Do an epar on headers and you will get:
This task will use the ut, date, exposure, ra, dec, and epoch
parameters to calculate and add to your headers the mid-observation
UT (utmid), the sidereal time (st), effective airmass (airmass),
and Julian Date (jd).
Make sure that the new parameters make sense. In particular, check
that the new airmass is close to the original airmass, and that the
julian date is correct.