Dome offsets and flat fields

Introduction: pre-March 2013 it was noticed that there could be small mis-alignments when the dome was moved into position
for daily flat fields and/or DECal scans. A piece of safety tape was placed so that it would show proper alignment when
the dome was moved into proper position. The telescope operator, as part of the procedures, now checks for this proper
alignment. Indeed, some of the early daily flat fields (before the procedure) show a "rim" about 1/3 of a CCD wide
with response lower by about a percent.

It is difficult to diagnose whether the observation might be due to dome mis-alignment as the screen illumination comes from
four discrete stations mounted on the outer ring and the screen itself is nearly a perfect Lambertian reflector (brightness
is the same no matter the view onto the focal plane). This series of studies attempts to look at the effect of the dome flats
versus small offsets in the dome position to see whether the illumination pattern matches some of the early problematic daily flats.

DocDB 7024 (Mar 2013 study)