Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14
 |
 |
 |
 |
 |
|
| Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14 | |||||
| ||||||
1.1 Instrument OverviewChapter 1:
Introduction
1.1.1 Field-of-View
1.1.2 Spectral Filters
1.1.3 Quantum Efficiency and Exposure Limits
1.1.4 CCD Detector Technology
1.1.5 UV Imaging
1.1.6 Aberration Correction and Optical Alignment
1.2 Which Instrument to Use: WFPC2, ACS, NICMOS, or STIS?
1.2.1 Comparison of WFPC2 and ACS
1.2.2 Comparison of WFPC2 and NICMOS
1.2.3 Comparison of WFPC2 and STIS
1.3 History of WFPC2
1.4 The Previous vs. Current Generation: WF/PC-1 vs. WFPC2
1.5 Organization of this Handbook
1.6 What's New in Version 6.0 for Cycle 11
1.7 What's New in Version 7.0 for Cycle 12
1.8 What's New in Version 8.0 for Cycle 13
1.9 What's New in Version 9.0 for Cycle 14
1.10 WFPC2 Handbook on the WWW
1.11 The Help Desk at STScI
1.12 Further Information
1.1 Instrument Overview
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 1150Å to 10500Å. 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 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.
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 (see also 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 Strömgren u, v, b, and y filters.
Narrow band filters include those for emission lines of Ne V (3426Å), CN (~3900Å), [OIII] (4363Å and 5007Å), He II (4686Å), H
(4861Å), He I (5876Å), [OI] (6300Å), H
(6563Å), [NII] (6583Å), [SII] (6716Å and 6731Å), and [SIII] (9531Å). 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 3767Å, 3831Å, 3915Å, and 3993Å. The Methane quad contains filters at 5433Å, 6193Å, 7274Å, and 8929Å. Finally, there is a set of narrow band "linear ramp filters" (LRFs) which are continuously tunable from 3710Å to 9762Å; these provide a limited field-of-view with diameter ~10".
At ultraviolet wavelengths there is a solar-blind Wood's UV filter (1200-1900Å). 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.
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
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.
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. 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 either negligible or can be calibrated, 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 7000Å. In the UV (1100-4000Å) 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 front side 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 (Modulation Transfer Function) perhaps caused by scattering in the front side electrode structure. The effect is to blur images and decrease the limiting magnitude by about 0.5 magnitudes.
1.1.5 UV Imaging
WFPC2 had a design goal of 1% photometric stability at 1470Å 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 1700Å. 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 are not available for GO commanding.
1.2 Which Instrument to Use: WFPC2, ACS, NICMOS, or STIS?
In this section we compare briefly the performance of HST instruments with imaging capability in the UV to near-IR spectral range. These instruments include WFPC2 and STIS, as well as NICMOS, which was revived through the installation of the cryo-cooler, and the Advanced Camera for Surveys (ACS), which was installed during the HST Servicing Mission 3b1. Important imaging parameters for all instruments are summarized in Table 1.1.
Table 1.1: Comparison of WFPC2, ACS, NICMOS, and STIS Instrumental Imaging Parameters.Parameter WFPC2 ACS NICMOS STIS Wavelength range 1150Å - 11,000Å WFC: 3700 Å - 11000 Å HRC: 2000 Å - 11000 Å SBC: 1150 Å - 1700 Å 8000Å - 25,000Å FUV-MAMA: 1150Å - 1700Å NUV-MAMA: 1700Å - 3100Å CCD: 2000Å - 11,000Å Detector Si CCDs CCDs (WFC, HRC) MAMA (SBC) HgCdTe arrays CCD, MAMAs Image Format 4 x 800 x 800 WFC: 2 butted 2048x4096 HRC: 1024x1024 SBC: 1024x1024 3 x 256 x 256 1024 x 1024 Field-of-view and pixel size 150" x 150" @ 0.1" /pix 34"x 34" @ 0.046"/pix (1) WFC: 202"x202" @0.05" /pix HRC: 29"x26" @0.028"x0.025" /pix SBC: 35"x31" @0.033"x0.030" /pix NIC1: 11"x 11" @ 0.043" /pix NIC2: 19" x 19" @ 0.075" /pix NIC3: 51" x 51" @ 0.2" /pix MAMAs: 25" x 25" @ 0.024"/pix CCD: 51" x 51" @ 0.05" /pix (2) Read noise 5 e- WFC: 5 e- HRC: 4.7 e- SBC: 0 e- 30 e- MAMAs: 0 e- CCD: 5.4e- Dark current 0.002[WF2] - 0.006[PC] e- /s/pix WFC: 0.002 e-/s/pix HRC: 0.0025 e-/s/pix SBC: 1.2x10-5 e-/s/pix <0.1 e- /s/pix NUV-MAMA: 0.0001 e- /s/pix FUV-MAMA: 7x10-6 e- /s/pix CCD: 0.004 e- /s/pix Saturation 53,000 e- WFC: 80,000 e- HRC: 140,000 e- SBC: 100 counts/s/pix 200,000 e- MAMAs: 100 counts/s/pix CCD: 140,000 e-
1"L"-shaped field-of-view using 3 CCDs with 0.1" pixels, and one CCD with 0.046" pixels.
