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Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14

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3.9 Apertures


The WFPC2 camera configuration and filter set require a substantial number of apertures for full utilization of its capabilities. All possible aperture and filter combinations are given in Table 3.14.

Each camera has an associated 'optimum' aperture close to the geometric center of its field-of-view (FOV). These positions have been adjusted to reflect CCD performance following SMOV and to allow for pyramid vignetting. The aperture designations are WF2, WF3, WF4, and PC1 for the individual cameras and WFALL for the three-WFC combination. WFALL is located close to the pyramid apex in WF3 (see Figure 3.12). Observers are expected to place small or unresolved targets on these apertures. Note that normally all four CCDs are read out even if a specific CCD is selected with an aperture. This is discussed in Section 2.8. The positions of these apertures may be updated if bad pixels, etc., appear on the CCDs.

In cases where the observer does not want to use the current 'optimum' centers, a complimentary set of apertures has been implemented specifically for this purpose. These locations remain fixed and correspond roughly to the geometric center of each camera's field-of-view. They are designated WF2-FIX, WF3-FIX, WF4-FIX, PC1-FIX, and WFALL-FIX. Observers are expected to place extended targets on these apertures.

As of July 2004 the 'optimum' apertures have remained identical to their 'FIX' counterparts, with the only exception being WF3 which has moved slightly from WF3-FIX.

An additional set of aperture names have been defined for use with the WFPC2 filters which require partial rotations. The characteristics and uses of these filters are discussed earlier in this chapter. In the nominal filter position, the three WFC segments of the [OII], Methane and Polarizer quad filters can be selected with an aperture for each camera corresponding to the optimum or geometric camera centers. The partially rotated quad filters, which generally fall into more than one camera, have been assigned apertures in the camera which provides the largest clear aperture. The pixel coordinates of these apertures will be reviewed on a regular basis to reflect changes in CCD and filter cosmetics. There are no analogous fixed apertures for the partially rotated filter configurations. The aperture name is generally the same as the (rotated) filter name. For the Wood's filters, the nominal filter position is used for the PC1 FOV only, while the rotated filter position is used for WFC observations. The linear ramp filters are unique because the ultimate location of the target will be determined from the central wavelength specified, and therefore only the generic aperture name LRF is required.

Occasionally the V2-V3 coordinates of the WFPC2 apertures are updated to correct slow drifts of the HST focal plane relative to the spacecraft (V1, V2, V3) system. Table 3.15 shows this history. The V2-V3 coordinates prior to 1996 day 127 for any aperture can be derived by setting (V22,V32) to the values in Table 3.14, and then computing the earlier coordinates. The V2-V3 coordinates after 1997 day 335 can also be computed in a similar maneuver.

Table 3.14: Aperture Definitions. The pixel coordinate system uses pixel numbers (row, column) for the CCD in use. See Figure 3.12 or Figure 1.1 for the definition of the V2-V3 coordinate system.
Aperture Name
Filter Name
CCD
Location
CCD Pixel
Coordinates1
X
Y
V2
V3
PC1
PC Optimum center PC
420
424.5
2.160
-30.490
WF2
WF2 Optimum center WF2
423.5
414
-51.530
-5.920
WF3
WF3 Optimum center WF3
4362
424.52
-0.150
48.470
WF4
WF4 Optimum center WF4
423
421
54.830
-6.320
WFALL
WF3 Optimum near apex
1333
1493
2.020
7.920
PC1-FIX
PC Fixed center PC
420
424.5
1.810
-30.900
WF2-FIX
WF2 Fixed center WF2
423.5
414
-51.530
-5.920
WF3-FIX
WF3 Fixed center WF3
416.5
424.5
1.230
47.100
WF4-FIX
WF4 Fixed center WF4
423
421
54.830
-6.320
WFALL-FIX
WF3 Fixed near apex
1333
1493
2.020
7.920
FQUVN33 FQUVN33 WF2 Optimum for FOV
292
520
-49.924
10.802
POLQN33 POLQN33 WF2 Optimum for FOV
292
520
-49.924
10.802
POLQN18 POLQN18 WF2 Optimum for FOV
3804
2004
-33.280
-17.717
POLQP15P POLQP15 PC Optimum for FOV
200
680
-13.057
-31.643
POLQP15W POLQP15 WF2 Optimum for FOV
500
260
-45.892
-22.069
FQCH4NW2 FQCH4N WF2 Optimum for FOV
602
608
-77.669
-5.102
FQCH4NW3 FQCH4N WF3 Optimum for FOV
602
608
0.928
72.995
FQCH4NW4 FQCH4N WF4 Optimum for FOV
640
386
67.687
11.323
FQCH4N33 FQCH4N33 WF2 Optimum for FOV
264
436
-41.997
6.950
FQCH4N15 FQCH4N15 PC Optimum for FOV
420
424.5
2.164
-30.494
FQCH4P15 FQCH4P15 PC Optimum for FOV
400
312
5.129
-26.221
F160BN15 F160BN15 WF3 Optimum for FOV
436
424.5
-0.153
48.470
1V2-V3 coordinates in effect 1996 day 127 (May 6) to 1997 day 335 (December 1). CCD pixel positions unchanged.
2CCD pixel position in effect after 1994 day 101 (April 11).
3WFALL and WFALL-FIX "meta-chip" coordinates are (903,904).
4CCD pixel position in effect after 1995 day 86 (March 27).

Table 3.15: Updates to (V2,V3) Positions of WFPC2 Apertures.
Date in Effect
V2
V3
Rotation
1994 day 101 - 1996 day 105
V20
V30
PA0
1996 day 105 - 1996 day 127
V21 = V20 - 0.12"
V31 = V30 + 0.11"
PA1 = PA0 + 0.14
1996 day 127 - 1997 day 335
V22 = V20 + 0.46"
V32 = V30 + 0.39"
PA2 = PA0+ 0.14
> 1997 day 335
V23 = V20 + 0.67"
V33 = V30 + 0.61"
PA3 = PA0+ 0.14

Figure 3.12: Precise CCD Alignments and Primary Aperture Locations. "FIX" apertures are in the same locations, unless otherwise indicated. Dashed lines show vignetted region along CCD boundaries. Short lines and "X"s in outer corners indicate directions of CCD bloom and OTA diffraction spikes, respectively. Origin of the (V2, V3) system is at the origin of the plot axes, with V2, V3, and U3 exactly along diagonal lines as marked. (V2,V3) system is post-1996 day 127. CCDs have pixel (1,1) located where the four CCDs overlap.


 

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