From:		J.J. Drake
To:		SOT, CAL, User Support, Mission Planning, LETG-TST, 
Subject:	Default SIM Z offsets for LETG,HETG + ACIS-S observations
Date:		28/10/99

The effects of CTI in ACIS-S FI chips can be ameliorated by aquiring
spectra as near to the framestore region as possible.  The nominal aim
points for each grating are both currently defined as the z coordinate
centre of the array. Of some use for observation set-up in cases where
no observer-preferred, or otherwise defined, z coordinate aim-point
exists is a set of default z offsets for LETG,HETC+ACIS-S observations
to attempt to minimize CTI effects.

Based on analysis of the various error budgets, dither parameters and 
spatial characteristics of HETG and LETG dispersed spectra, the 
following default offsets have been provisionally adopted

Configuration	Mode	SIM z (mm)	SIM z (motor steps)
HETG+ACIS-S	TE	-3		-1193
HETG+ACIS-S	CC	-4		-1591
LETG+ACIS-S	TE	-9**		-3580
LETG+ACIS-S	CC	-10		-3977

Assumptions:

 -  +/-30 arcsec error budget for target aquisition
 -  16 arcsec ACIS-S dither, peak-peak
 -  ~1mm background region for TE mode on -z side of spectra traces
 -  0mm background region for CC mode on -z side of spectra traces

Other effects, such as small clocking angles of grating dispersion
axes with respect to their ideal alignments on the detector have
been taken into account but are small.

** Note that the earlier memo circulated on the LETG offset adopted for 
p25 was -10mm for both CC and TE exposures; here -10mm is stated as 
-3977 motor steps rather than the previous memo's -3978 to be
consistent with the routine /data/ascmp2/eriksen/focus/zsim2motor