|Space Telescope Science Institute|
|FGS Instrument Handbook Cycle 23|
In this section, we describe several special requirements which are often needed in an FGS Phase II observing proposal.Many visit-level special requirements are outlined in the Phase II Proposal Instructions. Those most applicable to FGS programs are:
• Definition: The angle from North to the +U3 axis of the HST measured in the direction of +U2 (counter clockwise when North is up and East is to the left). The ORIENT special requirement is useful for orienting HST so that astrometric reference stars fall within the FGS FOV or for aligning a binary star’s position angle relative to an FGS axis. Along with the ORIENT angle, a tolerance must be specified.
• Calculation: Two angles must be known in order to calculate a special ORIENT angle: the angle from North to the eigenaxis of the target or target field (measured in the direction of East), and the angle between the +U3 axis to the FGS +YPOSTARG axis measured in the direction of +U2. (Note that the values are different for FGS1r and FGS3.)
• Accuracy: The HST roll angle precision depends on the relative guide star position errors and the FGS alignment calibration errors (when two guide star FineLock is used). The pre-designated roll angle for a two-guide star FineLock tracking will be accurate to ≤ 0.04 degrees.
• Recommendations when ORIENT is used in the proposal:
- Explain, in the “Additional_Comments” text section of the proposal, the method used to calculate ORIENT so that STScI schedulers and instrument scientists understand (and can defend the requirement.
- ORIENT is considered a Special Scheduling Request and as such, must be justified in the proposal and will affect the schedulability of the visit. Setting the ORIENT tolerance to as large a range as possible (and still be within the bounds of the scientific requirements) will help to lessen the scheduling impact.
• Definition: Timing links between visits are fairly common for FGS observations. For parallax programs, timing requirements are generally invoked so that a target field is observed at times of maximum parallax factor (every six months). Trans mode observations may also make use of timing requirements, such as observing a binary system at specific orbital phases. Timing links on the Visit level include: BEFORE, AFTER, BETWEEN [dates], GROUP [the following visits] WITHIN [xxx hours, or xxx orbits], etc. See the Phase II Proposal Instructions for the complete list.
• Note: Timing requirements place restraints on the schedulability of the visit. Specify the largest tolerance on the timing constraint that the science can accommodate.All available exposure-level special requirements are described in detail in the Phase II Proposal Instructions. Of these, the following three types are most often used in connection with FGS observations.
• Definition: The ΔX and ΔY offset of a target from the standard aperture reference position are the POS TARG coordinates. They are specified in arcseconds and with respect to special coordinate systems which are illustrated and described in Chapter 2 and in the Phase II Proposal Instructions. (Note, the POS TAR reference frame is not in the same coordinate system as the FGS detect reference frame.) POS TARG is used to position a target at various points in the FGS FOV. For example, a common use in Pos mode observations is to place the science target at a position in the FOV such that reference stars will also be within the FOV. In Trans mode, since calibrations are only available at the center reference point, the non default (0,0) use of POS TARG is not recommended.
• Definition: The position and orientation of the spacecraft will be held constant over the course of all exposures within a given visit when the special requirement SAME POS AS is invoked. This requirement is virtually always used for Pos mode visits.
• Definition: To ensure that all exposures within a visit are scheduled in the same orbit, the SEQ NON-INT special requirement should be used, otherwise the system may divide the exposures over several orbits, requiring guide star re-acquisitions and incurring instrumental overheads taking up to 12 minutes, as well as exceeding the number of orbits awarded by the TAC to the proposal.