All the FITS header keywords that appear in the dopplergram data are explained here. Any combination of FITS header keywords CAMERA and T_REC is unique for the both DRMS series hmi.V_45s and hmi.V_720s. Further explanation on WCL keyword descriptions can be found in this paper.
The HMI (Camera 2) instrument takes 72 filtergrams to construct a single dopplergram. Six filtergrams are taken in six positions across the nominal 6173.3 Å spectral line; each filtergram is taken in two polarization states (right circularly polarized(RCP), or Stokes I-V, and left circularly polarized(LCP), or I+V). Assuming that the absorption line is a Gaussian and that the HMI filter transmission profiles are delta functions, the Fourier coefficients can be calculated and are then used to estimate the line-of-sight magnetic field strength B. The MDI-like algorithm for calculating the dopplergrams is available here.
The wavelength dependence calibration tests for magnetograms are outlined here. The image quality calibration methods are outlined in this paper. The polarization calibration methods are outlined here.
Look-up tables and polynomial correction have been applied to the Dopplergrams. All the Line-of-Sight Observables show fluctuations in the values with a twenty four hour cadence. This is an instrumental effect.