Subtract solar gravitational redshift from twilight sky (yes or no)
If subgrav=yes and specsky=yes, subtract a correction for the sun's
gravity (0.636) from the barycentric and heliocentric corrections
computed by the program. If savebcv=yes, BCV, HCV, and SGRV are
written to the image header. HCV, the velocity of the sun relative
to the spectrograph with the gravitational correction subtracted,
should be used as the spectrum correction
when cross-correlating twilight spectra with XCSAO.