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Analyses of historical data suggest that the solar diameter may vary with time with an amplitude of a few tenths of a second of arc. The High Altitude Observatory has constructed a special purpose telescope, the Solar Diameter Monitor, designed to detect any such changes. The telescope is an f/50 transit instrument with an aperture of 10 cm and is almost completely automated to avoid observer bias. Each day at solar noon, it measures the sun's horizontal diameter by timing the solar disk transit time and the vertical diameter by comparing the image size to that of a stable length standard. Preliminary estimates suggest that these observations will allow a test of the solar diameter's constancy at the 1-sec of arc/century level in an observing time of 3-5 years.

A photoelectric polarimeter for measuring line profiles in all four Stokes parameters has been built and operates on the SPO 40 cm coronagraph in a joint project with Sacramento Peak Observatory. A description of the optical and electronic systems and the calibration scheme is presented. Performance parameters determined from observations are also given. The polarimeter package consisting of a pair of KDP's, a quarter wave plate, and a polarizing beam splitter is located at the prime focus of the coronagraph. Modulation of the KDP's encodes polarization information into intensity signals that are electronically detected. The scanning of the spectrum, accomplished by rotating the grating, permits Stokes line profiles to be recorded on magnetic tape for processing. The instrument can be used to scan any line from 3900 to 7000 ~ with a spectral resolution of 0.01 ~. Polarizations as small as 0.001% are detectable. The polarimeter and observing system are computer controlled.