  The Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics
Observatory (SDO) observes the Sun at the Fe I 6173 {\AA} line and returns
full-disk maps of line-of-sight (LOS) observables including the magnetic flux
density, velocities, Fe I line width, line depth, and continuum intensity.
These data are estimated through an algorithm (the MDI-like algorithm,
hereafter), which combines observables obtained at six wavelength positions
within the Fe I 6173 {\AA} line. To properly interpret such data it is
important to understand any effects of the instrument and the pipeline that
generates these data products. We tested the accuracy of the line width, line
depth, and continuum intensity returned by the MDI-like algorithm using various
one-dimensional (1D) atmosphere models. It was found that HMI estimates of
these quantities are highly dependent on the shape of the line, therefore on
the LOS angle and the magnetic flux density associated with the model, and less
to line shifts with respect to the central positions of the instrument
transmission profiles. In general, the relative difference between synthesized
values and HMI estimates increases toward the limb and with the increase of the
field; the MDI-like algorithm seems to fail in regions with fields larger than
approximately 2000 G.
  Instrumental effects were investigated by the analysis of HMI data obtained
at daily intervals for a span of three years at disk center in the quiet Sun
and hourly intervals for a span of 200 hours. The analysis revealed
periodicities induced by the variation of the orbital velocity of the
observatory with respect to the Sun, and long-term trends attributed to
instrument adjustments, re-calibrations and instrumental degradation.
