Space Telescope Science Institute
2010 WFPC2 Data Handbook
help@stsci.edu
Table of Contents Previous Next Index Print


WFPC2 Data Handbook > Chapter 1: WFPC2 Instrument Overview > 1.1 WFPC2 Physical Configuration

1.1 WFPC2 Physical Configuration
The WFPC2 field of view was located at the center of the HST focal plane, as shown in Figure 1.1. Figure 1.2 shows a schematic of its optical arrangement: the central portion of the f/24 beam coming from the OTA is intercepted by a steerable pick-off mirror attached to the WFPC2 and is diverted through an open port entry into the instrument. The beam then passes through a shutter and interposable filters. A total of 48 spectral elements and polarizers are contained in an assembly of 12 filter wheels.
The light falls onto a shallow-angle, four-faceted pyramid, located at the aberrated OTA focus. Each face of the pyramid is a concave spherical surface, dividing 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 optically-flat mirror to a Cassegrain relay that forms a second field image on a charge-coupled device (CCD) of 800 x 800 pixels. Each of these four detectors is housed in a cell sealed by a MgF2 window, which is figured to serve as a field flattener.
Figure 1.1: WFPC2 Field of View Prior to Servicing Mission 4.
The aberrated HST wavefront was corrected by introducing an equal but opposite error in each of the four Cassegrain relays. An image of the HST primary mirror was formed on the secondary mirrors in the Cassegrain relays. The spherical aberration from the telescope’s primary mirror was corrected on these secondary mirrors, which are extremely aspheric; the resulting point spread function was quite close to that originally expected for WF/PC-1.
Figure 1.2: WFPC2 Optical Configuration
WFPC2 Optical Configuration, showing the light path for one of the four Cassegrain relays after the main beam is split into four parts at the pyramid
The optics of three of the four cameras, the Wide Field Cameras (WF2, WF3, WF4), are essentially identical, with a final focal ratio of f/12.9. The fourth camera, known as the Planetary Camera (PC or PC1), has a focal ratio of f/28.3.
Figure 1.3 shows the field of view of WFPC2 projected on the sky:
The U2, U3 axes (same as the -V2, -V3 axes) are defined by the nominal Optical Telescope Assembly (OTA) axis, which was near the center of the WFPC2 FOV.
The readout direction is marked with an arrow near the start of the first row in each CCD. Note that it rotates 90 between successive chips. The readout direction of the four CCDs was defined such that the origin of the CCD was at the corner of the chip pointing towards the apex of the WFPC2 pyramid. (In the STSDAS pixel numbering system, the CCD origin is located at the lower left corner of the CCD image.)
The X, Y arrows mark the coordinate axes for any POS TARG commands1 that may have been specified in the proposal; the HST Phase II Proposal Instructions for Cycle 16 elaborates on the use of this requirement.
Figure 1.3: WFPC2 Field of View Projected on the Sky
The position angle of V3 on the sky varies with pointing direction and observation epoch, and is recorded in the calibrated science header keyword PA_V3. Note that for WFPC2, the PA_V3 is offset 180 from any ORIENT that may have been requested in the HST proposal (the optional special requirement ORIENT, if used, is specified in the Phase II proposal but is not recorded in the WFPC2 image headers).
The orientation of each camera on the sky, i.e., position angle of the y-axis of each detector, is provided by the ORIENTAT group keyword in the image headers. (Note: this is not the same as the ORIENT special requirement used in Phase II proposals.) The geometry of the cameras and the related image header keywords are explained in greater detail in Chapter 2.
Table 1.1: Camera Configurations
800 x 800
The Planetary Camera (PC1) provided a field of view sufficient to obtain full disk images of all planets except Jupiter. The PC1 CCD pixels undersampled the point spread function by a factor of two at 5800 . For the WF cameras, their pixel scales were over a factor of two larger than the PC1, and thus undersampled the image by a factor of four at visual wavelengths. Some of the sampling lost to these large pixel scales can be recovered by combining images that were executed with sub-pixel offsets (by using dither patterns or the POS TARG special requirement). Additional information about dithering is provided in The MultiDrizzle Handbook.
As a result of the aberration in the primary beam, the light from sources near the pyramid edges was divided between adjacent chips. Consequently, the lower columns and rows of the PC1 and WF chips are strongly vignetted, as shown in Table 1.2. The CCD xy (column, row) numbers given in this table are approximate to the 1 - 2 pixel level due to geometric distortion in the camera.
Table 1.2: Inner Edges of the WFPC2 Field Projected Onto CCDs
Start Vignetted Field (Zero Illumination)
Start Unvignetted Field (100% Illumination)
x > 0 and y > 8
x > 44 and y > 52
x > 88 and y > 96
x > 26 and y > 6
x > 46 and y > 26
x > 66 and y > 46
x > 10 and y > 27
x > 30 and y > 47
x > 50 and y > 67
x > 23 and y > 24
x > 43 and y > 44
x > 63 and y > 64
The WFPC2 Static Archive provides a drizzled image product with the four CCDs mosaicked into a single frame at the WF3 pixel scale. This provides a convenient “quick-look” image of the entire field of view. However, this image does not have the full photometric corrections applied, and should generally not be used for detailed analyses. Alternatively, the STSDAS task wmosaic provides another way to produce a “quick-look” image with the four CCDs mosaicked together. This task, in its default mode, will combine the four chips into a 1600 x 1600 pixel image at the resolution of the Wide Field cameras. Neither of these “quick-look” products support the full resolution of the Planetary Camera channel.
Finally, a comment about readout modes. There are two observation modes available on WFPC2: FULL and AREA. The mode used for a given observation is recorded in the image header keyword MODE. In FULL mode, each pixel is read out individually, while in AREA mode, pixels are summed in 2 x 2 boxes before they are read out. The advantage of AREA mode is that the readout noise for the “larger” pixels (6 e per pixel) is nearly the same as the readout noise for unsummed pixels (5 e per pixel). Thus, AREA mode can be useful in observations of extended sources when the primary source of noise is readout noise, as is often the case in the far UV.
In practice, observers have made very limited use of the AREA mode capability; less than 0.1% of all WFPC2 images in the Archive were taken in AREA mode. As a result, AREA mode calibration is not supported at the same level as FULL mode. Although reference files such as biases, darks, and flatfields are available for AREA mode images, they may not provide the best calibration as they are not updated and improved as frequently as the FULL mode reference files. Researchers using AREA mode images that require a very high level of calibration should consult the list of best available reference files, and consider generating their own AREA mode reference files. For example, it might be possible to make more accurate reference files by re-binning the FULL mode reference file appropriate for their observation epoch. See Section 3.5 for more information on how to manually recalibrate WFPC2 data. For assistance, questions, or problems, contact the HST helpdesk at help@stsci.edu.

1
POS TARG, an optional special requirement in HST Phase II proposals, places the target in offsets (specified in units of arc seconds) from a specified aperture.


Table of Contents Previous Next Index Print