COSMICAL PHOTOGRAPHY So many different subjects requiring special treatment are included in the general class of celestial or cosmical objects that for the majority of them it will be more satisfactory to describe the methods of photographing them under their respective special headings (which see below). As regards the actual photographic details of procedure, however, much that is common to all celestial photography may most conveniently be detailed here.
Apparatus.—This will vary from the hand camera to the powerful and specially adapted mechanical camera of the astronomer ; they will all, however, have in common the feature of being focused for parallel rays, or, as it is usually termed, set for infinity. The reason for this is that for all practical purposes all celes tial objects are so far distant from the earth that any differences between their respective distances are inappreciable. This fact will to many constitute a considerable simplification, as undoubtedly the use of a " fixed focus " camera admits of apparatus being efficiently used in a condition which would be much too rough for photographing objects whose distances were different. In photographic language there is no " depth of focus " difficulty in celestial photography, and the flatter the field of defini tion given by the lens the more satisfactory will its performance be. This leads at once to the fact that for photographing large areas of sky the modern anastigmat type of lens, giving critical definition over a large angle, is the most efficient instrument.
In many cases the question of cost may serve to prohibit the employment of a sufficiently large lens camera—or refractor, as it is usually termed—and work of the highest type of accuracy has been done with concave mirror cameras. These involve only the optical working of one sur face, and thus for a given sum the instrument may be of a much greater power. Two kinds of reflector have been principally used : speculum metal and silvered glass. The former were excel lent, but when they tarnished it was exceedingly difficult to re-polish them without altering the shape of the reflecting surface and thus destroy ing the definition of the image. Most of the reflecting telescope cameras now in use consist of a surface of glass accurately ground to a parabolic form and then coated over with an exceedingly thin film of pure silver. This offers
the important advantage that when it becomes tarnished it can be dissolved away in ki few minutes and replaced very quickly with a new film without in any way interfering with the shape of the glass surface.
Plates.—For most kinds of celestial photo graphy, excepting that of the sun and moon, it is advisable to employ the fastest plate obtain able, provided that the grain is not noticeably prominent. On account of the subjects being generally of special coloration, the isochromatic or panchromatic plates now so easily obtainable should be preferred to the non-colour sensitive brands, as without them it may be found im possible to render differences which are quite easily noticeable to the eye.
Development. —This will, in general, be exactly the same process as would be used for ordinary terrestrial photography. Again except ing photographs of the sun and moon, the general tendency will be for under-exposure to be experienced, so that the treatment recom mended for this should be the normal procedure for astronomical work if harshness in the results is to be avoided. Any of the standard developers may be employed with practically equally good result. Pyro soda, metolquinol, and rodinal are all used by some of the best workers, and there is little to choose between them. In general, no bromide or other restrainer should be used ; the negative should be thin and full of detail rather than dense and contrasty. To further this, development should never be over-done, as it is practically impossible to remedy it by subse quent reduction, whereas a thin negative, with detail, may be gradually intensified and re intensified, by means of mercuric chloride and ferrous oxalate, until the requisite density is obtained.
For copies all the usual media are available. Lantern slides of good astronomical subjects make beautiful and interesting records.
(For details of various subjects see " Comets, Photographing," " Corona Photography," " Eclipses," "Lightning," "Moon," "Nebul2e," "Rainbow," "Stars," " Sun," etc.) C. P. B.