WFPC2 Instrument Handbook for Cycle 11

Apertures

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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 apex in WF3 (see Figure 3.11). 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 CCD Orientation and Readout. 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.

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 3.11 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

1 V2-V3 coordinates in effect 1996 day 127 (May 6) to 1997 day 335 (December 1). CCD pixel positions unchanged. 2 CCD pixel position in effect after 1994 day 101 (April 11). 3 WFALL and WFALL-FIX "meta-chip" coordinates are (903,904). 4 CCD 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 3.11: 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.