Harvard astronomers, who for 200 years have studied the stars and planets through the telescope, the spectroscope, and the photograph, are developing a new group of electronic instruments for searching the heavens.
Until the last decade, nearly everything we know about the planets, stars, and galaxies came to us on light waves in the narrow, visible portion of the spectrum. Astronomers have likened the visible region of the electromagnetic spectrum--which stretches from short-wave gamma rays at one end and the long radio waves at the other--to a tiny window looking out on the universe. Most of the revolution in modern astronomy stems from the design of instruments that have opened up new "windows" in the electromagnetic spectrum.
Many of these telescopes, cameras, radiometers, spectrometers and spectrographs have been built, and others are being built by astronomers at the Harvard College Observatory and the closely associated Smithsonian Astrophysical Observatory. The laboratories on Observatory Hill have become modern electronic workshops.
Some of the devices built in Cambridge are for use with telescopes and other instruments at the half-dozen or so stations in the United States and abroad where University astronomers are working; others are designed to observe the universe from satellites above the earth's obscuring atmosphere. The following is a report on three of these new electronic instruments, each of which peers through a different window in the electromagnetic spectrum: the infrared, the far ultraviolet, and the radio.
How Hot the Moon
Nearly all that we know about the moon has come to us in visible sunlight, reflected from the moon toward the earth. There are other, longer waves that travel invisibly between the moon and the earth, which can add knowledge about our satellite: the infrared, or radiated heat, rays. Although thermal photography is not new, the Harvard College Observatory has recently built one of the most sensitive instruments in existence for making thermal "pictures" of the moon.
The instrument, about the size of a TV camera, is called a radiation pyrometer. It was built under the supervision of Donald H. Menzel, Director of the Harvard College Observatory, with the collaboration of Hector Ingrao, Research Engineer and Lecturer on Astronomy, who designed the device. The pyrometer is a product of the Observatory's Infrared Laboratory, established with funds from the National Aeronautics and Space Administration.
The pyrometer, which has already performed well on the 61-inch reflector telescope at Harvard's Agassiz Station, measures the temperature of a small area of the moon's surface at a time. On a map of the moon some five feet in diameter, this area is about the size of a postage stamp. As the pyrometer scans the moon from side to side and from top to bottom, the record of intensity maps the distribution of temperature over the whole disc of the moon.
The heart of the instrument is a tiny square "thermistor," one-tenth of a millimeter on a side, small than a pin head, and attached to two hairlike platinum wires. The electrical resistance of the thermistor changes in response to tiny fluctuations in temperature. Special filters allow only a very narrow band of the infrared, between eight and 14 microns, to fall on the thermistor. The filter rejects the shorter infrared waves omitted by the sun and reflected by the moon. The earth's atmosphere, where water vapor, ozone and carbon dioxide absorb other portions of the infrared, also acts as a sort of filter. However, one can easily derive the temperature of the moon from the intensity of infrared radiation in the band transmitted between eight and 14 microns.
Attached to the pyrometer is a 35-millimeter camera accurately aligned to take pictures of the exact area of the moon's surface from which the thermistor is recording. As the pyrometer scans, the camera and the thermistor record alternately.
The moon's thermal profile will be compared with a conventional map of the moon to see of any correlation exists between the temperature variations and such surface features as mountains, "seas," and craters.
Russian astronomers have reported signs of volcanic activity on the moon, and Menzel hopes to arrange with Russian observatories to report immediately to the Cambridge Observatory any signs of volcanic activity so that the observation may be corroborated, using the Harvard pyrometer.
In other experiments, the pyrometer will measure how fast the surface of the moon cools. From the cooling rate, astronomers hope to estimate the size of the particles composing the moon's surface--rocks or dust.
Under favorable conditions, the thermistor can detect a change in temperature on the moon of less than one degree Fahrenheit. The Infrared Laboratory has invented and built another detector, called a ferroelectric bolometer, which promises to be even more sensitive. It should be able, for example, to detect the heat given off by a human hand flashing across it from across the room.
The Flares of the Sun