736 HANDBOOK OF PHOTOGRAPHY 



renders this form of telescope for the photography of nebulae the instrument par 

 excellence. 



Whether the refractor or the reflector is employed for photography, the funda- 

 mental requisite is a stable mounting so designed and mechanized as to make possible 

 the continued registration of a given star field on the emulsion of the plate, in spite of 

 the diurnal motion of the sky. 



Mechanical Aids for "Following." — Since in all latitudes except at the equator 

 and the poles, the diurnal motion of a celestial object has both a vertical and horizontal 

 component, the only practical mounting for celestial photography is the so-called 

 "equatorial" form. In the equatorial mounting of a telescope, the principal axis, 

 known as the polar axis, is fixed so that it is with high accuracy parallel to the direction 

 of the axis of the earth at any given observing station. The polar axis of the tele- 

 scope mounting, therefore, will be directed to the true pole of the sky about which all 

 the stars appear to revolve. If a motor or clockwork with suitable gearing engages 

 with this axis, it will be so rated that the telescope will make one complete revolution 

 about this axis in the course of one sidereal day. The sidereal day is the period of the 

 rotation of the earth with respect to a fixed star and is shorter than the ordinary solar 

 day by 3 min. 56 sec. 



In order that the celestial camera or telescope may be directed to any part of the 

 sky, it must be possible to turn the instrument about an axis at right angles to the 

 polar axis. This second axis gives a movement of the telescope tube north and south 

 along the great circle, passing through the celestial poles. Since the stellar coordinate 

 representing the angle of departure of a star from the celestial equator is known as 

 declination, the axis which provides for the motion of the telescope in declination is 

 known as the "declination" axis. When the telescope has once been directed to the 

 object in the sky to be photographed, it will be firmly clamped in declination. If the 

 image is then centered on the plate and the driving clock of the telescope is allowed to 

 turn the polar axis, the object will continue to maintain its approximate position with 

 respect to the center of the plate. 



For exposures of but a few minutes on bright star fields when the focal length of 

 the telescope is comparatively short, the instrument may be left to itself to record the 

 photography of the star field. The instrument in this sense is practically automatic. 

 Such self -operating instruments have been in nightly use as "patrol" cameras at 

 Harvard and elsewhere. 



For prolonged exposure with instruments of large focal length, some auxiliary- 

 guiding device is necessary to compensate for irregularities in the clockwork and for 

 the variations in refraction and atmospheric disturbances encountered by the star- 

 light in reaching the plate. Such auxiliary "guiding" may be performed by utilizing 

 a visual telescope of the same or greater focal length made integral with the mounting 

 of the photographic telescope. This auxiliary telescope provided with a pair of 

 "cross wires" or intersecting spider threads at the focus of the eyepiece may be utilized 

 for detecting the slightest trace of movement of the star from the central position in 

 the field. Mechanical slow motions attached to the axes of the telescope and under 

 manual or electrical control may be used for keeping the star image in exact agreement 

 with the intersection of the cross wires. A satisfactory continued registration of the 

 star image in the field of such a "finder" will result in perfect registration of the image 

 on the photographic plate. 



In the employment of large instruments with very limited fields, one often needs 

 a guiding telescope of as large an aperture as the photographic one if faint stars are to 

 be employed for guiding purposes. The cost of such double construction, to say noth- 

 ing of the added mechanical difficulties involved, necessitates the employment of some 

 alternative mechanism. Hence came the development of the double-slide plateholder. 