2Field-of-view is up to 51" x 51" if no filter is used, and down to 12" x 12" for some neutral density filters.
1.2.1 Comparison of WFPC2 and ACS
Advantages of each instrument may be summarized as follows.
- Wider field of view in the UV - effective area of 134"x134" vs. 35"x31".
- Wider field of view in many narrow band filters - effective area of 134"x134" vs. up to 40"x70" (ACS LRFs).
- Wider field of view in broad band optical filters - effective area of 202"x202" vs. 134"x134".
- Factor of ~2 better sampling of the PSF.
- Higher detective efficiency (factor of 2-10 depending on wavelength). Table 1.2 compares the detective efficiency for WFPC2 and ACS filters with similar band passes.
- True solar blind imaging in the UV due to the MAMA detector.
- Coronagraphic capability.
For projects using optical broad band filters, ACS is better suited due to its wider field of view, better sampling of the PSF, and higher throughput.
For projects using UV and narrow band filters the choice may depend on source size. For relatively compact objects, ACS is better due to the better PSF sampling and higher throughput and solar blind performance. For larger objects, e.g., the large planets Jupiter and Saturn, and diffuse galactic nebula such as the Orion and Eagle Nebulae, the larger field of view of WFPC2 makes it competitive.
Table 1.2: Comparison of WFPC2 and ACS Filters.
Effic'y3
Effic'y3
1Relative efficiency for ACS vs. WFPC2 for wide-field imaging. Defined as [(ACS FOV area)x(ACS efficiency)] / [(WFPC2 FOV area) x (WFPC2 efficiency)]. For WFPC2 we have reduced FOV for the missing "L" shaped region around PC1.
2The full WFPC2 FOV is a 150" x 150" L-shaped region, with area equivalent to a 134" x 134" square, which we use for comparisons to ACS.
3Efficiency near filter pivot wavelength; includes HST+instrument+filters.
4For ACS the full WFC FOV is 202"x202", the full HRC FOV is 26"x29", and the full SBC FOV is 31" x 35". When using the narrow band ramp filters the larger WFC FOV gets reduced, depending on the FOV location. There are three possible locations: inner region (45"x85"), middle region (65"x100"), and outer region (60"x85").
1.2.2 Comparison of WFPC2 and NICMOS
Both WFPC2 and NICMOS are capable of imaging at wavelengths between ~8000Å and ~11,000Å. At longer wavelengths NICMOS must be used; at shorter wavelengths WFPC2, STIS, or ACS must be used. Table 1.3 compares the detective efficiency of WFPC2 and NICMOS in the wavelength region where both instruments overlap in capabilities. Count rates for a V=20 star of spectral class A0 are given for all filters at common wavelengths; the signal-to-noise (S/N) is also given for a 1 hour exposure of this same star. For bright continuum sources WFPC2 and NICMOS offer similar efficiency over the spectral range from 8800Å to 10,500Å; the choice of instrument will likely depend on other factors such as field size and details of the passband shape. However, for very faint sources, the lower read noise of WFPC2 (5e- for WFPC2 vs. 30e- for NICMOS) should prove advantageous.
