Basic photometric reductions below image.
My calculated magnitude difference using SX Windows Photometry program on this occasion was "0.43".
from "Object" "Comparison star" and "Sky Background" reductions.
Calculation.
QSO V = 15.78
Ref Star "D" = 15.82 in "B"
Subtract B-V = .62
= 15.20
ADD = 0.43 (magnitude difference 22.5UT on 07/02/2002.
Magnitude of object = 15.63, so 15.63 from 15.78 = "0.15"
of a magnitude brighter on this occasion.
NB: As I have so often mentioned before filters U.B.V.R. are required for accurate measurements.
John R Fletcher. Mount Tuffley Observatory.UK.