The Delta a photometric system has be designed by Maitzen to detect photometrically
peculiar stars (Ap). The original system is based on photoelectric photometry.
The da CCD system is the realisation of the original da system
with a CCD detector.
It uses three filters g1, g2 and y and the parameter a is defined as
a = g2 - [(g1 + y) / 2]].
Delta a is computed relatively to a linear relation : a versus (B-V) or (b-y) or (g1-y).