  The Helioseismic and Magnetic Imager (HMI) instrument is a major component of
NASA's Solar Dynamics Observatory (SDO) spacecraft. Since beginning normal
science operations on 1 May 2010, HMI has operated with remarkable continuity,
e.g. during the more than five years of the SDO prime mission that ended 30
September 2015, HMI collected 98.4% of all possible 45-second velocity maps;
minimizing gaps in these full-disk Dopplergrams is crucial for helioseismology.
HMI velocity, intensity, and magnetic-field measurements are used in numerous
investigations, so understanding the quality of the data is important. We
describe the calibration measurements used to track HMI performance and detail
trends in important instrument parameters during the mission. Regular
calibration sequences provide information used to improve and update the HMI
data calibration. The set-point temperature of the instrument front window and
optical bench is adjusted regularly to maintain instrument focus, and changes
in the temperature-control scheme have been made to improve stability in the
observable quantities. The exposure time has been changed to compensate for a
15% decrease in instrument throughput. Measurements of the performance of the
shutter and tuning mechanisms show that they are aging as expected and continue
to perform according to specification. Parameters of the tunable-optical-filter
elements are regularly adjusted to account for drifts in the central
wavelength. Frequent measurements of changing CCD-camera characteristics, such
as gain and flat field, are used to calibrate the observations. Infrequent
expected events, such as eclipses, transits, and spacecraft off-points,
interrupt regular instrument operations and provide the opportunity to perform
additional calibration. Onboard instrument anomalies are rare and seem to occur
quite uniformly in time. The instrument continues to perform very well.
