Dear _______,

As part of CXC support of operation of the Chandra mission, we are
assigning interface contacts for all observers. I will be that contact
for you throughout the uplink cycle (i.e., from acceptance of the
observing proposal until the observational data has been received on 
the ground) of the subject observation.

Your proposed observation of ____ (Sequence #####, ObsId ####) using the
HRC is scheduled to be observed the week of DD-MMM-YYYY.

To prepare the command load for your observation, it would be helpful
for you to participate in the planning process.  To this end, please
review and confirm the various settings of the instrument by checking
the following pages:

        http://cxc.harvard.edu/cgi-gen/mp/target.cgi?####
        http://cxc.harvard.edu/cgi-gen/target_param.cgi?####

Please check these pages for overall accuracy, and the following in
particular:
        * the Target Name,
        * the target coordinates [RA and Dec] 
              (in decimal degrees or sexagesimal),
        * the Exposure Time (in kilo-seconds),
        * the target offset [Offset Y, Z] (in arcminutes),
        * the defocus [Focus] (in mm),
        * the focal plane position [Sim-Focus; esp. HRC-S/No Grating] 
              (in mm),
        * the translational position of the detector [Z-SIM] (in mm),
        * the roll constraint [Roll Angle] (in degrees),
        * the time constraint [Temporal Window]
        * the operational mode of the detector [SI Mode]

In the event that you discover anything is incorrect in the
observation set-up, please inform me of the appropriate changes. 
To assist you in the review process, information on the calibration of
the HRC can be found at:

	http://cxc.harvard.edu/cal/Hrc/detailed_info.html

Because this information is periodically updated, we recommend that
you familiarize yourself with the various applicable models
available there each time you review an observation. If everything is
correct, please let me know that you are satisfied with the current
set-up. (Please also email a copy of all correspondence to the Chandra
Uplink Support archive at cus@head-cfa.harvard.edu.) Please return any
such corrections or confirmation your satisfaction by DD-MMM-YYYY.

We also would like to make you aware-- if you are not already-- of
relevant information which may or may not affect your observation.

There is a wiring error in the HRC that can lead to incorrect times
being assigned to events. The HRC team has developed an operating mode
using the HRC-S that recovers the ~16 microsecond resolution given in
the Proposal Guide. When the HRC is not in this new timing mode the
timing resolution is given by the inverse of the total event rate. The
new timing mode for HRC consists of using the HRC-S detector in what
is called S_TIMING Mode.  In this mode, only the center segment of the
HRC-S is active, and the on-board event filtering is disabled. While
the lack of on-board filtering increases the telemetered background
count density compared to the HRC-I, the use of only the center
segment mediates the overall amount of background events collected
such that the background contribution to the total and telemetered
event rates is ~50-70 counts per sec. The HRC-S, when used in
S_TIMING, provides about 7 arc minutes Field of View in one direction,
and 30 arc minutes along the long axis of the segment. The center
segment of HRC-S has about the same sensitivity as the HRC-I (see the
Proposal Guide for details). Note that due to the nominal use of the
LETG with the HRC-S, S_TIMING Mode requires a Sim-Focus offset of -0.1
mm to be in focus. For more information on the HRC timing error and
its fix see:

	http://cxc.harvard.edu/cal/Hrc/timing.html

We would also like to remind you that there is a limit of events that
can be telemetered of 184 cts/s on the total counting rate in the
detector-- from your source, background and any sources coincidentally
within the field. This is due to the telemetry throughput.  The
primary impact is on the photometric accuracy, as the dead time
corrections themselves are only accurate to ~10% for "mild"
saturation (i.e., 184 - ~250 counts per second). The HRC count rate
response to point sources becomes non-linear at rates much less than
this (i.e. at ~20 - 30 counts per second).

In order to assist in the identification and analysis of the sources, we 
have recently developed an RMF for the HRC-I, see

	http://cxc.harvard.edu/cal/Hrc/RMF/

If any of these issues affect the viability of your observation, or if
the details of the observing plan are incorrect or inadequate, please
contact me at the CXC as soon as possible.

Regards,
_______