Observables Processing for the Helioseismic and Magnetic Imager Instrument on the Solar Dynamics Observatory
NASA's Solar Dynamics Observatory (SDO) spacecraft was launched 11 February 2010 with three instruments onboard, including the Helioseismic and Magnetic Imager (HMI) After commissioning, HMI began normal operations on 1 May 2010 and has subsequently observed the Sun's entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two 4096x4096$4096 \times 4096$ cameras, from which are computed arcsecond resolution maps of photospheric observables that include line of sight velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters [I,Q,U,V$I, Q, U, V$ Two processing pipelines have been implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level 1 filtergrams, one that computes line of sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that computes averages on a 720 second cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter element uncertainty and nonuniformity, as well as Sun spacecraft velocity. We detail the functioning of these two pipelines, explain known issues affecting the measurements of the resulting physical quantities, and describe how regular updates to the instrument calibration impact them. We also describe how the scheme for computing the observables is optimized for actual HMI observations. Initial calibration of HMI was performed on the ground using a variety of light sources and calibration sequences. During the five years of the SDO prime mission, regular calibration sequences have been taken on orbit to improve and regularly update the instrument calibration, and to monitor changes in the HMI instrument. This has resulted in several changes in the observables processing that are detailed here. The instrument more than satisfies all of the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity.