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NICMOS Aperture Definitions

 Each HST Science Instrument requires its own local coordinate system and apertures to support both target acquisition and small angle motions (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).

Aperture Definitions

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 coronographic acquisition.

Standard Apertures

The names of the defined apertures are listed in Table 4.4 along with a description of their function and their initial location.

Table 4.4: 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-CORON

 Center of Coronographic Mask

NIC2-ACQ

 Center of Mode 2 ACQ region

NIC3

 Optimal center of Camera 3

 140,135

NIC3-FIX

 Geometric center of Camera 3

 128,128


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