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Wide Field and Planetary Camera 2 Instrument Handbook

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1.1 Instrument Overview


Wide Field and Planetary Camera 2 (WFPC2) was placed aboard the Hubble Space Telescope in December, 1993 during the first servicing mission (SM1). It is due to be replaced by Wide Field Camera 3 (WFC3) during SM4, currently scheduled for Fall, 2008.

The instrument is a two-dimensional imaging photometer, located at the center of the Hubble Space Telescope (HST) focal plane and covers the spectral range between approximately 1150┼ to 10500┼. It simultaneously images a 150" x 150" "L"-shaped region with a spatial sampling of 0.1" per pixel, and a smaller 34" x 34" square field with 0.046" per pixel. The total system quantum efficiency (WFPC2+HST) ranges from 4% to 14% at visual wavelengths, and drops to ~0.1% in the far UV. Detection of faint targets is limited by either the sky background (for broad band filters) or by noise in the read-out electronics (for narrow band and UV filters) with an RMS equivalent to 5 detected photons. Bright targets can cause saturation (>53000 detected photons per pixel), but there are no related safety issues. The sections below give a more detailed overview.

1.1.1 Field-of-View

The WFPC2 field-of-view is divided into four cameras by a four-faceted pyramid mirror near the HST focal plane. Each of the four cameras contains an 800x800 pixel Loral CCD detector. Three cameras operate at an image scale of 0.1" per pixel (F/12.9) and comprise the Wide Field Camera (WFC) with an "L" shaped field-of-view. The fourth camera operates at 0.046" per pixel (F/28.3) and is referred to as the Planetary Camera (PC). There are thus four sets of relay optics and CCD sensors in WFPC2. The four cameras are called PC1, WF2, WF3, and WF4, and their fields-of-view are illustrated in Figure 1.1 (also see Section 7.8). Each image is a mosaic of three F/12.9 images and one F/28.3 image.

Figure 1.1: WFPC2 Field-of-View Projected on the Sky. The readout direction is marked with arrows near the start of the first row in each CCD. The X-Y coordinate directions are for POS-TARG commands. The position angle of V3 varies with pointing direction and observation epoch, and is given in the calibrated science header by keyword PA_V3.


 

1.1.2 Spectral Filters

The WFPC2 contains 48 filters mounted in 12 wheels of the Selectable Optical Filter Assembly (SOFA). These include a set of broad band filters approximating Johnson-Cousins UBVRI, as well as a set of wide U, B, V, and R filters, and a set of medium bandwidth Str÷mgren u, v , b , and y filters.

Narrow band filters include those for emission lines of Ne V (3426┼), CN (~3900┼), [OIII] (4363┼ and 5007┼), He II (4686┼), H (4861┼), He I (5876┼), [OI] (6300┼), H (6563┼), [NII] (6583┼), [SII] (6716┼ and 6731┼), and [SIII] (9531┼). The narrow band filters are designed to have the same dimensionless bandpass profile. Central wavelengths and profiles are uniformly accurate over the filter apertures, and laboratory calibrations include profiles, blocking, and temperature shift coefficients.

There are also two narrow band "quad" filters, each containing four separate filters which image a limited field-of-view. The UV quad contains filters for observing redshifted [OII] emission and are centered at 3767┼, 3831┼, 3915┼, and 3993┼ (see Section 3.4). The Methane quad (Section 3.6) contains filters at 5433┼, 6193┼, 7274┼, and 8929┼. Finally, there is a set of narrow band "linear ramp filters" (LRFs) which are continuously tunable from 3710┼ to 9762┼; these provide a limited field-of-view with a diameter of ~10" . More information on the LRFs can be found in Section 3.3.

At ultraviolet wavelengths, there is a solar-blind Wood's UV filter (1200-1900┼); please see Section 3.7 for more information on the Woods filter. The UV capability is also enhanced by control of UV absorbing molecular contamination, the capability to remove UV absorbing accumulations on cold CCD windows without disrupting the CCD quantum efficiencies and flat field calibrations, and an internal source of UV reference flat field images.

Finally, there is a set of four polarizers set at four different angles, which can be used in conjunction with other filters for polarimetric measurements. However, due to the relatively high instrumental polarization of WFPC2, they are best used on strongly polarized sources (>3% polarized). Sources with weaker polarization will require very careful calibration of the instrumental polarization. For more information on the polarizers, please see Section 3.5.

1.1.3 Quantum Efficiency and Exposure Limits

The quantum efficiency (QE) of WFPC2+HST peaks at 14% in the red, and remains above 4% over the visible spectrum. The UV response extends to Lyman wavelengths (QE~0.1%). Spherical aberration correction is achieved via internal optics.

Exposures of bright targets are limited by saturation effects, which appear above ~53000 detected photons per pixel (for setting ATD-GAIN=15), and by the shortest exposure time which is 0.11 seconds. There are no instrument safety issues associated with bright targets. Detection of faint targets is limited by the sky background for broad band filters at visual wavelengths. For narrow band and ultraviolet filters, detections are limited by noise in the read-out amplifier ("read noise"), which contributes an RMS noise equivalent to ~5 detected photons per pixel for an ATD-GAIN of 7.

1.1.4 CCD Detector Technology

The WFPC2 CCDs are thick, front-side illuminated devices made by Loral Aerospace. They support multi-pinned phase (MPP) operation which eliminates quantum efficiency hysteresis. They have a Lumogen phosphor coating to give UV sensitivity; the on-orbit performance of the detectors is discussed in Chapter 4. Technical details may be summarized as follows:

1.1.5 UV Imaging

WFPC2 had a design goal of 1% photometric stability at 1470┼ over a month. This requires a contamination collection rate of less than 47 ng cm-2 month-1 on the cold CCD window. Hence, the following features were designed into WFPC2 in an effort to reduce contaminants:

  1. Venting and baffling, particularly of the electronics, were redesigned to isolate the optical cavity.
  2. There was an extensive component selection and bake-out program, and specialized cleaning procedures.
  3. Molecular absorbers (Zeolite) were incorporated.

The CCDs were initially operated at -77░C after launch, which was a compromise between being as warm as possible for contamination reasons, while being sufficiently cold for an adequate dark rate. However, at this temperature significant photometric errors were introduced by low-level traps in the CCDs. This problem with the charge transfer efficiency of the CCDs has been reduced since 23 April 1994 by operating the CCDs at -88░C, but this leads to significantly higher contamination rates than hoped for. On-orbit measurements indicate that there is a decrease in throughput at a repeatable rate of ~30% per month at 1700┼ (see Section 6.11). Monthly decontamination procedures are able to remove the contaminants completely and recover this loss. As of Cycle 12, the interval between decontaminations has been increased from 30 days to approximately 49 days.

1.1.6 Aberration Correction and Optical Alignment

WFPC2 contains internal corrections for the spherical aberration of the HST primary mirror. These corrections are made by highly aspheric surfaces figured onto the Cassegrain relay secondary mirror inside each of the four cameras. Complete correction of the aberration depends on a precise alignment between the OTA pupil and these relay mirrors.

Mechanisms inside WFPC2 allow optical alignment on-orbit. The 47░ 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 actuated fold mirror (AFMs) 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 were not available for GO commanding.)


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