An entire book could be written on this issue.
For details, see
the first (and still only) comprehensive in-flight
calibration of all 9 CDS channels:
Del Zanna, G., et al., 2001, A&A 379, 708-734.
Solar EUV spectroscopic observations with SOHO/CDS. I. An in-flight
calibration study
Also, see the book:
The Radiometric Calibration of SOHO, ISSI Scientific Report,
2002, Pauluhn,A., Huber M.C.E., von Steiger,R. eds.
And my WWW pages:
http://www.damtp.cam.ac.uk/user/astro/gd232/cds/
The CDS routine NIS_CALIB performs the conversion from
photon-events/pixel/s to the calibrated
default units `photons/cm2/sec/arcsec2'.
The conversion factor from arcsec2 to steradians is simply
(p/(180 ×602))2 @ 2.35 ×10-11,
while the conversion from photons to ergs is hc/l @ 1.986×10-8/l, where l is in Ångstroms.
The conversion to per Ångstrom is accomplished simply by dividing the data
by the pixel width in Ångstroms.
There are several factors which determine this final conversion
of the data from photon-events to absolute units.
