A Fabry-Perot Interferometer For Accurate Measurement Of Temporal Changes In Stellar Doppler Shift.

Abstract

We are making accurate observations of the changes in Doppler shift of stellar absorption lines. The scrambling of incident light by an optical fiber and the stability of wavelength calibration by a tilt-tunable Fabry-Perot etalon provide immunity to systematic errors. A cross-dispersed echelle spectrograph spatially separates the orders of constructive interference transmitted through the etalon. Selecting several echelle diffraction orders in the vicinity of 4250-4750 A, which are imaged on a CCD, about 350 Points on the profile of the stellar spectrum are sampled by successive orders of interferometric transmission through the etalon. At 4300 A each interference order is 47 milliangstroms wide and the sample points are 0.64 A apart, causing distinct, widely-spaced monochromatic images of the entrance aperture to be formed in the focal plane of the camera. Changes in Doppler shift modify the relative intensities of these images, in proportion to the slope of the spectral profile at each point sampled; for simplicity, the data are being analyzed only for changes in radial velocity rather than its absolute magnitude. With an argon emission line lamp the interferometer is calibrated to better than 2 parts in 100 million; this corresponds to + 6 meters/sec in Doppler shift. These calibrations show instrumental variations of + 12 meters/sec on a time scale of months; observations of stars are corrected for such changes. The internal repeatability of observations of the differential Doppler shift of Alpha Tau (K5 III; B=2.4) is + 6 meters/sec for each exposure of 224 square meter-seconds, and + 8 m/s for Beta Gem (KO III, B = 2.1). These exposures are obtained in 7 minutes with a 0.91-meter telescope. The external repeatability (day-to-day differential accuracy) of nightly averages of stellar observations is + 20 meters/second; improvements to the instrument are planned to reduce this value.