Calibration of SOT Dopplergrams

Y. Katsukawa, K. Ichimoto, T. Sekii, T. Tarbell, and SOT team

Narrowband Filter Imager (NFI) on SOT provides Dopplergrams which are images of Doppler velocity and critically important in observations of photospheric dynamics and helioseismology. The Doppler shift of a spectral line is derived with 4 narrow-band images uniformly spaced through the line. The primary photospheric line used for Dopplergrams is Fe I 5576 A which is a line insensitive to Zeeman effect. We made a LUT for the 5576A Dopplergrams to get actual Doppler velocities from outputs of the Dopplergrams using an atlas spectrum and simulated transmission profiles of the tunable filter on SOT. Using data sets taken in the Sun light test, we evaluated accuracy of the Dopplergrams by comparing the rotational speed of the Sun with the predicted one. There was a little systematic error in the velocity obtained by SOT, but the error was less than 20% of the predicted rotational velocity. We will also report the effects of Doppler motion of the sattellite and temperature variation in orbit.

Correspondence

Yukio Katsukawa (yukio.katsukawa@nao.ac.jp), National Astronomical Observatory of Japan

presentation

poster