260 PROCEEDINGS OF SECTION A. 



6,— ON SOME OF THE DIFFICULTIES OCCURRING 

 IN THE PHOTOGRAPHIC CHARTING OF THE 

 HEAVENS. 



By R. L. J. ELLERY, F.B.S., C.M.G. 



[^Abstract.'] 



It is found from the experimental work at llie Melbourne 

 Observatory that the exposure necessary to obtain a 

 measurable image of a star the diameter of this image 

 must be over ^" ; if it be of less diameter we find the 

 image of insufficient density to measure with certainty. 

 This is larger than some astronomers state is sufficient 

 according to their experience, but I think they must be 

 satisfied to measure a disc that can be measured only v/ith 

 some uncertainty, unless indeed different instruments give 

 different results. We have come to the conclusion that with 

 our telescope no disc is black enough to measure unless it is 

 ^"'16. If this be true generally, we have the diameter of 

 the disc as a measure of the density, aud the question of 

 duration of exposure for the 11th or 14th magnitude would 

 be settled by directing that the images be ^"'16 diameter. 

 Taking this as a basis we find that under best condition of 

 sky a white 9th magnitude star is well impressed in 30 sees., 

 but to get ordinary 9th magnitude stars under ordinary 

 atmospheric conditions requires an exposure of 60 sees. 

 Excluding all decidedly over exposed stars or stars of the 

 red type (which upset every law yet formulated for this 

 purpose), the diameters of the images are proportional to the 

 square root of the times of the exposure. 



From the measure of a great many stars it appears that to 

 gain one photometric magnitude (light ratio 2*51 2) the time 

 of exposure must be increased 3" 16 times, or 10 times to 

 gain 2 magnitudes; that is, if a 9th magnitude requires 

 60 sees, to give a measurable disc (3'^'75) it will take 10 

 minutes' exposure to get a measurable disc of the 11th 

 magnitude. Still, as I have already stated, it has been 

 decided to expoee plates destined for the catalogue (11th 

 magnitude) only 5 or 6 minutes. This decision leads me to 

 the belief either that discs which we find unsafe to measure 

 are accepted as sufficient, or that our telescope has some 

 optical constant which will have to be taken into account. 



