748 HANDBOOK OF PHOTOGRAPHY 



exposed, this characteristic seldom appears and the round images can scarcely be 

 distinguished from those taken with the refractor or lens camera. 



Determination of Focus. — Unlike the sun, moon, planets and an occasional comet, 

 the images of stars on photographic plates give no delineation of the object photo- 

 graphed since even the largest stars at the distances encountered in astronomical 

 photography have an angular diameter far below the resolving powers of either the 

 telescope or the photographic plate. The star images obtained vary in size depending 

 upon the brightness of the star photographed. Each image, of course, represents the 

 reduction of a substantial number of silver grains acted upon by the light forming the 

 diffraction image of the source. As the exposure is prolonged, the area of reduction 

 of the silver grains spreads more or less uniformly in all directions, thus enlarging the 

 latent image. In order to produce images of the faintest stars, it is necessary that 

 the plate be in the exact focus of the lens or mirror employed. The focus for stellar 

 work may be determined either by (1) trial and error or (2) calculation from extra-focal 

 images. 



In the trial-and-error method a series of exposures of a few seconds are made on a 

 bright star, starting with the plate well within the known focus of the objective or 

 mirror, and making successive exposures after changing the focal setting by 1 or 2 mm. 

 imtil the final exposure is made with the plate well outside the known focus. By 

 turning one of the micrometer screws of the plate carrier between each exposure, a 

 suitable separation of the multiple images can be secured. 



After the plate has been developed, a series of star images will be observed of 

 varying size, depending upon the distance of the plate from the focus at the time of 



each exposure. If the focal settings for the 

 exposures have been appropriate, there will 

 appear two or three of the smallest images of 

 about the same size. By interpolation, there- 

 fore, the focal setting for the smallest image or 

 position of sharpest focus is thus determined. 



It is important to record the temperature 



of the air surrounding the camera, for, owing to 



the expansion and contraction of the telescope 



with temperature, the focus thus determined 



will not be correct for any other operating 



temperature. If care is taken to provide a 



series of focal settings for widelj"- different 



Fig. 7. Diagram showing thermometer readings, a curve may be drawn 



method of determining focal length ^^^ ^ -^^^ instrument which thereafter will 



CI telescope by extra focus method. , . 



show the proper setting of the plate for any 



temperature encountered under working conditions. It is obvious that a calibrated 

 scale of some sort should be provided, along with an appropriate index attached to 

 the plateholder to indicate relative settings of the plate with respect to the optical 

 system of the instrument. 



It is often desirable to determine the photographic focus of a given telescope or 

 astronomical camera by calculation from the results of two exposures, one of which 

 is made well inside the focal plane and the other of which is made well outside the focal 

 plane. 



In determining the focus by the extra-focal method, a cardboard diaphragm is 

 placed in front of the lens or mirror with two openings at the extremities of a diameter. 

 In the case of a lens of 10-in. aperture the openings in the cardboard screen may be 

 cleanly cut round holes of about 1 cm. in diameter. Pimches such as are used for 

 cutting wads for gun cartridges or those used by sailmakers for punching holes in 

 canvas to receive grommets serve as excellent tools for the purpose. 