Both instruments have polarimetry capability, but the WFPC2 polarizers are not viable above 8000Å; above this wavelength NICMOS must be used for polarimetry. We note that the ACS WFC is optimized for the far red and has polarimetric capability.
Table 1.3: Comparison of WFPC2 and NICMOS Count Rates for a V=20 A0 Star.Instrument Filter Mean
Wavelength
(Å)Effective
Width (Å)Count Rate (e- s-1) SNR in 1 hour1 WFPC2 F785LP 9366 2095 14 215 F791W 8006 1304 30 314 F814W 8269 1758 33 333 F850LP 9703 1670 7.1 150 FQCH4N (Quad D) 8929 64 0.47 34, 292 F953N 9546 52 0.21 19, 152 F1042M 10,443 611 0.20 18, 152 LRF3 8000 9000 9762 105 113 126 1.5 0.64 0.23 66 40 20 NICMOSe F090M4 8970 1885 17.4 89 F095N4 9536 88 0.883 9.2 F097N4 9715 94 1.19 12 F108N4 10,816 94 1.17 9.9 F110W (Camera 1) 11,022 5920 73 170 F110W (Camera 2) 11,035 5915 83.7 290 F110W (Camera 3) 11,035 5915 75.9 390
1WFPC2 SNR assuming two 1800s exposures for cosmic ray removal. NICMOS SNR for central pixel of PSF.
2Values given for WFC (0.10" pixels) and PC (0.046" pixels).
3LRF filter is continuously tunable from 3710Å to 9762Å. LRF field-of-view is 10"x10".
4These NICMOS filters are available only on Camera 1 which has 11"x11" field-of-view.
4e. The NICMOS ETC performs S/N calculations for the brightest pixel with the detector temperature at 77.1oK.
1.2.3 Comparison of WFPC2 and STIS
Both WFPC2 and STIS are capable of imaging over the same wavelength ranges between ~1150Å and ~11000Å. At much longer wavelengths NICMOS must be used.
Advantages of each instrument may be summarized as follows.
- Wider field-of-view, effective area of 134" x 134" vs. 50" x 50" or less.
- Greater selection of filters, including polarizers.
- Bright Targets: WFPC2 has no bright target safety issues, and can give useful data on faint targets near very bright objects. STIS MAMAs can be damaged by bright objects.
- Much higher UV throughput.
- True solar blind imaging in UV due to MAMA detectors. WFPC2 CCDs are very sensitive to filter red-leak.
- PSF sampling: STIS offers 0.024" pixels vs. 0.046" on WFPC2.
- High time resolution is possible (
~125µs) with the MAMA detectors. Also the STIS CCD may be cycled on ~20s time scale using a sub-array.
In general, WFPC2 has a much greater selection of filters and wider field-of-view than STIS, but STIS has greater detective efficiency in the UV and for its long-pass and unfiltered modes. Table 1.4 compares the detective efficiency for WFPC2 and STIS filters with similar bandpasses. For UV imaging STIS is greatly superior due to higher throughput and insensitivity to filter red-leak; only if some detail of a WFPC2 filter bandpass were needed, would it be a viable choice.
For both [OII] 3727Å and [OIII] 5007Å imaging STIS has much higher QE and is preferred, unless the larger WFPC2 field-of-view is an important factor. The WFPC2 [OIII] filter is wider than its STIS counter-part, which may also be useful for redshifted lines. For broad-band imaging the unfiltered and 5500Å long-pass modes of STIS again have higher efficiency than WFPC2, though with reduced field-of-view.
Table 1.4: Comparison of WFPC2 and STIS Detector Efficiencies.Instrument Filter Mean
Wavelength
(Å)Bandpass
FWHM
(Å)1Peak QE2 WFPC2 F122M 1420 100 0.11% STIS F25LYA 1216 85 0.32% WFPC2 F160BW 1492 500 0.07% STIS 25MAMA (FUV) 1370 320 4.5% WFPC2 F255W 2586 393 0.5% STIS 25MAMA (NUV) 2220 1200 3.1% WFPC2 F375N 3738 42 0.9% STIS F28X50OII 3740 80 3.7% WFPC2 F502N 5013 37 5.8% STIS F28X50OIII 5007 5 11% WFPC2 F606W 5935 2200 14% STIS F28X50LP ~73003 2720 12% STIS 50CCD ~5800 4410 15%
1Note that definition of FWHM is different from "effective width" elsewhere herein.
