B. Light:
presented the comparisons that he and S. Wheelock have been doing
for R2 vs. R2-R1 positions coming out of KAMPHOT and FREXAS.
He showed an offset in inscan positional difference, delta y, for
bright stars in ascending vs. descending scans in the FS28 scan; the average
positional difference between these is about 0.2 camera pixels. There is
also an offset from zero for both ascending and descending positions.
This offset has been known about for a long time and is accounted for in
the pipeline. It is thought to be due to shifting of the secondary affecting
the R1 more than the R2 data; the effect is not a software issue since the
same results are found from FREXAS and KAMPHOT.
Bob then demonstrated that the offset varies in size from one scan to another.
He has found no dependence of the size of the offset with airmass, but
speculated that there may be a possible dependence on temperature. A
similar, though smaller, effect is seen between ascending vs. descending
scans in cross-scan pixel position differences, delta x: about 0.1 camera
pixels.