NICMOS Instrument Handbook for Cycle 11

NICMOS Aperture Definitions

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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).

The basic philosophy of the NICMOS aperture definitions follows that used by WF/PC-1 and WFPC2. 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, while for observers with smaller targets the second type is recommended.

Additional apertures are defined in Camera 2 for use in the Mode 2 coronagraphic acquisition.

The names of the defined apertures are listed in Table 6.1 along with a description of their function and their initial 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).

Table 6.1: NICMOS Aperture Definition

Aperture Name	Description	Position (detector pixels)
NIC1	Optimal center of Camera 1	162,100
NIC1-FIX	Geometric center of Camera 1	128,128
NIC2	Optimal center of Camera 2	149,160
NIC2-FIX	Geometric center of Camera 2	128,128
NIC2-CORON1	Center of coronagraphic hole	-
NIC2-ACQ	Center of Mode 2 ACQ region	157,128
NIC3	Optimal center of Camera 3	140,135
NIC3-FIX	Geometric center of Camera 3	128,128

1 NIC2-CORON aperture position not given here as it is time dependent and automatically determined onboard for each coronagraphic acquisition.