43.3 WF/PC-1 Instrument Basics
Figure 43.1 shows a schematic of the optical arrangement of the WF/PC-1. The central portion of the optical telescope assembly (OTA) f/24 beam was intercepted by a steerable pick-off mirror attached to the WF/PC-1 and diverted through an open port entry into the WF/PC-1. The beam then passed through a shutter and interposable filters. A total of 48 spectral elements and polarizers were contained in an assembly of 12 filter wheels. The light then fell onto a shallow-angle, four-faceted pyramid located at the aberrated OTA focus. Each face of the pyramid was a concave spherical surface. The pyramid divided the OTA image of the sky into four parts. After leaving the pyramid, each quarter of the full field-of-view was relayed by an optical flat to a Cassegrain relay that formed a second field image on a charge-coupled device (CCD) of 800 x 800 pixels. Each of these detectors was housed in a cell filled with 0.1 atmosphere of argon sealed by a MgF2 field flattener.
In total, the WF/PC-1 contained eight relay mirror-repeater-CCD trains, four for the wide-field camera and four for the planetary camera. To place the desired camera into operation, the pyramid was commanded to rotate into one of two fixed orientations separated by 45 degrees. The Wide Field cameras (WF) produced a final focal ratio of f/12.9 while the Planetary Cameras (PC) produced f/30.
Table 43.1 summarizes the configuration of the two cameras (also see the OV/SV Report, 1992 and the WF/PC-1 Instrument Handbook, 1992). The Wide Field chips are numbered 1 through 4, the Planetary Camera chips 5 through 8.
|Camera||Pixels||Field of View||Scale||f/ratio|
|WFC||800 x 800 x 4 chips||2.6´ x 2.6´||0.102"||12.9|
|PC||800 x 800 x 4 chips||66" x 66"||0.043"||30|
Two readouts for each camera were available: FULL and AREA mode (given in the MODE header keyword). FULL mode was the default, with full single-pixel resolution. In full mode, each line of science data contained two 16-bit words of engineering data followed by 799 16-bit numbers as read from the chip, followed by 12 overclocked pixels which were used to determine the bias level (BLEV) correction (see "Calibration Details" on page 45-7. During the initial steps of the routine pipeline processing, the science data was reformatted and separated into the raw (.d0h/.d0d) science data (800 x 800 pixels) and the engineering data along with the 12 bias columns (.x0h/.x0d).
AREA mode-which was not used often (only 50 images, 35 of them externals, out of 15,746 total WF/PC-1 images)-was a 2 x 2 pixel summation. In this case, the two words of engineering data were followed by only 400 16-bit numbers read out from the chips; no overclocking was done. In this mode, the BLEV correction was determined from the second column of engineering data. The least significant bit (from odd rows only, even rows contained engineering data) was averaged, then multiplied by two.
The field of view of the WF/PC-1 charge-coupled devices (CCDs) on the sky is illustrated in Figure 43.2; some features of the chips are summarized below.
- Each chip was read out from the corner nearest the central point where all chip corners meet; the arrows indicate the readout direction. The column and row (line) numbers increased from the center outwards; the direction of blooming (along columns) was parallel to the arrows.
- An optically inactive edge (about 25 pixels wide) bordered the two sides of each chip that were adjacent to another chip. The resultant area on the sky covered by all four chips was approximately 1543 x 1543 pixels for the wide field camera (WFC) (154.5" x 154.5") and 1531 x 1531 pixels for the planetary camera (PC) (65.8" x 65.8").
- A region of low reflectance on the pyramid (called the Baum spot) with 0.1% reflectance of the rest of the pyramid can be found in PC 8 (x=416, y=417) and in WF 4 (x=195, y=197), about 1.2" (~28 pixels) in diameter.
- The Kelsall spots, a series of eleven pinholes along each of the common pyramid edges used for image registration, were illuminated only during special calibration observations.
- Read noise was around 13 rms electrons per pixel; gain was approximately 7.5 electrons per digital number (DN) (see the WF/PC-1 Instrument Handbook, Version 3.0 page 34). The images from each chip are oriented such that north shifts by roughly 90 degrees from chip to chip. The orientation of each chip (i.e., the direction of north) is stored in the ORIENTAT group keyword; the north task can be used to obtain the position angle of the image. Calibrated WF/PC-1 images retain a residual geometric distortion (about 1 pixel, corner to corner). The wmosaic task can be used to geometrically correct and align the four chip images (see Chapter 3 for more details). Figure 43.3 shows the image produced after wmosaic has been used to properly align and geometrically correct the four sub-images and produce a single mosaic image.