Evening star. [volume] (Washington, D.C.) 1854-1972, 1956-12-16

stitution are the United States
Bureau of Standards. the Naval
Observatory and the Westing-
house Corp. Several European
observatories are working along
the same lines. Instruments built
on the same principle already
have proved successful, In lin
tied way.
Matter of Photography
Modern astronomy is largely a
matter of photography. The Mt.
Palomar astronomer actually
need never see star with his
own eyes. The light from a lu-
minous object-star or conglom-
Grafton Of stars-is concentrated
by the mirror of the rejecting
telescope and falls on a photo
graphic plate. This light darkens
grains of silver iodide in the
emulsion with which the plate is
covered. The pattern of these
darkened grain against the gray
white background of the plate
constitutes the picture, Just as
in SNAPSHOT. The more grain
darkened the greater the detail,
Light is divided into extreme
~ minute units - some say
packages known as photons.
II has been determined that 900
photons are required, on the
average, to darken one grain.
Every grain darkened can be
considered as one bit of recorded
information about the object
photographed The "quantu ef-
flciency," as the astronomers say,
Is about tenth Of one per cent.
Changed Into Light
But when light strikes a light
sensitive metal, or photocathode
the sensitizing part Of tele-
vision tube-it elects from the
surface one electron for every 10
photons. These electrons can be
transformed into light which can
be focused on the plate. Thus
the Photographic efficiency of
the luminescence from an ex-
ternal object is multiplied 100
times. With the light gathered
by the mirror Of the 200-inch
telescope as much could be &C
complished as with that which
could be gathered by an impos-
sible 2.000-inch telescope.
One instrument Incorporating
the design has been built and
used by Prof, Andre Lallemond
of the Paris Observatory on the
Pic du Midi in the French
Pyrenees. Prof. Lallemond, by
the way. must be considered the
Gallileo of astronomy's new age.
With the instrument. however
crude and inefficient in its pres
ent stage. he has obtained clear
pictures of naked eye stars. He
has demonstrated that the idea
is workable. A telescope built
according to his model also has
been used successfully at the
Yerkes Observatory of the Uni
versity of Chicago.
Gases Poison" Plate
Naturally the designers Of the
television- telescope are encount-
ering difficulties. The worst, to
date, has been that gases aris-
ing from a Photographic emul-
sion very quickly poison" the
sensitive cathode plate so that
it no longer reacts to light. Plate
and cathode must be kept in
separate compartments. There
must be a filter between the two
through which electrons can
pass but not gases.
Prof. Lallemond solved this
problem. after a fashion. He
brought together a photocathode
and a Photographic plate in a
single vacuum tube. TIE cath
ode. however, was enclosed in a
glass capsule until the last pos
sible moment. At the instant of
exposure. however. the capsule
was broken by a magnet-con-
trolled hammer. and at the same
time the cathode was pulled, also
by a magnetic device. in front
of the exposure window.
With the design upon which
Carnegie Institution and Bureau