HST Data Handbook for WFPC2

Chapter 1:
WFPC2 Instrument
Overview

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The Wide Field and Planetary Camera 2 (WFPC2) is a two-dimensional imaging device covering a wavelength range from Lyman- to about 1 µm. Built at the Jet Propulsion Laboratory by an Investigation Definition Team (IDT) headed by John Trauger, WFPC2 was the replacement for the first Wide Field and Planetary Camera (WF/PC-1) and includes built-in corrections for the spherical aberration of the HST Optical Telescope Assembly (OTA). The WFPC2 was installed in HST during the First Servicing Mission in December 1993. An early IDT report of the WFPC2 on-orbit performance can be found in Trauger et al. (1994, ApJ, 435, L3), and a more detailed assessment of its capabilities can be found in Holtzman et al. (1995, PASP, 107, page 156 and page 1065).

The WFPC2 field of view is located at the center of the HST focal plane; figure 1.1 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 then 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.

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 spherical aberration from the telescope's primary mirror is corrected on these secondary mirrors, which are extremely aspheric; the resulting point spread function is quite close to that originally expected for WF/PC-1.

Figure 1.1: WFPC2 Optical Configuration
 

The optics of three of the four cameras - the Wide Field Cameras (WF2, WF3, WF4) - are essentially identical and produce 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 1.2 shows the field of view of WFPC2 projected on the sky; the U2,U3 axes are defined by the "nominal" Optical Telescope Assembly (OTA) axis, which is 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 degrees between successive chips. The x,y arrows mark the coordinate axes for any POS TARG commands¹ that may have been specified in the proposal; the Proposal Instructions elaborate on the use of this requirement.

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 degrees from any ORIENT that may have been requested in the HST proposal; as an optional parameter, ORIENT can be found in the proposals but is not recorded in the WFPC2 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. The geometry of the cameras and the related group and image keywords are explained in greater detail in chapter 2.

Figure 1.2: WFPC2 Field of View Projected on the Sky
 
Table 1.1: Camera Configurations

Camera	Pixels	Field of View	Scale	f/ratio
PC	800 x 800	36" x 36"	0.0455" per pixel	28.3
WF2, 3, 4	800 x 800	80" x 80"	0.0996" per pixel	12.9

 

The Planetary Camera (PC) provides a field of view sufficient to obtain full disk images of all planets except for Jupiter, though the pixels undersample the point spread function of the telescope and camera optics by a factor of two at 5800 Å. The WF pixels are over a factor of two larger, and thus undersample the image by a factor of four at visual wavelengths. It is possible to recover some of the sampling lost to these large pixels by image dithering, i.e., taking observations at different sub-pixel offsets. A short discussion of dithering is provided in section 5.5; more detailed information is available in the HST Dither Handbook.

The readout direction of the four CCDs is defined such that in IRAF pixel numbering (origin at lower left corner), the origin of the CCD lies at the corner of the chip pointing towards the apex of the WFPC2 pyramid (see figure 1.2). As a result of the aberration of the primary beam, the light from sources near the pyramid edges is divided between adjacent chips. Consequently, the lower columns and rows of the PC and WF chips are strongly vignetted, as shown in table 1.2. The CCD x,y (column, row) numbers given in this table vary at the 1-2 pixel level because of bending and tilting of the field edge in detector coordinates due to geometric distortion in the camera.

Table 1.2: Inner Field Edges of Field Projected Onto CCDs

Camera	Start Vignetted Field	Contiguous Field	Start Unvignetted Field
PC1	x>0 and y>8	x> 44 and y>52	x>88 and y>96
WF2	x>26 and y>6	x>46 and y>26	x>66 and y>46
WF3	x>10 and y>27	x>30 and y>47	x>50 and y>67
WF4	x>23 and y>24	x>43 and y>44	x>63 and y>64

 

The STSDAS task wmosaic provides a convenient way to piece the four groups into a single image comprising the full WFPC2 field of view. This task, in its default mode, will combine the four chips into a large, 1600 x 1600 pixel image at the resolution of the Wide Field cameras, resampling the pixels and correcting for the chip overlap, rotations, and geometric distortion. Consequently, resolution will be lost in the PC, whose pixels are rebinned to the resolution of the Wide Field Cameras (a factor 2.3 coarser).

While the images produced by wmosaic are usually adequate for presentations and the identification of interesting features, they are not recommended for science uses because of the loss of resolution and photometric accuracy associated with data resampling, especially in the PC.

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 readout noise for the larger pixels is nearly the same as for the unsummed pixels: 6e^- vs. 5e^- 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 are 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 are advised to consult the list of best available reference files, and consider generating their own area mode calibration files in order to manually recalibrate their data. See section 3.4 for more information on how to manually recalibrate WFPC2 data. For assistance, questions, or problems, e-mail the HST helpdesk at help@stsci.edu.

¹ POS TARG, an optional special requirement in HST observing proposals, places the target an offset of POS TARG (in arc sec) from the specified aperture.