ASTRONOMICAL PHOTOGRAPHY 747 



ammonia just prior to exposure. The procedure recommended is to bathe the plate 

 for 1 min. in a 4 per cent solution of 28 per cent ammonia. To avoid fogging, the 

 temperature should be maintained below 55 °F. and the plate dried as rapidly as possi- 

 ble immediately afterward. In the use of a fan, care should be exercised to see that 

 the air is dust free, such as will be the case if the procedure is carried out in a suitably 

 constructed drying box. If plates so hypersensitized are not used with reasonable 

 promptness after the hypersensitizing process, they should be kept in a refrigerator 

 until used. On account of their low humidity, mechanically cooled refrigerators are 

 to be preferred to those employing natural ice. The refrigerated plate should be 

 brought to normal temperature 24 hr. before use to prevent the condensation of mois- 

 ture upon the emulsion during exposure. 



Exposixres.— Except in the case of photography of the sun, moon, and planets, 

 exposures in celestial photography may vary over a range extending from a few min- 

 utes to many hours, depending upon the faintness of the object photographed. There 

 is an old adage current in the early days of photography, "Expose for the shadows; 

 the high lights will take care of themselves." This may be paraphrased for the celes- 

 tial photographer, "Expose for the faintest stars desired, and let the brightest take 

 care of themselves." The exposure time, therefore, for a celestial photographer 

 depends entirely upon the equipment available, speed of the plate, and the magnitude 

 of the faintest stars which it is desirable to record. 



• For the faintest extra-galactic nebulae on which theories of the expanding universe 

 depend, the largest telescopes available necessitate exposures equivalent to several 

 nights of observations. 



When an exposure of greater length than the duration of darkness in a single night 

 is needed, the slide of the plateholder is closed, the plateholder, however, being left in 

 place, and the instrument undisturbed until the following night. At the beginning 

 of the next period of darkness, the telescope is again directed to the region in question. 

 The same guiding star is brought exactly to the same intersection of the cross wires in 

 the guiding eyepiece. Then the slide of the plateholder is withdrawn and exposure 

 continued. This procedure is not at all uncommon in the case of photographing 

 spectra of faint objects. 



In the case of direct photography, there is a certain amount of illumination of the 

 night sky which introduces a slight fogging of the background of the plate. To pro- 

 long the exposure so that this fog obliterates the faintest star images recordable is 

 obviously futile. Hence for a given type of emulsion and a given form of telescope, the 

 light of the night sky usually sets a limit to the maximum practical exposure period. 



If groups of brighter stars only are desirable, exposure may be restricted to as short 

 an interval as will produce a measurable image of the given stars. The smaller and 

 better defined the star image becomes, the more satisfactory the result. When the 

 plate is exposed for stars of faint magnitude, the stars of the second and first magnitude 

 will be grossly overexposed, producing a large blotch on the plate which will often 

 obscure faint stars in the immediate vicinity of the brighter ones. Often a rotating 

 sector is employed during the exposure to cut down the light of an interfering bright 

 star. 



It is appropriate to mention here the marked characteristics on the image 

 of brighter stars found on the images of the photographic plate taken with a reflector 

 as compared with a refractor. In the case of a reflector, the brighter images are 

 characterized by a diffraction pattern which tends to produce a square image, often 

 taking the form of a Greek cross, as the exposure is prolonged. The square image 

 and the cross in the case of the reflector are due to the diffraction pattern of light 

 produced as the result of the finlike supports at the upper end of the tube of the reflec- 

 tor that hold the auxiliary mirror in the axis of the telescope. For stars not over- 



