Trouble on Palomar

The 200-inch Hale telescope on Palomar Mountain is ailing. Installed last January, it has thus far taken no scientifically valuable photographs of the stars. Dr. Ira S. Bowen, director of the observatory, announced this week that it might not be in working order until next fall.

The telescope's most serious trouble is a "bulge" in the massive mirror. The outer edge, 18 inches inward from the rim, is 20 millionths of an inch too high. This infinitesimal error, observable only by the most refined tests, is enough to make the telescope useless for serious astronomical purposes. Until it is removed or corrected, the distant nebulae a billion or so light-years away will keep their privacy.

The mirror is 24 inches thick at the edge, and looks absolutely rigid. But, judged by the delicate standards of astronomy, it is almost as flexible as cellophane, changing its shape appreciably (in millionths of an inch) with each new position as the telescope swings with the stars. In cells on the under side of the mirror (see cut) are 36 complicated mechanisms, each with 1,100 parts, designed to control this out-of-focus warping by varying pressure.

When the mirror left the optical shop in Pasadena, observatory authorities knew that its edge was "high," but they counted on the supports to correct the fault. The bulge turned out to be too big for the supports to handle.

The telescope also has another ailment. Complicated tests have proved that the edge of the mirror responds to temperature changes more quickly than the center. This makes the glass expand and contract unevenly, throwing the mirror's curvature out of whack.

Undiscouraged, Dr. Bowen points out that the 100-inch telescope on Mt. Wilson did not work acceptably for a year and a half. He predicts that the 200-inch will eventually be fixed. The first step will be the installation of a set of small fans to keep the temperature more uniform around the mirror. The mirror's outer edge may also be insulated with aluminum foil to make it respond more slowly to temperature changes.

With the temperature difficulties eliminated, the supports may be able to correct the pesky bulge. If not, the mirror will be taken out of the telescope and the bulge polished away--a six-months' job that cannot be started before spring. At intervals, the mirror would be put back into the telescope for the only conclusive kind of testing: on the stars themselves.

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