P. M. MERIFIELD* (editor) J. M. SAARI R. 'vV. SHORTIlILL R. L. WlLDEY D. E. 'vVILHELl\'lS R. S. \VILLIAMS, JR. Interpretation of Extraterrestrial o - ) I I I ! Imagery KILOMETERS FRONTISPIECE. The multiringed basin of Mare Orientale. Lunar Orbiter IV moderate resolution photograph. Framelet width is about 90 km. Framelet orientation varies among the various Orbiter missions. (See page 489.) Note: Jorth is at the top in this and succeeding lunar photographs. (NASA photo.) Earth based telescopes; Photogeologic mapping of the Moon; Thermal mapping of the Moon; Lunar and planetary probes; Geologic interpretation of Lunar Orbiter photos; Photographic sensors for detecting extraterrestrial life. (A bslract on next page) TNTROD C'1'I0N * Chairman, Subcommittee V II, Photo Jnter­ HREE YEA RS HA VE elapsed si nce progress pretation Committee, American Society of Photo­ in the field of space imagery has been re­ grammetry, 1964-1967. Present address: Earth T Science Research Corp., Santa iVlonica, Calif. ported on to the members of the society (Van 90405. The authors are identified in their respective Lopik, Merifield, et at, 196.1), and a review of sections. 477 478 PHOTOGRAMMETRIC ENGINEERING some of the major projects that have been Ii te imagery of the earth is the subj ect of a undertaken is deemed appropriate. Terrestri­ subsequent paper to be published in this jour­ ally based telescopic photography and in­ nal. Al though this paper is intended to encolll­ frared imagery are continuing to be utilized as pass the broad spectrum of the subject, it can fundamental tools in the study of celsetial not be considered comprehensi\·e. Many proj­ bodies. Knowledge of the lunar and Martian ects ha\·e been reported elsewhere, and in such surfaces has been increased many fold by instances, the reader is referred to the ap­ imagery returned from Ranger, Surveyor, propriate literature. Lunar Orbiter and Mariner spacecraft. Satel- ABSTRACT: Imagery interpretation is playing an important role in the investi­ gation of celestial bodies. Astronomers utilize terrestrially based photography in the identification and mensuration of celestial objects such as galaxies, plane­ tary nebulae, supernova remnants and sun spots. Telescopic photographs of the Moon are being used to construct 1: 1,OOO,OOO-scale maps of the lunar surface, and Earth-based infrared images of the Moon provide information on the dis­ tribution of hot spots and aid in tlte analysis of thermal contour maps. Snrveyor photography has yielded data 011 the size, shape and distribution of lunar sur­ facefragments as well as the mechanical and photometric properties of lunar soil. Illterpretation of Lunar Orbiter photographs cOlltinnes to provide '/lew illsight into the origi'/l and history of lunar snrface feat ures. TERRESTRIALLY BASED TELESCOPES PHOTOGRAPHIC IMAGERY Dl CLASSICAL ASTRO:-,rOMY* T HE CAMERA THAT provides the astronomer design culminated in the last century in the with imagery is simply a large telescope in construction of the Yerkes 40-inch and Lick whose focal plane is placed a photographic 36-inch objective lenses, are usually most plate. The prj mary use categories of the de­ aberrant in chromatism and sphericity, while veloped plate are: (1) recognition and classi­ at sufficient distance from the optical axis fication of particular forms in the image de­ astigmatism becomes important. Modern, tail, (2) image photometry, and, the most large, fast reflecting telescopes have para­ time-honored of all, (3) the determination of boloidal surfaces so that perfect imaging is accurate relative positions of objects on the theoretically possible until such distance from celestial sphere. The observational difficulties the optical axis is achieved as allows coma to which confron t these endeavors are dom­ quickly set in. Except for telescopes designed inated by the principal problems of (1) specifically for wide-angle imagery, much use resolution and (2) radiative signal-to-noise is made of the naturally OCCUlTing coma-free ratio. The nearer objects are sufficien tly field. 'v\Then the 200-inch Hale telescope was bright that these two difficulties may be built, at f/3.3, the coma-free field was too considered independently, but fainter objects, small for adequately precise iris-diaphragm such as the outer regions of galaxies, presen t a photometry of stellar images, and a corrector situation in which the desired goals compete lens was designed by Ross to enlarge this fIeld with one another in a direct trade-off, because (Bowen, 1960a). the faster emulsion of a given type is always The epi tome of wide-angle telescopes is the the grainier one. true Schmidt camera, which makes use of the Most of the well-known astronomical fact that a spherical mirror has no optical axis telescopes are sufficiently large that off-axis and cannot, therefore, if a concentrically aberrations are of limited concern for the curved focal surface is used for the photo­ practical sizes of