SUN, PHOTOGRAPHING THE Very little interest attaches to photographs of the sun taken with cameras fitted with lenses of ordinary focal lengths, but good results may be obtained by means of medium or high-power telephoto equipments. If the equivalent focal length is sufficient to give an image about i in. or more in diameter, the photographs would probably be of value for scientific records of the phenomena taking place on the sun's surface. In the case of the photographer merely wishing to obtain a chance record, there is no necessity for providing any mounting, or, in fact, any accessories beyond some means of holding the camera steadily in the direction of the sun, and an exposure shutter giving the most rapid expo sure it is possible to make. Use any good brand of slow plates, preferably of the fine-grain variety, and develop with a rather hard deve loper, as in most cases over-exposure will be experienced in spite of the rapid shutter.
If records of more perfect astronomical value are desired, the only difference will be an improve ment of the mounting of the camera, to allow of repeated exposures without constant readjust meat of the image, and for large-scale work an increase of the aperture and focal length of the lens system employed. In most, cases an equa torial mounting will be found necessary, with clock-driven mechanism adjusted to the solar rate of movement. For the telephoto camera, however, either a positive or a negative secondary magnifier may be employed, the actual system chosen depending on the choice of the operator.
Both methods are in constant use, and give practically equally good results, although, of course, the positive magnifier makes the appa ratus more cumbersome. With such cameras the photographs will show simply the white round disc of the sun, with, at times, groups of dusky spots showing in belts across the middle regions. By means of a special spectroscopic attachment, called the spectroheliograph, it is now possible to screen off all light from the plate, except that of one particular colour, and on photographs thus taken it is found that certain patches of the surface are rendered much more prominent than on the ordinary pictures in integrated sunlight. Instruments of this type are now installed at all the chief observatories of the world.
(For notes on the phenomena to be photo graphed round the sun on special occasions, see under the headings " Corona Photography," and " Eclipses, Photographing.") Any part of a P.O.P. print that is white or too light and devoid of detail may be toned to a pale grey by sunning-down. A shield is cut and laid over the parts of the print that it is desired to protect, the print being put in a printing frame under a piece of plain glass.