http://space.mit.edu/ASC/calib/planning.text Observation Planning Considerations for Chandra HETGS Observations September 10, 2002 OBSERVING SCHEDULES AND TIMESCALES If your target is in the long term schedule (LTS, available at http://cxc.harvard.edu/longsched.html), information is needed by 7 weeks in advance of an observation. The target will not be scheduled earlier, but could be scheduled later, if necessary, due to other demands on the scheduling process. Please remember that these schedules are PLANS and are subject to change. The LTS is usually updated every week. Note that all time and roll constraints will be honored. One may request roll or scheduling "preferences", which are usually met, but they are not guaranteed. If your target is in in a "pool" week, it means it is in a pool of observations which have no constraints and are visible any time. These are used to fill in the weekly schedules to increase observing efficiency. Because these can be scheduled at any time, we request review of the configuration as soon as possible, so that we might use them. (We regret that it is inevitable that some of these will not be done until almost the very end of the observing cycle). It is extremely difficult to change observation parameters within 3 weeks of the observation as the short-term schedules are developed from the weekly observing plans. Therefore, please contact us EARLIER than 7 weeks prior to when the target is scheduled to be observed. ***Warning*** Observations that have NOT been signed off by the contact scientist may not be included in OR lists so they may not be scheduled as listed in the LTS. The LTS has not yet been developed for AO4 targets so we advise reviewing the observation parameters as early as possible. See http://space.mit.edu/ASC/calib/hetg_GO_info.html for a discussion of issues relating to setting up observations and http://space.mit.edu/ASC/calib/hetgcal.html for HETGS calibration updates. When reviewing your setup, please consider the several important issues discussed below. In particular: * the target name, * the target coordinates (in decimal degrees), * the exposure time (in kilo-seconds), * the target offset (in arcminutes) if applicable, * the defocus (in mm) if applicable, * the SIM-Z offset (in mm) if applicable, * the roll constraint (in degrees) if applicable, * the time constraint if applicable. To view the placement of the detector on the optical (DSS) or X-ray sky (if this region was observed by ROSAT), go to http://cxc.harvard.edu/targets Find your proposal using the form provided by filling in the "PI Name" "Sequence Number", etc. Then click "Sequence Number". (Or click on "sequence number" at the bottom of the form and locate and click your sequence number.) Finally, click the "DSS" or "ROSAT" buttons on the "Summary of Sequence Number" page. If you find errors in your instrumental set up, please inform your CXC contact scientist of the appropriate changes. If everything is correct, please let us know that you are satisfied with the current set-up. The contact scientist must sign off on all observation setups even if there are no changes. Parameter settings relevant to HETGS observations: SI Mode - This code for ACIS is determined by the ACIS parameters selected and may be identical to previously used ACIS SI Modes. This code is not set directly by users. Offset Y - Used to offset point along the dispersion direction to fine tune chip-gap locations, for example. We generally recommend moving +0.33 arcmin to avoid placing 0th order on a node boundary. Offset Z - generally preferable to use "Z-Sim" instead of "Offset Z". An offset in Z may be needed if the desired SIM aimpoint cannot be reached due to fid light conflict, which occurs for Z-SIM offsets of -11 to -8 mm. SIM Z - Useful to move aim point on ACIS-S array closer to framestore area to reduce FI CTI effects. The nominal move is -3 mm but one may move more if the low energies of the dispersed HEG spectrum may be sacrificed. For a -3 mm shift, a 747 row subarray can be used without losing source events. ACIS Exposure Mode - TE or CC -- using CC mode increases timing resolution but data analysis can be difficult as the MEG and HEG events are not readily discerned. Event TM Format - For telemetry reasons, we generally recommend "Faint" instead of "Very Faint"; "Graded" should be used only if the count rate is very high. Frame Time - Generally 3.2 sec for HETGS; may be reduced for pileup reduction if subarrays or CC mode are used. Ignored unless a longer exposure is desired than the minimum allowed for a given subarray. For a 747 row subarray, the frame time is 2.5 s. Standard Chips - "Y" for normal ACIS-S array operation. I0 - I3 - Ignored if "Standard Chips" is set to "Y". If "N", then specify the desired chips. ACIS-I chips may be combined with ACIS-S chips up to a total of 6. S0 - S5 - see I0 - I3 Subarry Type - Generally "None" for HETGS observations. If a Z-SIM shift of 3 mm is used, one may request a "CUSTOM" subarray of 747 rows starting at row 1. Other custom subarrays may be constructed with potential loss of dispersed events. Duty Cycle - For use in Alternating Frame mode. This Number mode must be selected and used with care. Tprimary The main objective is to reduce pileup in 0th order Tsecondary for a small fraction (usually < 5%) of the observation. There are restrictions on the possible values. Event Filter - Events may be vetoed from the telemetry based Lower on energy. This may be useful for telemetry Range reduction when, for example, sources are cut off at low energies due to the ISM. Window Filter - Spatial window parameters may be useful for Start Row reducing telemetry rate on a bright source; Start Column note that "sample rate" allows some events Height to be telemtered, e.g., from zero-order region. Width It is generally recommended that *some* events from Lower Energy zeroth order be retained to facilitate data analysis Energy Range and so that instrument problems may be diagnosed more Sample Rate easily on the ground. CHANGES NOT PERMITTED: Be advised, the following changes in your observing program are not usually permitted: a) Adding constraints (e.g. roll angle or a time constraint). b) A change in exposure time or target, or instrument switches that would deviate from, or be inconsistent with, your proposed scientific goals.