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: http://www.stsci.edu/instrument-news/handbooks/wfpc2/W2_15.html
Дата изменения: Wed Jul 24 04:09:00 1996 Дата индексирования: Tue Feb 5 06:01:51 2013 Кодировка: Поисковые слова: dra |
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.
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
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.
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.