(mpg) The brightness of a star measured with a blue-sensitive (orthochromatic) photographic plate. This magnitude is poorly defined because different amounts of ultraviolet light are included depending on whether a refractor or an aluminized reflector is used. Although this magnitude is now obsolete, it can be roughly related to the B magnitude by the approximation
B ≈ mpg + 0.11
Subjects: Astronomy and Astrophysics.