2Includes instrument and OTA.
35500Å long pass filter.
1.3 History of WFPC2
The original Wide Field and Planetary Camera (WF/PC-1) served as the prototype for WFPC2. In many respects the two instruments are very similar. Both were designed to operate from 1150Å to 11000Å, both use 800x800 CCD detectors, and both provide spatial samplings of ~0.045" and ~0.1" per pixel. The development and construction of WF/PC-1 was led by Prof. J. A. Westphal, Principal Investigator (PI), of the California Institute of Technology. The instrument was built at the Jet Propulsion Laboratory (JPL) and was launched aboard HST in April 1990. It obtained scientific data until it was replaced by WFPC2 during the first servicing mission in December 1993.
Because of its important role in the overall HST mission, NASA decided to build a second Wide Field and Planetary Camera (WFPC2) as a backup clone of WF/PC-1 even before HST was launched. WFPC2 was already in the early stages of construction at JPL when HST was launched. After the discovery of spherical aberration in the HST primary mirror, it was quickly realized that a modification of the WFPC2 internal optics could correct the aberration and restore most of the originally expected imaging performance. As a result, development of WFPC2 was accelerated. Dr. J. T. Trauger of JPL is the project PI for WFPC2 and led the Investigation Definition Team (IDT2).
The WFPC2 completed system level thermal vacuum (SLTV) testing at JPL in April and May 1993. Between June and November there were payload compatibility checks at Goddard Space Flight Center (GSFC), and payload integration at Kennedy Space Center (KSC). WF/PC-1 was replaced by WFPC2 during the first servicing mission in December 1993. WFPC2 was shown to meet most of its engineering and scientific performance requirements by testing conducted during the three month Servicing Mission Observatory Verification (SMOV) period following the servicing mission. The General Observer community has had access to WFPC2 since the start of Cycle 4 in January 1994.
WFPC2 accurately corrects the HST spherical aberration, is a scientifically capable camera configured for reliable operation in space without maintenance, and is an instrument which can be calibrated and maintained without excessive operational overhead. It incorporates evolutionary improvements in photometric imaging capabilities. The CCD sensors, signal chain electronics, filter set, UV performance, internal calibrations, and operational efficiency have all been improved through new technologies and lessons learned from WF/PC-1 operations and the HST experience since launch.
WFPC2 SMOV requirements were developed by the IDT, GSFC, and the STScI to include: verification of the baseline instrument performance; an optical adjustment by focusing and aligning to minimize coma; the estimation of residual wave front errors from the analysis of star images; a photometric calibration with a core set of filters (including both visible and UV wavelengths); and the evaluation of photometric accuracy and stability over the full field with the core filter set. The results of these studies are documented in Holtzman, et al., 1995a and 1995b, and are summarized in this Handbook.
Despite these successes, the first years of scientific operation of WFPC2 have revealed a number of relatively minor instrumental defects that were not expected from the pre-launch testing. These include a low-level charge transfer inefficiency, a higher than expected level of scattered light around bright objects, and variable and lower than expected ultraviolet (UV) efficiency. In addition, we have come to understand the instrument more fully -- particularly in terms of its overall photometric performance, geometric distortion, scale and alignments, hot pixels, and CCD traps. All of this new information is described here.
1.4 The Previous vs. Current Generation: WF/PC-1 vs. WFPC2
For historical reasons, it is useful to offer comparisons between the current WFPC2, and its predecessor WF/PC-1, which was returned to Earth in December 1993.
- Field format: WF/PC-1 contained 8 cameras and CCDs, each CCD having 800 x 800 pixels. Four were used in the Planetary Camera mode (0.043" pixels), and four were used in the Wide Field Camera mode (0.10" pixels). The two pixel formats were selected by rotating the pyramid mirror by 45°. WFPC2 budget and schedule constraints forced a reduction from 8 to 4 camera channels in August 1991. WFPC2 contains only 4 CCDs; the pyramid mirror is fixed and the 4 cameras are physically located in the bays occupied by the WF/PC-1 WFC.
