What is the reference for the mag to counts calibration?

[jeanconn]

We reference the CALDB file, but not process or docs, so I'm not sure where to grab that.

chandra_aca/chandra_aca/transform.py

Line 192 in 3724f56

Based on $CALDB/data/chandra/pcad/ccd_char/acapD1998-12-14ccdN0002.fits