L. Acton  8-Nov-95
                            			   (Rev. 11-Nov-95)

ORBITAL VARIATION OF YOHKOH POINTING & COMPUTATION OF TERMINATOR OFFSET

This figure displays the variation of Yohkoh pointing over a 3 day period
during which no offpoints took place.  The data were derived from running
SXT_CEN on the pfi roadmap structure for the period:

	5-NOV-95  18:21:53 to 7-NOV-95  22:24:45

This effect has been known for some time.  Note that the peak to peak E-W
(X) variation of about 3-4 FR pixels is substantially smaller than the N-S
(Y) variation of 8-10 FR pixels.

The purpose of this analysis was to compute a bias to include in the
program PLOT2OFFSET which is used to determine offset pointings for
terminator images.  Because terminator images are always taken at the end
of a orbital day they have a systematic offset from the average pointing --
yet they are referenced to the average pointing through LASTSFD which may
be acquired at any time-to-FMS position.  Another approach would be to
choose reference SFDs just before sunset.  However, because of other
sources of random pointing error, and the need to change tested software, I
think it is better to include this systematic offset in the pointing
determination. 

Here are the average values of these pointing data (FR pixels).

			  		   X		   Y
	Tim2fms<60 (1251 cases) 	537.430		593.183
	Tim2fms>60 (27 cases)		537.153		588.262
	Terminator offset		  0.277           4.921

The difference in X is small enough to ignore.  The Y offset is nearly 5
pixels and should be accommodated in the software.