STECF POA/FOS-001

FOS YBASE induced X-Shifts

Michael R. Rosa, Anastasia Alexov,
Paul Bristow & Florian Kerber

Abstract
Analysis of more than 5000 data sets in the FOS archive reveals that the
adjustment of YBASE values preformed during the operational life of FOS
resutted in a strongly correlated shift in the dispersion direction (X).
Because no simultaneous corrections were made to the dispersion relations,
ALL data in the FOS archive suffer from wavelength zero-point errors. These
errors are a step function in time and amount to up to 5 pixels (1.25
diodes) at the last epoch of YBASE updates (Nov 1996). The effect is
completely predictable and originates in the physics of electron optics in
the Digicon detectors. YBASEs (or Y-deflections) are effected through
addition of a vertical magnetic field induced by a pair of deflection coils.
With the parameters of E and B fields present, any B field at right angle to
the Digicon axis (Z) will result in a rotation of the electron beam by 17.6
deg and a movement parallel to the new coordinate system X/Y axes. The
appropriate physical model is described and will be part of the POA pipeline
for recalibration of the FOS science data archive.