- Aberration correction: WF/PC-1 contained no correction for spherical aberration of the OTA primary mirror. Only about 15% of light from a stellar target fell into the core of the PSF (diameter ~0.1"). WFPC2 incorporates corrective figures on the Cassegrain secondary mirrors inside the relay cameras, and as a result places ~60% of the light from a star inside a diameter of 0.1". Precise alignment of the OTA pupil on these mirrors is required to attain full correction of the spherical aberration. Hence the pick-off mirror (POM) is steerable in WFPC2, and three of the fold mirrors contain tip-tilt actuators.
- CCD Technology: Many properties of WF/PC-1 and WFPC2 CCDs are compared in Table 4.1. Many differences derive from the fact that the WF/PC-1 CCDs were thinned, backside illuminated devices whereas the WFPC2 CCDs are thick, front side illuminated devices. In the WF/PC-1 CCDs the active silicon layer was a free-standing membrane somewhat less than 10µm thick, with photons impinging directly on the silicon layer, without attenuation in the polysilicon gate structure built on the other ('front') side of the device.
- Quantum Efficiency Hysteresis (QEH): The WF/PC-1 CCD's required a UV flood procedure and continuous cold temperatures to avoid QEH and hence non-linearity. A UV flood was performed early in the WF/PC-1 mission, but could not be repeated due to problems with the HST magnetometers. This in turn limited the temperature range allowable during decontaminations, since high temperatures would remove the UV flood, which in turn severely limited UV science capabilities. Some QE instability was also seen, particularly in the B band, due to changes in the UV flood. WFPC2 CCDs support multi-pinned phase (MPP) operation which eliminates quantum efficiency hysteresis.
- Charge Transfer Efficiency: WF/PC-1 devices suffered from significant charge transfer efficiency (CTE) errors at image intensities below ~200 electrons per pixel. This was overcome by preflashing virtually all science images. WFPC2 devices have much less CTE error, and hence no preflash is used. However, low-level charge traps are present in the WFPC2 devices, and are increasing slowly with time. See the discussions in Photometric Anomalies: CTE and Long vs. Short for details of WFPC2 CTE behavior.
- Detector MTF: The WFPC2 Loral devices do suffer from poorer CCD detector MTF than the WF/PC-1 CCDs, perhaps caused by scattering in the front side electrode structure. The effect is to blur images and decrease the limiting magnitude by about 0.5 magnitudes.
- Flat field quality: WF/PC-1 CCDs were chemically thinned devices and therefore varied in thickness across the field-of-view causing large features in the flat fields. WFPC2 CCDs are un-thinned and the intrinsic response is uniform to ~3% across the field.
- DQE: The WFPC2 CCDs have intrinsically lower QE than WF/PC-1 CCDs above 4800Å, which is due to attenuation by front side electrode structures.
- Gain switch: WF/PC-1 had only a single analog-to-digital converter gain setting of 8 e- DN-1 which saturated at about 30,000e-. Two gains are available with WFPC2: a 7 e- DN-1 channel which gives reasonable sampling of the 5e- read noise, and which saturates at about 27,000e-, and a 14 e- DN-1 channel which saturates at about 53,000e- and extends the useful dynamic range.
- Quantization: The systematic analog-to-digital converter errors that were present in the low order bits on WF/PC-1 have been largely eliminated, contributing to a lower effective read noise in WFPC2.
- Calibration Channel: WF/PC-1 contained a solar UV flood channel which was physically in the location of the present WFPC2 calibration channel. This transmitted solar UV light into the camera to provide a UV flood capability.
- Entry Port: The WF/PC-1 camera was sealed by an afocal MgF2 window immediately behind the shutter. The WFPC2 entry port is open.
- Chronographic Capability: WF/PC-1 contained a low reflectance spot on the pyramid (known as the Baum spot) which could be used to occult bright objects. This has been eliminated from WFPC2, since the spherical aberration severely reduces its utility.
