Hubble Space Telescope Primer for Cycle 11

4.6 Wide Field and Planetary Camera 2 (WFPC2)

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The WFPC2 has three "wide-field" CCDs, and one high-resolution (or "planetary") CCD. Each CCD covers 800 x 800 pixels and is sensitive from 1200 to 11,000 Å. All four CCDs are exposed simultaneously, with the target of interest being placed as desired within the FOV.

The three Wide Field Camera (WFC) CCDs are arranged in an "L"-shaped FOV whose long side projects to 2.5', with a projected pixel size of 0.10". The Planetary Camera (PC) CCD has a FOV of 35" x 35", and a projected pixel size of 0.0455". The WFC configuration provides the larger FOV, but undersamples the cores of stellar images; the PC configuration samples the images better, but has the smaller FOV.

A variety of filters may be inserted into the optical path. Polarimetry may be performed by placing a polarizer into the beam. A ramp filter exists that effectively allows you to image a ~10" region in an arbitrary 1.3% bandpass at any wavelength between 3700 and 9800 Å, by appropriately positioning the target within the FOV.

4.6.1 Charge Transfer Efficiency (CTE) Performance

The WFPC2 CCDs are undergoing a gradual deterioration in performance due to exposure to the space radiation environment. This leads to a gradual reduction in the CTE during readout, which is most apparent for observations of faint stellar targets observed against little or no background light. Examples of images with little background light would include short exposures (< 30 sec.) in broad band filters, or exposures in UV and narrow band filters. A significant effort is being made to study these effects, and formulae exist to correct stellar photometric results in simple situations. Nonetheless, you should be aware that this will be an important source of uncertainty for some observations. For further information please consult the WFPC2 Instrument Handbook.

4.6.2 Comparing WFPC2 to ACS

While ACS will clearly have advantages over WFPC2, there are some observing programs for which WFPC2 will continue to be the instrument of choice. Such programs would include, but are not limited to:

• Narrowband imaging across large fields of view.
• UV survey and/or parallel observations over large fields of view.
• Continuations of monitoring programs with a substantial baseline of WFPC2 observations.
• Precision astrometry.

In addition, due to the longer readout times and larger data volumes expected for ACS, programs requiring many short exposures taken over a short period of time may benefit from the use of WFPC2. WFPC2 is a well-established, proven instrument which will be better suited for some programs; for further details and assistance in evaluating the trade-offs, please see the ACS Instrument and WFPC2 Instrument Handbooks.