Radial Velocity Measurements of the Sun made with a Birefringent Filter

摘要

A Simple technique is presented for making velocity measurements on the sun. This uses two simultaneous images formed at the exit of a Lyot birefringent filter made by substituting the final polarizer in the filter with a Wollaston polarizing beam splitter. With the Wollaston axes of polarization at 45 deg. with respect to the axis of the polarizer for nominal filter tuning on a spectral line the two images primarily represent intensity in wavelengths equal distance on either side of the line. The images are recorded photographically and digitized in registration with respect to the common image field stop; I measure that the frame registration is better than 0.025 arcseconds on the sun across its field. Since the two images are simultaneous they can be directly subtracted without differences due to seeing. I present a new method for fitting the density-intensity calibration of the film and demonstrate its effectiveness in providing a linear measure of intensity and a well defined measure of velocity.