ASTRONOMY: F. SCHLESINGER 13 
SPHERICAL ABERRATION IN ASTRONOMICAL OBJECTIVES 
DUE TO CHANGES OF TEMPERATURE 
By Frank Schlesinger 
ALLEGHENY OBSERVATORY. UNIVERSITY OF PITTSBURG 
Presented to the Academy, November 5, 1914 
In September of this year the 30-inch objective for the Thaw Photo- 
graphic Refractor was installed at the Allegheny Observatory. The 
objective was made by the John A. Brashear Company of this city. 
Before it was removed from the optical shop it was tested in a number of 
ways by the makers and by me. These tests implied an almost per- 
fect figure, and also indicated that the material in the objective is of 
unusual excellence. 
When the objective was first tested at the telescope on actual images 
of stars, a considerable amount of positive spherical aberration was found 
to be present; that is, the focal lengths of the outer zones were found to 
be shorter than for the inner and intermediate zones. The tests were 
repeated on as many different occasions as possible, with the result that 
while positive aberration was found to be always present, it varied in 
amount from night to night and even in the course of a single night. A 
study of the data thus obtained indicates that the aberration arises from 
effects of temperature and that it is more pronounced when the tempera- 
ture is falling rapidly. 
To remove this aberration the following expedients were employed. 
During the day-time, when the telescope is not in use, the objective 
is enclosed by a metallic cover. With the tube of the telescope this 
cover acts as a kind of temperature-case, throughout which the tempera- 
ture is roughly uniform. The observer removes this cover about one 
hour before beginning to observe in the evening, and at the same time he 
starts a small electric fan mounted near the upper end of the tube. This 
fan blows a continuous current of air into the tube through a large vent 
just below the rear surface of the objective; two similarly placed vents 
permit the air to escape from the tube. In this way the warmer air 
that has accumulated during the day is expelled and the rear surface of 
the objective cools as rapidly as the front. The upper (flint) lens of 
the objective is separated from the lower (crown) lens by a little less than 
half an inch. The iron cell in which the two are mounted has been 
pierced by six holes half an inch in diameter; these permit the air to gain 
access to the space between the two components. 
At the close of the night the observer covers the objective, closes the 
three vents and inserts cork stoppers in the six lateral holes. 
