WFPC2 Instrument Handbook for Cycle 11
Wide Field and Planetary Camera 2 (WFPC2) is a two-dimensional imaging photometer which is 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 5% to 14% at visual wavelengths, and drops to ~0.5% in the far UV. Detection of faint targets will be limited by either the sky background (for broad filters) or by noise in the read-out electronics (for narrow 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.
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 (see also CCD Position and Orientation on Sky). 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.
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┼. The Methane quad 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 diameter ~10".
At ultraviolet wavelengths there is a solar-blind Wood's UV filter (1200-1900┼). 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.
Quantum Efficiency and Exposure Limits
The quantum efficiency (QE) of WFPC2+HST peaks at 14% in the red, and remains above 5% over the visible spectrum. The UV response extends to Lyman wavelengths (QE~0.5%). Internal optics provide a spherical aberration correction.
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.
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. Details may be summarized as follows:
- Read noise: WFPC2 CCDs have ~5e- RMS read noise which provides good faint object and UV imaging capabilities.
- Dark noise: Inverted phase operation yields low dark noise for WFPC2 CCDs. They are being operated at -88░C and the median dark current is about 0.0045 e- pixel-1 s-1.
- Flat field: WFPC2 CCDs have a uniform pixel-to-pixel response (<2% pixel-to-pixel non-uniformity) which facilitates accurate photometric calibration.
- CTE: Low level charge traps are present in the WFPC2 devices at the present operating temperature of -88░C. For bright stellar images, there is a ~4% signal loss when a star image is clocked down through all rows of the CCD. Fainter images show a larger effect which also appears to increase with time. The effect can be as large as tens of percent for faint stars (few hundred electrons) seen against a low background (<0.1 DN) in data taken during later Cycles. For most typical applications, CTE is negligible or calibratable, and pre-flash exposures are not required. This avoids the increase in background noise, and the decrease in operational efficiency that results from a preflash.
- Gain switch: Two CCD gains are available with WFPC2, a 7 e- DN-1 channel which saturates at about 27000 e- (4096 DN with a bias of about 300 DN) and a 14 e- DN-1 channel which saturates at about 53000 e-. The Loral devices have a full well capacity of ~90,000 e- and are linear up to 4096 DN in both channels.
- DQE: The peak CCD DQE in the optical is 40% at 7000┼. In the UV (1100-4000┼) the DQE is determined by the phosphorescent Lumogen coating, and is 10 - 15%.
- Image Purge: The residual image resulting from a 100x (or more) full-well over-exposure is well below the read noise within 30 minutes. No CCD image purge is needed after observations of very bright objects. The Loral devices bleed almost exclusively along the columns.
- Quantization: The systematic Analog-to-Digital converter errors have been largely eliminated, contributing to a lower effective read noise.
- QEH: Quantum Efficiency Hysteresis (QEH) is not a significant problem in the Loral CCDs because they are frontside illuminated and use MPP operation. The absence of any significant QEH means that the devices do not need to be UV-flooded and the chips can be warmed monthly for decontamination purposes without needing to maintain a UV-flood.
- Detector MTF: The Loral devices do suffer from low level detector MTF perhaps caused by scattering in the frontside electrode structure. The effect is to blur images and decrease the limiting magnitude by about 0.5 magnitudes.
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:
- Venting and baffling, particularly of the electronics, were redesigned to isolate the optical cavity.
- There was an extensive component selection and bake-out program, and specialized cleaning procedures.
- 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 now a decrease in throughput at a repeatable rate of ~30% per month at 1700┼. Monthly decontamination procedures are able to remove the contaminants completely and recover this loss.
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 are not available for GO commanding.
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