The available spectral elements are listed in Table 10.2: Spectral Elements for the FGS. Only one filter can be used at a time. The
F583W is the recommended element for programs requiring the use of POS and TRANS modes combined because of the Optical Field Angle Distortion (OFAD) calibration, which is available only for
F583W.
F583W cannot be used if the target magnitude m<8.0.
F5ND must be used if m<8.0. Unless special S-curve calibration observations are requested, only certain combinations of filter and target color will be calibrated. Please refer to the Instrument Handbook for more information.
The exposure time (Texp) is the total photon-collecting time (in seconds) for all
Nscan scans. The time per individual scan line is
Texp/
Nscan. See the discussion in
STEP-SIZE above and the
FGS Instrument Handbook.
Enter the ORIENTation visit-level Special Requirement (see
ORIENTation <angle1> TO <angle2>) if the arrangement of the target or reference stars requires a particular orientation of the FGS field of view.
ORIENTation may be required to keep the targets in the field of view when more than one target is observed.
The POSition TARGet <X-value>,<Y-value> special requirement can be used to offset a target from the aperture reference position. For the FGS, the aperture reference frame is centered near the geometric center of the field of view, and is
not equivalent to the FGS detector reference frame. The two frames have different parities and origins (see
Figure 10.1 The FGS POS TARG and interferometer coordinate systems. and the FGS Instrument Handbook).
The SAME POSition AS <exposure> special requirement should be used to keep the position of the spacecraft held constant over the course of all observations within a visit. To ensure that all exposures within a visit are scheduled within the same orbit, use the
SEQuence <exposure-list> NON-INTerruptible (replaced by Exposure Group Containers in the APT User Interface) special requirement.