The limiting stellar magnitude attainable with a given instru ment, as shown by F. E. Ross, does not increase indefinitely as the exposure time is extended. In order that a perceptible image of a star may be recorded, the contrast between sky illumination, which is always present in some degree, and sky illumination plus brightness of the central diffraction disc of the star image, must not drop below a certain minimum value. The effect of sky illumina tion is determined by the ratio with the reservations ex pressed above the brightness of the optical star image varies as Increase in the linear dimensions increases the latter, but not the former ratio. These ratios, together with certain photo metric constants, determine the limiting magnitude attainable by prolonged exposure with any instrument. To photograph still fainter stars an instrument of larger aperture must be used.
The plate may be placed in the principal focus of the objective, and the image enlarged afterwards if desired ; but the amount of the subsequent enlargement is limited by the grain of the original negative, which is equally enlarged by the copying process. On the other hand, a lens or combination of lenses may be interposed between the objective and the plate to give an enlarged image on the latter. In the Greenwich photoheliograph the image of
the sun is enlarged on the plate in this way and the method is often used in planetary photography. For bright objects it is advantageous; but for fainter ones the exposures required may be impracticably long. The contrast also suffers, but this may be corrected by subsequent copying on plates of strong contrast.
In the early days of astronomical photography the inferior performance of the driving-clock of the equatorial was found to be a drawback, but this has been remedied, first by the introduc tion of electrical control and later by improved forms of driving mechanism of which the electro-motor controlled by a pendulum designed by W. P. Gerrish of Harvard is an example. The slight residual irregularities are eliminated by the use of the twin instru ment consisting of two parallel telescopes of nearly equal size in the same frame, one of which is used visually by the observer to guide; or otherwise by the double-slide plateholder and dupli cate guiding microscopes, to be used on two guiding stars on opposite sides of the field to detect the small rotation which may occur during exposures. (H. P. H.; F. H. S.)