Dopplergrams calibration

n this paper a procedure to calibrate dopplergrams is presented. This procedure is instrument independent and is based on the main topological characteristics of solar rotation. It is reliable in those regions on the solar disk where the sensitivity can be considered linear in the east-west direction (that is, out to 0.75 solar radius for most instruments) and, in principle, can be applied to all dopplergrams with medium spatial resolution (typically 8 arc sec pixel-1). The aim of this calibration is to correct any variation of the sensitivity to the velocity signal caused by instrumental changes. The procedure is currently applied to data obtained at the Solar Observatory at Kanzelhöhe (Austria) with a sodium magneto-optical filter. The procedure, applied to a one-day run, shows how the low-degree five-min oscillations are recovered from the raw integrated Doppler signal over the whole CCD.