- Contamination Control: Since launch, WF/PC-1 suffered from the accumulation of molecular contaminants on the cold (-87°C) CCD windows. This molecular accumulation resulted in the loss of FUV (1150-2000Å) throughput and attenuation at wavelengths as long as 5000Å. Another feature of the contamination was the "measles" - multiple isolated patches of low volatility contamination on the CCD window. Measles were present even after decontamination cycles, when most of the accumulated molecular contaminants were boiled off by warming the CCDs. In addition to preventing UV imaging, these molecular contamination layers scattered light and seriously impacted the calibration of the instrument. WFPC2 has far less contamination than WF/PC-1 owing to pre-launch cleaning and bake-out procedures, careful design of venting paths to protect the optical bench area, and inclusion of Zeolite molecular absorbers in the design. There is now a decrease in throughput of about 30% per month at 1700Å, but monthly decontamination procedures completely remove this material. This throughput drop is also highly predictable and can be calibrated out during photometric analyses.
1.5 Organization of this Handbook
A description of the instrument is contained in Chapter 2. The filter set is described in Chapter 3. CCD performance is discussed in Chapter 4. A description of the Point Spread Function is given in Chapter 5. The details necessary to estimate exposure times are described in Chapter 6. A summary of observation strategies is given in Chapter 7. Data products, standard calibration methods, and calibration plans are summarized in Chapter 8. A complete list of references is given in Chapter 9.
This document summarizes the performance of the WFPC2 as known in June 2004 after nine years of on-orbit calibration. Observers are encouraged to contact the STScI Help Desk, or to consult the WFPC2 WWW pages (see Section 1.10 below).
HST may transition to use of two instead of three gyroscopes at some point in the future. The present Instrument Handbook describes the state of WFPC2 as used with three gyroscopes. For a discussion of how a decrease to two gyroscopes will affect some characteristics of WFPC2 observing, please refer to the separate
Two-Gyro Handbook.1.6 What's New in Version 6.0 for Cycle 11
Major revisions since Version 5.0 may be summarized as follows:
- Anomalies: Added information on the shutter anomaly, the ORIENT bug, and the FR533N anomaly.
- Dark Current Evolution: New information on the latest results throughout this section.
- Dithering: Updated information on dithering from the new Dithering Handbook throughout this section.
- CTE: Updated CTE monitor figure to reflect the latest results. Added section on mitigating CTE effects.
- Photometry: Updated photometry monitor figure to reflect the latest results.
- Calibration: Added sections on the "On The Fly Reprocessing System" and the "Cycle 10 Calibration Plan".
- Updated references and index.
1.7 What's New in Version 7.0 for Cycle 12
Major revisions since Version 6.0 may be summarized as follows:
- Serial Clocks: Added updated information on the CLOCKS=YES mode.
- Filter Wheel Anomaly: Added updated information on the status of this anomalous rotational offset for many WFPC2 filters.
- CTE and "Long vs. Short": Provided the most up-to-date correction formulae for the CTE effect; Presented the latest conclusions on the cause of the apparent "Long vs. Short" anomaly.
- ETC: Described the new options available in the on-line Exposure Time Calculator.
- Flat Fields: Described the latest flat field reference files and their improved accuracy.
- Calibration Accuracy: Updated the overall accuracies achievable with the WFPC2 calibration.
- Updated references and index.
1.8 What's New in Version 8.0 for Cycle 13
Major revisions since Version 7.0 may be summarized as follows:
- The Phase II proposal preparation tool RPS2 has been replaced with the Astronomer's Proposal Tool (APT). APT is a new suite of powerful tools, including the Visual Target Tuner (see Section 7.8.1), Exposure Time Calculators for all HST instruments, and all Phase II proposal tools that were previously handled by the RPS2 software. Details on APT can be found on our web site at:
http://apt.stsci.edu/1.9 What's New in Version 9.0 for Cycle 14
Major revisions since Version 8.0 may be summarized as follows:
- Chapter4.8, Dark Backgrounds, Chapter5.11, Optical Distortion, and Chapter7.6, Dithering with WFPC2 have been updated with new information.
1.10 WFPC2 Handbook on the WWW
This Handbook will appear on the WFPC2 WWW pages accessible at:
http://www.stsci.edu/instruments/wfpc2/wfpc2_top.htmland will be updated as new information becomes available.