photographic plates. The sensitive surface, show coma or astigmatism classical large refracting telescopes whose (those aberrations which depend on size of field). Only spherical aberration is left, and * Contributed by Robert L. "Vildey, U. S. Geo­ this can be removed by figuring a corrector logical Survey, Flagstaff, Arizona. Publication plate, as an entrance pupil, as a figure of authorized by the Director, U. S. Geological Sur­ vey. revolution based on a calculated fourth- INTERPRETATION OF EXTRATERRESTRIAL IMAGERY 479 degree polynomial (Bowen, 1960b). Of course, afford the increased observing time required the reA ector plate does have an axis, bu t to reach the minimum detectable brightness because it is not a focusing de"ice its effects (Sandage and I'd iller, 1966). A typical emul­ are quite weak. sion used in o!Jsen'iug faint sources such as The angular resolution achievable in faint stars, distant galaxies, and gaseous astronomy is usually limited by the turbu­ nebulae would be an Eastman '<odak type lence in the atmosphere as a disturbance to 103a or lIa emulsion, whereas for brighter its wa"e-propagating characteristics (Stock objects like the :\loon, a much less grainy and Keller. 1960). The air ought to act, emulsion such as 649 or e,"en Panatomic X ideally, as an optically Aat filter p'"eceding the can be considered. One of the techniques of objective. A Auctuation in density is a Auctua­ avoidi ng the trade-off between the problem of tion in refractive index, however, and so this signal-to-noise (speed) and resolu tion (grain­ is not the case. The average size of the seeing iness) is the use of an image tube (Kron, 1966; tremor disk might be 1 to 2 seconds of arc at a Kron and Papiashvili, 1962) which adopts an reasonably good observatory site, while from electron-focu ed photo-emissi"e surface as an three to seven nights a year it would become intermediary to improve the basic quantum as small as 0.2 to 0.3 arc-sec in diameter. efficiency of the photographic process. Apart from seeing, there is a fundamental Some of the uses of imagery for morpho­ resolution limit imposed by the Fraunhofer logical classification have been: classification diffraction of the objective mirror, acting as of galaxies into a supposed evolutionary se­ an aperture stop. But this cannot be achie"ed quence (Sandage, 1961 a; Arp, 1966); di­ unless the primary mirror is precisely figured. tinction between distended gaseous nebulae, And even a mirror which is initially correct planetary nebulae, and supernova remnants can be strongly disturbed by thermal shock­ (Abel et al., 1964); distinction between open or flexure through improper support (Meinel, star clusters and globular clusters (\\'ildey, 1960). The requisi te condi tions become 1961); classification of sunspot groups; dis­ harder to fulfill as the mirror diameter in­ tinction bet"'een primary and secondary creases. Smaller mirrors can be figured to impact craters on the j\ loon (S hoemaker, achieve consistently their diffraction limit 1962); interpretation of lunar features, such under normal nightti me operati ng condi tions. as volcanic craters, domes, sinuous rilles, and The diameter of diminishing returns begins to garbens; geologic mapping by surficial expres­ set in at about 30 inches, and a 6O-inch tele­ sion (Shoemaker and Hackman, 1962); and scope which achieves its diffraction limit is the iden tification of i\ Iartian canali. quite rare. Although the diffraction limit of The second category-photographic pho­ the 200-inch Hale is 0.035 arc-sec, its opti­ tometry of celestial imagery-is untrust­ mum figure limit is about 0.3, whereas the worthy unless great care is taken to ensure U. S. Geological Survey's 30-inch telescope uniformity of exposure and de"elopment and has a 0.18 arc-sec diffraction limit and 0.20 preferably to calibrate simultaneously with has been achieved in practice. photoelectric observations of detailed parts In photographic imagery, the signal-to­ of the object. The photoelectric observations noise ratio in brightness comparisons depends are calibrated, in turn, to starts of constant on quantum detectivity (essentially emulsion luminosity, and when sufficiently compre­ graininess), the actual brightness level, and hensive they crosscheck the overall spa­ exposure time. Certainly a low f/ratio tial uniformity in photometric responsivit)'. (usually implying a large mirror) improves Lunar photometry may be used to determine the signal-to-noise '"atio achieved in a certain constraints for models of the lunar surface period of time, but because of the saturation structure and as a lunar geologic mapping tool phenomenon of a photographic emulsion (Wildey and Pohn, 1964; Pohn and Wildey, coupled wi th the importance of reci proci ty 1967). Similarly, an important constraint on failure at typical astronomical brightness atmospheric models is the predicted limb­ levels, the fain test signal that can be detected darkening of the planet in question.