The design of a dew detector for use on astronomical telescopes is described. The sensing element for the detector is a piece of printed-circuit board etched with a grid pattern of metallic and nonmetallic areas. The sensor grid pattern is produced on the circuit board by photoetching from a positive contact-printing mask. The relation between the electrical resistance and the relative humidity of the surface of the sensor is studied. The alarm circuit for signaling when the resistance of the sensor drops to a selected value is examined. The detector has been tested on the 3.0-telescope of the Lick Observatory; it is noted that the detector has saved 9 hours of observation time when the humidity was above 93 percent.