FGS Instrument Handbook for Cycle 11

Special Requirements

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In this section, we describe several special requirements which are often needed in an FGS Phase II observing proposal.

Visit-Level Special Requirements

Many visit-level special requirements are outlined in the Phase II Proposal Instructions. Those most applicable to FGS programs are:

Spacecraft Orientation ORIENT:

• Definition: The angle from North to the +U3 axis of the HST measured in the direction of +U2. The ORIENT special requirement is useful for arranging the targets optimally in a field or for aligning an eigenaxis of a science target with an FGS axis with the goal of maximizing the scientific return. 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 (measured in the direction of East), and the angle between the +U3 axis to the FGS +Y_POSTARG axis measured in the direction of +U2. The latter input angle is given in Table 7.3 of the Proposal Instructions and in Chapter 2. (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 understands the requirements.
  • 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.

Timing Requirements:

• Definition: Timing links between visits are fairly common for FGS observations. For Position Mode exposures, timing links may be necessary for parallax work in which a subsequent exposure should be taken six months after the previous exposure. Transfer Mode observations may also make use of timing requirements: if a calibration exposure is to be made, it is often prudent to schedule the calibration exposures close in time to the science observation. 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.

Exposure-Level Special Requirements

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.

POS TARG:

• Definition: The X and Y 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. POS TARG is used to position a target at various points in the FGS FOV. For example, a common use in Position Mode observations is to place the science target at a position near the center while also accommodating the largest number of reference targets. In Transfer Mode, since calibrations are only available at the center reference point, the POS TARG is not recommended.
• Recommendations when POS TARG is used in the proposal:
  • The POS TARG reference frame is centered (0,0) at the standard reference position of the aperture and is not equivalent to the FGS detector reference frame (the two frames have different parities and different origins). See Chapter 2.
  • The POS TARG specification refers to the final offset position of the target (and not to the motion of the telescope to carry the target to its destination).

SAME POS AS:

• Definition: The position and orientation of the spacecraft may be held constant over the course of all observations within a given visit by use of the exposure-level special requirement SAME POS AS. This requirement is virtually always used for Position Mode visits.

SEQ NON-INT:

• 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, each of which takes several minutes to execute (time which could be spent on target).

Do not hesitate to explain the use of any special requirements in the proposal text. The more explanation, the easier for STScI to understand your requirements and schedule the proposal