focused by the gravitational field of 
the lens galaxy, and it is also affected 
by the gravitation of the galaxy cluster 
as a whole. Because the lens is not 
a simple spherical object, not the per- 
fectly shaped work of an -optician but 
a real galaxy of stars, the light bending 
is more complex than the simple math- 
ematical case that Walsh, Carswell, 
and Weymann had considered. Also, 
the B image is actually seen through 
the lens. These two circumstances ac- 
count for the fact that the A and 
B images are not equally spaced around 
the lens galaxy. 
During the year following the dis- 
covery of the double quasar, many 
radio observations were made of it. 
The most impressive results, however, 
were obtained in January and Feb- 
ruary of 1980, by means of a technique 
known as Very Long Baseline Inter- 
ferometry (VLBI). In VLBI a celestial 
radio source is observed simultane- 
ously with antennae at widely sepa- 
rated locations on the earth. The re- 
ceived signals at each antenna are re- 
corded on magnetic tape, along with 
very precise timing information from 
an atomic clock. Then the tapes from 
the various observatories are collected 
and analyzed by computer. The net 
result is a measurement of the size 
and structure of the radio source, with 
a precision comparable to that which 
a single radio telescope could provide 
only if its diameter were comparable 
to the transcontinental spacing be- 
tween the VLBI antennae. 
The VLBI measurements of the 
double quasar were obtained with an- 
tennae in Germany, Sweden, the 
United Kingdom, and West Virginia. 
They showed that the two images, A 
and B, are not simply little round dots 
in the sky. Instead, each consists of 
a tiny bright core with an elongated 
jet extending to the northeast. Thus, 
the two apparent members of the dou- 
ble quasar are virtual carbon copies 
of each other, maintaining a precise 
resemblance even when scrutinized 
with the most powerful technique at 
the astronomer’s command. 
About a year after the double qua- 
sar was discovered, a second case of 
gravitational imaging was found. The 
team leader in this case was Wey- 
mann. Although by that time a num- 
ber of astronomers were on the lookout 
for additional examples of gravita- 
tional imaging with galaxies and qua- 
sars, the second discovery was purely 
accidental. Weymann and several 
other observers were conducting rou- 
tine studies of quasar spectra. While 
observing the fairly well-known quasar 
PGl 115+08, Weymann noticed that 
during a moment of exceptionally 
good viewing, the blurry image on the 
TV monitor of the 90-inch Steward 
Observatory telescope suddenly re- 
solved into three components. One 
member of this triple quasar is much 
brighter than the other two, and all 
three are within three arc seconds of 
one another, making them even more 
closely spaced than the two images 
of the double quasar. Once again, how- 
ever, subsequent studies with the Mul- 
tiple Mirror Telescope showed that 
the three apparent quasars have iden- 
tical spectra and red shifts, indicating 
that this is indeed another sport of 
nature: three gravitational images of 
a single quasar to befuddle astrono- 
mers who have difficulty enough in 
accounting for the remarkable prop- 
erties of even a single garden variety 
quasar. 
Stephen P. Maran is a senior staff sci- 
entist in the Laboratory for Astron- 
omy and Solar Physics at NASA's 
Goddard Space Flight Center in 
Greenbelt, Maryland. 
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