does for snapshot exposures around 1/100 sec.

    Fortunately, emulsion makers have discovered methods of reducing low-intensity reciprocity failure (LIRF). Kodak spectroscopic plates treated in this way are distinguished by an "a" in their designations: see the curve labelled lIa-O in Figure 1. While the sensitivities of 11-0 and lIa-O are similar for short exposures, lIa-O is nearly 10 times more sensitive than II-O at exposures of an hour. Much of the work described in this article is aimed at reducing still further thc LIRF of special a-type emulsions.

    1.2 Evolution of Photographic Sensitivity Figure 2 shows the progress of photographic sensitivity since the invention of the daguerreotype in 1839. Major revolutions occurred around 1851 and 1875 with the introduction of the wet plate and dry plate processes. During the last 100 years progress seems to have occurred by slow evolution, rather than by revolution, and in recent decades the curve seems to be app