flashing sequence. A short time later the 'postcalibration' is 
performed. Useable stars to the 9th magnitude are recorded by 
the Astrotar, to the 8th magnitude by the Aviotar, and to the 
Tth magnitude by the Aviogon. The final plate thus consists 
of a series of images of the missile-borne flashing light re- 
corded against a background of hundreds of superb reference 
points provided by the stellar images. Inasmuch as the stellar 
images vary wldely in size and density, one has little diff 
in obtaining a large selection of control consisting exclusively 
w 
of stellar images whose photographic characteristics closely 
match those of the flashing light images. When such care is 
taken in the selection of the control, the problem of personal 
bias in measuring is essentially eliminated, for personal bias 
would then tend to be the same for both the flashing light ime ages 
and the measured control. 
2 
The most widely used missile-borne flashing lig 
m, 
nt sources 
consist of pyrotechnic flares and high intensity strobe lamps. 
The most powerful pyrotechnic flare currently employ 
work, the "Daisy", weighs about an ounce and is sufficiently 
brilliant to be recorded at distances in excess of 5,000 nautical 
miles by a camera of 4 inch aperture employ ring an infra-red sensi- 
tive emulsion. An advantage of flares is that they entail but a 
Small weight penalty as long as the number of flashes required 
is not large. Their main disadvant cages are relatively long flash 
durations (3 to 5 milliseconds) and the fact that they must be 
10