Pickering wedge for photometer

A wedge for visual stellar photometry

Stellar photometry—the measurement of a star's brightness—is an exacting and error-prone undertaking even with today's sophisticated instruments. Photometric measures are strongly influenced by local observing conditions and Pickering photometer instrument characteristics. Differences in individual visual perception, an inherent problem when eyes are the detectors, made repeatable measurements even more difficult.

E. C. Pickering, astronomer and director of the Harvard College Observatory, designed a photometer that he hoped would make measurements more accurate and repeatable. He proposed using the device in a cooperative program to measure faint stars at several observatories. Lick agreed to participate and was provided one of the instruments—Photometer No. 3 (left)—in 1900. The photometer contained a small electric lamp which projected an "artifical star" onto a piece of glass so that it appeared next to the actual star to be measured. The wedge pictured above was slid in front of the lamp, gradually dimming it until the brightness of the artificial star matched that of the real one. The position of the wedge was then read from a scale, and the brightness of the target star estimated.

Lick Historical Collections, catalog nos. SO000288 (wedge) and SO000272 (photometer)