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