1.11 The Help Desk at STScI
STScI maintains a Help Desk whose staff quickly provide answers to any HST-related topic, including questions about WFPC2 and the Cycle 13 and 14 proposal process. The Help Desk staff has access to all of the resources available at the Institute. They maintain a database of frequently asked questions and answers, so that many questions can be answered immediately. The Help Desk staff can also provide copies of STScI documentation, in either hardcopy or electronic form, including Instrument Science Reports and Instrument Handbooks.
Questions sent to the Help Desk are usually answered within two business days. Usually, the Help Desk staff will reply with the answer to a question, but occasionally they will need more time to investigate the answer. In these cases, they will reply with an estimate of the time needed to reply with the full answer.
We ask that you please send all initial inquiries to the Help Desk. If your question requires a WFPC2 Instrument Scientist to answer it, the Help Desk staff will put a WFPC2 Instrument Scientist in contact with you. By sending your request to the Help Desk, you are guaranteed that someone will provide a timely response.
To contact the Help Desk at STScI:
- Send e-mail:
help@stsci.edu- Phone: 1-410-338-1082
The Space Telescope European Coordinating Facility (ST-ECF) also maintains a Help Desk. European users should generally contact the ST-ECF for help; all other users should contact STScI.
To contact the ST-ECF Help Desk in Europe:
- Send e-mail:
stdesk@eso.org.1.12 Further Information
The material contained in this Handbook is derived from ground tests and design information obtained by the IDT and the engineering team at JPL, and from on-orbit measurements. Other sources of information are listed below. For a complete reference list please see References.
http://www.stsci.edu/public/p2pi.html3 andhttp://www.stsci.edu/hst/programs/http://www.stsci.edu/instruments/wfpc2/Wfpc2_dhb/WFPC2_ longdhbcover.html3.http://www.stsci.edu/instruments/wfpc2/Wfpc2_serv/post_ serv.html3http://www.stsci.edu/stsci/meetings/cal97/proceedings.h tml3.http://www.stsci.edu/hst/HST_overview/documents/calwork shop/workshop2002/3.http://www.stsci.edu/hst/acs/performance/cte_workgroup/ cte_papers.html
- STSDAS Users Guide, (April 1994, version 1.3).3
- The Reduction of WF/PC Camera Images, Lauer, T., P.A.S.P. 101, 445 (1989).
- The Imaging Performance of the Hubble Space Telescope, Burrows, C. J., et. al., Ap. J. Lett., 369, L21 (1991).
- Interface Control Document (ICD) 19, "PODPS to STSDAS"
- Interface Control Document (ICD) 47, "PODPS to CDBS"
- The Wide Field/Planetary Camera in The Space Telescope Observatory, J. Westphal and the WF/PC-1 IDT, IAU 18th General Assembly, Patras, NASA CP-2244 (1982).
- The WFPC2 Science Calibration Report, Pre-launch Version 1.2, J. Trauger, editor, (1993). [IDT calibration report]
- White Paper for WFPC2 Far-Ultraviolet Science, J. T. Clarke and the WFPC2 IDT (1992)3.
- The Performance and Calibration of WFPC2 on the Hubble Space Telescope, Holtzman, J., et al., P.A.S.P., 107, 156 (1995).
- The Photometric Performance and Calibration of WFPC2, Holtzman, J., et al., P.A.S.P., 107, 1065 (1995).
- Charge-Transfer Efficiency of WFPC2, B. Whitmore, I. Heyer, S. Casertano, PASP, 111, 1559 (1999).
- The Institute's WFPC2 World Wide Web page at address:
http://www.stsci.edu/instruments/wfpc2/wfpc2_top.html
- The Institute's WFPC2 Space Telescope Analysis Newsletter (STAN), which is distributed monthly via e-mail, and provides notification of any changes in the instrument or its calibration. To subscribe, send e-mail to
help@stsci.edu. Previous issues are at:1The FOC also had UV imaging capability, but it has been physically replaced by ACS.http://www.stsci.edu/instruments/wfpc2/wfpc2_stan.html
2The members of the IDT are: John T. Trauger, Christopher J. Burrows, John Clarke, David Crisp, John Gallagher, Richard E. Griffiths, J. Jeff Hester, John Hoessel, Jon Holtzman, Jeremy Mould, and James A. Westphal.
3These documents may be requested by e-mail fromhelp@stsci.edu.
|
|||||
|
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
