M 
REPORT OF COMMISSION V GV-37 
zz 111 Tronsparency of Wratten Filter No12 
“Tz Sensitivity of Eastman G-Emulsion 
M4000 5000 6000 7000 8000 
| 1 | | 1 — Qmm 
1 zia 
to objective 
  
— 20 
—30 
Fic. 3. Focal curve of the Sproul 24 inch objective. 
5. MEASUREMENT 
Long-focus astrometric problems are customarily studied in rectangular 
coordinates x and y, which coincide closely with the directions of right ascension 
and declination of the equatorial coordinate system. For an angular extent « 
on the celestial sphere, a third order distortion o?/3 between plane and angular 
portrayal exists, which is ordinarily negligible because of the limited areal 
extent of the field used. For an extended stellar path, an increasing disparity 
between rectangular and spherical coordinates will develop which may be repre- 
sented to a high approximation as a slowly changing orientation of the equa- 
torial with respect to the rectangular coordinate system, amounting to 
A0 = + Ax sin 6 
where Ax is the projection of the path on the x-coordinate and à the declination. 
Because of atmospheric refraction, any vertical angular distance suffers a 
minute contraction, for which the first order term is represented by the factor 
R(1+tan®{); any horizontal distance, measured along the great circle suffers a 
contraction R. For a wide range in wave-lengths in the visual spectrum, R is 
close to 60", hence the scale reduction in the zenith amounts to .00029. 
The positions of star images in any one coordinate may be conveniently ob- 
tained with the aid of a long-screw measuring machine. The plate is mounted 
on a carriage which can be moved up and down in a plane, at an angle with the 
horizontal, and is oriented by two or more star images. The plate is viewed 
with a low-power (say 20X) microscope mounted on a carriage by means of a 
nut which fits a horizontal long-precision screw having a pitch of, say, one 
millimeter; by turning the screw and moving the carriage up and down, all 
portions of the plate may be viewed. The microscope contains a vertical wire 
(and a horizontal one to indicate the center of the field), for bisecting the star 
images. At one end the screw is provided with a graduated dial, so that the 
amount of turning, and hence the distance moved, can be read and recorded to 
.001 millimeter (one micron). It is customary to carry out averages and further 
calculations to .0001 mm. 
Two successive settings are made on each image; the central star is usually