Dear _______,

I will be the Uplink Support Interface (USINT) contact for your
LETGS/ACIS-S observation of ______ (Sequence ######) throughout
the "uplink cycle", i.e., until your photons are collected by Chandra. 
I am contacting you now to request that you confirm your observation
set-up by DD-MMM-YYYY.

Your confirmation ensures that your observation set-up will be ready when 
the satellite command loads are built.  It is important to note that after 
the above date it will not be possible to change any of the observation 
parameters (unless your observation is a TOO and updates are inherently 
unavoidable).  The scheduling process becomes more complicated each year 
because of the increasing number of thermal control issues, and Mission 
Planning needs all observation configurations confirmed early in the Cycle.

If you have any questions concerning your observation or its set-up, please 
do not hesitate to contact me. Correspondence via email is preferred (cc to 
"cus@head-cfa.harvard.edu"), but you may also contact me by phone or FAX 
(see below).  If the set-up is correct as it is, I still require your 
confirmation.

Your observation is currently scheduled for the week of DD-MMM-YYYY.

You can check your observation parameters in the on-line OBSCAT by going to
http://cxc.harvard.edu, clicking on "Target Search Page," and then using
the search facility to find your "Summary of Sequence Number" page, or by 
following these links:

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

Please check ALL parameters carefully, especially the following:

        * the target name,
        * the target coordinates (in decimal degrees),
        * the exposure time (in kiloseconds),
        * the target offset (in arc minutes),
        * the defocus (in mm),
        * the SIM-Z (detector translation, in mm),
        * the roll constraint (in degrees),
        * the time constraint,
        * the ACIS mode.

Once this Cycle's preliminary Long Term Schedule has been created, you can 
view the orientation of the detector on the sky in the optical and X-ray bands 
by clicking on the "DSS" (Digital Sky Survey), "ROSAT" (pointed observation, 
if any), and "RASS" (ROSAT All-Sky Survey) buttons at the bottom of the 
"Summary of Sequence Number" page.  Please note that if your observation is 
a TOO, the roll angle will of course be unknown; the DSS, ROSAT, and RASS 
images will be shown assuming a roll angle of 0 degrees.  The range of 
permitted roll angles vs date is illustrated via the "Roll/Visibility" link.

I strongly urge you to consult the "Checking your LETG/ACIS-S Obscat setup" 
page at http://cxc.harvard.edu/cal/Letg/ACIS_params/, which contains helpful 
and up-to-date information on optional chips (see below), subarrays (useful
to reduce pileup), and pointing/wavelength offsets.  Observers are especially 
advised to check the Y offset: the observatory aimpoint slowly drifts over
time, which affects where 0th order and dispersed spectral features fall 
with respect to ACIS chip edges.

You have probably already been asked by the Chandra Director's Office to 
consider marking one or more ACIS CCDs as Optional.  Please note that chips 
S0 and S5 are useless for providing 1st order LETG data; those outermost 
chips are only useful if you intend to analyze higher-order spectra, or 
perhaps in other very special circumstances.  The LETG/ACIS-S Obscat setup
web page (http://cxc.harvard.edu/cal/Letg/ACIS_params/) and the Spectrum
Visualization Tool (http://cxc.harvard.edu/cgi-gen/LETG/alp.cgi) are helpful 
in determining which CCDs may be unnecessary.

Also note that observations using fewer CCDs are less likely to be split into 
multiple pieces because of their greater scheduling flexibility.  Further 
information is available in the POG, section 6.19.1/Choosing Optional CCDs
(http://cxc.harvard.edu/proposer/POG/html/ACIS.html#tth_sEc6.19.1).

Please be advised that the following changes in your observing program are
not generally permitted:

        * adding any constraints,
        * changing exposure time or target,
        * adding or removing a grating, or
        * switching between ACIS and HRC or LETG and HETG so that
          the target and instrument configuration end up being the same
          as in someone else's observing proposal.

If you request any changes to your set-up, I will confirm these with you 
by e-mail (or telephone), and after about two days you should see these 
updated parameters in the on-line OBSCAT database.

Again, if you have any questions at all about the set-up and execution
of your LETGS observation please do not hesitate to contact me. 

Best wishes,

_______