Instrument Description

2.3 Overall Instrument Description

The Wide-Field and Planetary Camera, illustrated in
Figure 2.1, occupies the only radial bay allocated to a scientific instrument. Its field-of-view is centered on the optical axis of the telescope and it therefore receives the highest quality images. The three Wide-Field Cameras (WFC) at F/12.9 provide an "L" shaped field-of-view of 2.5x2.5 arcminutes with each 15 mm detector pixel subtending 0.10" on the sky. In the Planetary Camera (PC) at F/28.3, the field-of-view is 35" x 35" , and each pixel subtends 0.046" . The three WFCs undersample the point spread function of the Optical Telescope Assembly (OTA) by a factor of 4 at 5800Å in order to provide an adequate field-of-view for studying galaxies, clusters of galaxies, etc. The PC resolution is over two times higher. Its field-of-view is adequate to provide full-disk images of all the planets except Jupiter (which is 47" in maximum diameter). The PC has numerous extra-solar applications, including studies of galactic and extra-galactic objects in which both high angular resolution and excellent sensitivity are needed. In addition to functioning as the prime instrument, the WFPC2 can be used for target acquisition in support of other HST instruments, or for parallel observations.

Figure 2.2 shows the optical arrangement (not to scale) of the WFPC2. The central portion of the OTA F/24 beam is intercepted by a steerable pick-off mirror attached to the WFPC2, and is diverted through an open entry port into the instrument. The beam then passes through a shutter and filters. A total of 48 spectral elements and polarizers are contained in an assembly of 12 filter wheels. Then the light falls onto a shallow-angle, four-faceted pyramid located at the aberrated OTA focus, each face of the pyramid being a concave spherical surface. The pyramid divides the OTA image of the sky into four parts. After leaving the pyramid, each quarter of the full field-of-view is relayed by an optical flat to a Cassegrain relay that forms a second field image on a charge-coupled device (CCD) of 800x800 pixels. Each detector is housed in a cell that is sealed by a MgF2 window. This window is figured to serve as a field flattener.

The aberrated HST wavefront is corrected by introducing an equal but opposite error in each of the four Cassegrain relays. An image of the HST primary mirror is formed on the secondary mirrors in the Cassegrain relays. (The fold mirror in the PC channel has a small curvature to ensure this, and is why the PC magnification changed from F/30 in WF/PC-1 to F/28.3 in WFPC2.) The spherical aberration from the telescope's primary mirror is corrected on these secondary mirrors, which are extremely aspheric. The point spread function is then close to that originally expected for WF/PC-1.

Figure 2.2: WFPC2 Optical Configuration

The single most critical and challenging technical aspect of applying a correction is assuring exact alignment of the WFPC2 pupils with the pupil of the HST. If the image of the HST primary does not align exactly with the repeater secondary, the aberrations no longer cancel, leading to a wavefront error and comatic images. An error of only 2% of the pupil diameter produces a wavefront error of 1/6 wave, leading to degraded spatial resolution and a loss of about 1 magnitude in sensitivity to faint point sources. This error corresponds to mechanical tolerances of only a few microns in the tip/tilt motion of the pick-off mirror, the pyramid, and the fold mirrors.

Mechanisms inside WFPC2 allow optical alignment on-orbit; these are necessary to insure correction of the OTA spherical aberration. The beam alignment is set with a combination of the steerable pick-off mirror and actuated fold mirrors in cameras PC1, WF3 and WF4. The 47 degrees pick-off mirror has two-axis tilt capabilities provided by stepper motors and flexure linkages, to compensate for uncertainties in our knowledge of HST's latch positions (i.e., instrument tilt with respect to the HST optical axis). These latch uncertainties would be insignificant in an unaberrated telescope, but must be compensated for in a corrective optical system. In addition, three of the four fold mirrors, internal to the WFPC2 optical bench, have limited two-axis tilt motions provided by electrostrictive ceramic actuators and invar flexure mountings. Fold mirrors for the PC1, WF3, and WF4 cameras are articulated, while the WF2 fold mirror has a fixed invar mounting. A combination of the pick-off mirror and fold mirror actuators has allowed us to correct for pupil image misalignments in all four cameras. Since the initial alignment, stability has been such that mirror adjustments have not been necessary. The mechanisms are not available for GO commanding.

The WFPC2 pyramid cannot be focused or rotated. WFPC2 is focused by moving the OTA secondary mirror, and then COSTAR (or any future science instruments) is adjusted to achieve a common focus for all the HST instruments.

The four CCDs provide a 1600 x 1600 pixel field-format; three of the 800 x 800 CCDs have 0.1" pixels (WFC), and one has 0.046" pixels (PC). The CCDs are physically oriented and clocked so that the pixel read-out direction is rotated approximately 90 degrees in succession (see Figure 1.1 on page 2). The (1,1) pixel of each CCD array is thereby located near the apex of the pyramid. As a registration aid in assembling the four frames into a single picture, a light can be turned on at the pyramid to form a series of eleven fixed artificial "stars" (known as Kelsall spots or K-spots) along the boundaries of each of the quadrants. This calibration is normally done in a separate exposure. The K-spot images are aberrated and similar in appearance to the uncorrected HST PSF. The relative alignment of the four channels has been more accurately determined from star fields, and is stable over long periods, but the K-Spot images are useful for verifying the stability.

Figure 2.3: Cooled Sensor Assembly

Each CCD is a thick frontside-illuminated silicon sensor, fabricated by Loral Aerospace. A CCD, mounted on its header, is hermetically packaged in a ceramic-tube body that is filled with 1.1 atmosphere of argon to prevent degradation of the UV sensitive phosphor, and sealed with the MgF2 field flattener. This complete cell is connected with compliant silver straps to the cold junction of a thermo-electric cooler (TEC). The hot junction of the TEC is connected to the radial bay external radiator by an ammonia heat pipe. This sensor-head assembly is shown in Figure 2.3. During operation, each TEC cools its sensor package to suppress dark current in the CCD.

Figure 2.2: - WFPC2 Optical Configuration
Figure 2.3: - Cooled Sensor Assembly