STELLAR PnOTOGRAPny. 207 



Charts. 



In the formation of charts of the stars by photography, we have a definite 

 model to copy. It is not likely that any one will attempt to construct by eye 

 observations charts of any considerable portion of the sky which will be more 

 complete than those of Peters and Chacornac. If then charts equal to these can 

 be obtained by photography, it may be regarded as an entirely satisfactory solution 

 of the question. The area of these charts is 5^ square, and their scale is 6 cm. to 1°, 

 or three times the scale of the Durchmusterung. This scale corresponds to a focal 

 length of 343.7 cm. or 135.3 inches. Bat it is impossible, without enlaro-ement, to 

 print the finest details visible on a good photograph, and, if printed, they could not 

 be seen without a magnifying glass. The necessity of such a glass would greatly 

 interfere with the general utility of star charts, especially when they are to be 

 compared with the stars at night. Accordingly, the plan of enlarging the photo- 

 graphs does not seem objectionable, although some of the finer detail is lost. The 

 scale of the photographs taken with the telescope described on page 184 is 2 cm. 

 to 1°. If then they are enlarged three times, their scale will be the same as that 

 of the charts named above. Lenses ai'e made for ordinary photographic purposes 

 which will include a field of view of 60", or even 90°, without serious distortion. A 

 photograph of the stars is, however, a far severer test. The distortion becomes 

 perceptible even at a few degrees from the centre. With a single achromatic lens, 

 the distortion is perceptible within a single degree ; but with the compound achro- 

 matic, such as that of the telescope just mentioned, a much larger angle may be 

 covered satisfactorily. The distortion at the sides of the plates, 5° from the centre, 

 is not very large ; at the corners of a plate 5° square, about 3°.5 from the centre, 

 the errors are so small that they will not seriously affect the value of a map. 



The advantages of this plan for constructing star charts are its economy and 

 the rapidity with which the work can be performed. When several exposures 

 are made on each plate, an error in one will ruin the whole. A single exposure of 

 one hour is here proposed, which also diminishes the danger of interruption by 

 clouds. The apparatus works automatically, and an observer is not needed who 

 shall continually correct the motion of the clockwork by watching a star through 

 an attached telescope. A great saving in fatigue is thus effected, and skilled labor 

 is not required, since the work may easily be reduced to a routine. 



The cost of continuing the work throughout the entire night would be small, 

 since it would only be necessary for the observer to change the plate and readjust 



