Space Telescope Science Institute   7.7.4 Polarizer Apertures  7.7.6 SBC Apertures

7.7.5 HRC Apertures


The HRC has an area of 1062 by 1024 including 19 physical overscan pixels at each end in the x direction. The active area is 1024 by 1024 pixels. The mean scales along the x and y directions are 0.028 and 0.025 arcseconds/pixel, thus providing a field of view of about 29 by 26 arcseconds in extent. The anisotropy and variation of scales is discussed in a later section of this handbook. The reference point for the aperture labelled HRC-FIX, and initially for HRC, is at the geometric center, (531,512). As with the WFC apertures, there may be reason to move the HRC reference point later.

The HRC is equipped with two coronagraphic spots, nominally 1.8 and 3.0 arcseconds in diameter and a coronagraphic finger, 0.8 arcseconds in width. Apertures HRC-CORON1.8, HRC-CORON3.0, and HRC-OCCULT0.8 are defined to correspond to these features. In addition we define a target acquisition aperture, HRC-ACQ designed for acquiring targets which are subsequently automatically placed behind a coronagraphic spot or the occultation finger. The positions of the coronagraphic spots have been found to fluctuate. Observations will need to incorporate a USE OFFSET special requirement to allow current values to be inserted at the time of the observation (see the Phase II Proposal Instructions).

A substantial region masked out by the occulting finger can be present in the HRC data ( Figure 7.6). The occulting finger is not retractable -- it will be in every HRC exposure. However as with any other detector feature or artifact, the "lost" data can be recovered by combining exposures which were suitably shifted with respect to each other. A dither pattern, ACS-HRC-DITHER-LINE has been defined for this purpose and spans the area flagged for the HRC occulting finger (~1.6 arcseconds or ~56 pixels wide), with an extra ~0.3 arcseconds or ~10 pixels of overlap. More details can be found in the Phase II Proposal Instructions.

Use of the prism PR200L requires specifying aperture HRC, but results in a reference point of [671,512] to optimally center the target coordinates within the somewhat vignetted prism field of view. Although the HRC direct imaging and PR200L prism apertures have the same name in APT, they are actually distinct, and HST executes a small angle maneuver between observations of a given target with them, to compensate for the positional deflection by the prism. One consequence is that the Special Requirement SAME POS AS cannot be used among mixed direct and prism exposures, as always with different apertures.

Figure 7.6: HRC coronagraphic finger and spots.

 
The HRC aperture parameters are summarized in Table 7.8.

Table 7.8: HRC aperture parameters.
APT Aperture Name
Active area
Extent
(arcsec)
Reference
pixel
Reference V2,V3
(arcsec)
x-axis angle
y-axis angle
HRC
1024 × 1024
29 × 26
(531, 512)
(206,472)
-84.1
0.1
HRC-FIX
1024 × 024
29 × 26
(531, 512)
(206,472)
-84.1
0.1
HRC-CORON1.8
200 × 200
6 × 5
(564,466)1
(205,471) 1
-84.1
0.1
HRC-CORON3.0
200 × 200
6 × 5
(467,794) 1
(208,479) 1
-84.2
0.1
HRC-OCCULTO.8
200 × 200
6 × 5
(443,791)
(209,477)
-84.2
0.2
HRC-ACQ
200 × 200
6 × 5
(735,575)
(200,474)
-84.1
0.0
HRC-512
512 × 512
15 × 13
(276,256)
(214,465)
-84.1
0.2
HRC-SUB1.8
512 × 512
15 × 13
(570,468)
(205,471)
-84.1
0.1
HRC-PRISM 2
744 × 1024
21 × 26
(671,512)
(214,471)
-84.2
0.2
1These values fluctuate and will be updated at the time of the observation.
2HRC-PRISM is automatically created by commanding software when spectral element PR200L and aperture HRC are selected in APT.

 

 7.7.4 Polarizer Apertures  7.7.6 SBC Apertures
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