Each HST Science Instrument requires its own local coordinate system
and apertures to support both target acquisition and small angle maneuvers (SAMs). Apertures are calibrated locations in the HST focal plane relative to the FGS frame. All acquisitions and SAMs are relative to apertures. Any location within the field of view of a NICMOS camera can be specified by the POSTARG
special requirement (described in the HST Phase II Proposal Instructions).
Each NICMOS camera has two primary apertures. One is positioned at
the geometric center of the detector and the other at an optimal
position close to the center. The first of these apertures is anchored to that fixed location, while the second may be moved in the future. In this way the optimal aperture may be shifted to avoid array defects, even if these are time dependent. Observers with large targets which fill the field of view of a particular camera are generally advised to use the first type of aperture, the “FIX” apertures, while for observers with smaller targets the second type is recommended.
The names of the defined apertures are listed in Table 6.1
along with a description of their function and their current location.
Observers should note that while apertures are defined by their pixel
position in each detector, displacements relative to the default aperture position given with POSTARG
are expressed in arcseconds (see the Phase II Proposal Instructions for further details). All current positions are available in the Science Instrument Aperture File (SIAF) for NICMOS at: