Wide Field Camera 3 (WFC3) is a fourth-generation imaging instrument that
replaced the extraordinarily successful WFPC2 and thereby ensures and enhances the imaging capability of HST
in the remaining years of its observing lifetime. WFC3 is the first HST
instrument developed as a facility instrument by the HST
Project. The primary design goal for WFC3 is to provide HST
with a high-sensitivity, high-resolution, wide-field survey capability covering a broad wavelength range, from the near-UV at 200 nm to the near-IR at 1700 nm.
|Infrared channel (IR): 136 ×
123 arcsecond field of view from 800−
1700 nm with a plate scale of 0.13 arcsec/pixel and a focal ratio of f
WFC3 occupies WFPC2’s spot in HST
’s radial scientific-instrument bay, where it obtains on-axis direct images. Light coming from the HST
Optical Telescope Assembly (OTA) is intercepted by the flat 45°
WFC3 pick-off mirror (POM) and directed into the instrument. A channel-select mechanism inside WFC3 then diverts the light to the IR channel via a fold mirror, or allows the light to enter the UVIS channel uninterrupted. Because of this design, only a single channel, either UVIS or IR, can be used at any one time, although it is possible to switch between them fairly quickly. Optical elements in each channel (anamorphic aspherical correctors) correct separately for the ~1/2 wave spherical aberration of the HST
primary mirror. Both channels also have internal flat-field illumination sources. Figure 1.1
shows a schematic diagram of the instrument’s optical and mechanical layout. The main characteristics of each channel are summarized in the following sections. For more detailed information, please refer to the WFC3 Instrument Handbook
, which gives a technical description of the instrument’s properties, performance, operations, and calibration.
Figure 1.1: Schematic optical layout of the WFC3 instrument. Note that for schematic simplicity, the incoming OTA beam and POM have been rotated into the plane of the optical diagram. The actual incoming OTA beam direction is into the page and then reflected by the POM into the instrument. Yellow indicates light from the OTA, which is sent into the camera by the pick-off mirror. The Channel Select Mechanism then either allows light to pass into the UVIS channel (blue path), or directs light into the IR channel (red path). Mechanisms and optics in both channels allow for focus and alignment, and correct for the OTA spherical aberration. Filters and grisms are contained in the UVIS selectable optical filter assembly (SOFA) and the IR filter selection mechanism (FSM). The UVIS channel has a mechanical shutter, while the IR channel is shuttered electronically by the detector. Light is detected by either the UVIS CCDs or the IR focal-plane array. A separate subsystem provides flat-field illumination for both channels.
The UVIS channel
employs a mosaic of two 4096 ×
2051 e2v Ltd. (formerly Marconi Applied Technologies Ltd.) CCDs, with ~0.040 arcsecond pixels, covering a nominal 162 ×
162 arcsecond field of view. These CCDs are thinned and back-illuminated devices cooled by thermo-electric cooler (TEC) stacks and housed in sealed, evacuated dewars with fused silica windows, nearly identical to the ones used for ACS. The spectral response of the UVIS CCDs is optimized for imaging from the near-UV at 200 nm to visible wavelengths at 1000 nm. The two CCDs are butted together but have a ~35-pixel gap between the two chips (~1.4 arcsec on the sky). The minimum exposure time is 0.5 sec for the UVIS detector. The dynamic range for a single exposure is ultimately limited by the depth of the CCD full well (~70,000 e–
), which determines the total amount of charge that can accumulate in any one pixel during an exposure without saturation.
The UVIS detector operates only in ACCUM
mode to produce time-integrated images. Cosmic rays affect all UVIS exposures, therefore observations should be broken into multiple exposures or dither patterns whenever possible, to allow removal of cosmic rays in post-observation data processing.
WFC3 recycles hardware used in WF/PC-1 to house the complement of filters for
the UVIS channel. The Selectable Optical Filter Assembly (SOFA) contains a stack of 12 wheels housing 48 physical elements covering the UV/Visible range: 42 full-frame filters, 5 quad filters (2×
2 mosaics providing 4 different bandpasses), and 1 grism, giving a total of 63 spectral elements. Each wheel has an open slot such that when an observation is taking place, the appropriate wheel is rotated to place the desired filter in the beam, while the other wheels place the open slot in the light path.
shows a schematic of the UVIS channel aperture projected onto the sky with respect to the V2/V3 reference frame. (For definitions of the coordinate systems in the figure, please refer to Section 6.4.3 of the WFC3 Instrument Handbook
.) The WFC3 optics cause the nominally square field of view of the UVIS detector to be projected onto the sky as a skewed rhombus, 162 arcsec on a side, with an angle of 86.1°
between the sides. This distortion affects both the photometric accuracy and astrometric precision of the UVIS images. For a thorough discussion of WFC3 geometric distortion, we refer the reader to Chapter 4
The IR detector employs a 1024 ×
1024 Teledyne (formerly Rockwell Scientific) low-noise, high-QE HgCdTe detector array with ~0.13 arcsecond pixels, covering a nominal 136 ×
123 arcsecond field of view. Only the central 1014 ×
1014 pixels are useful for imaging. The outer 5-pixels are used as reference pixels. The HgCdTe array is actively cooled by a six-stage TEC that keeps the detector at a nominal operating temperature of 145 K. The spectral response of the IR detector is optimized for imaging at near-IR wavelengths from ~800 to 1700 nm.
IR detectors, like the one used in WFC3, show higher dark current and read noise
than CCD detectors. In addition, IR detectors allow accumulated signal to be read out non-destructively multiple times, without affecting other pixels. This capability can be exploited to reduce the effective read-out noise significantly. Non-destructive readouts also allow recovering pixels affected by cosmic rays (CRs), since CR hits may be recognized and removed between adjacent reads.
The WFC3-IR detector is immune to the charge bleeding exhibited by CCDs at
high signal levels; however, saturation can still be a problem because pixels subject to the highest signal levels show higher dark-current rates (“image persistence”) in subsequent exposures. IR detectors do not show long-term on-orbit CTE degradation, because they do not employ the charge-transfer mechanism used in CCDs. However, they are intrinsically non-linear. Nevertheless, at low and intermediate count levels, the departure from linearity is quite modest and can be well calibrated.
The IR channel has a single filter wheel housing 17 spectral elements covering the
near-IR wavelengths: 15 filters and 2 grisms. An 18th slot contains a blank, opaque blocker. For IR observations, the requested element is simply rotated into the light beam. The IR channel operates only in MULTIACCUM
mode, identical to that of NICMOS.
Figure 1.3 shows a schematic of the IR channel aperture projected onto the sky with respect to the V2/V3 reference frame. (For definitions of the coordinate systems in the figure, please refer to Section 6.4.3 of the WFC3 Instrument Handbook.) The IR focal plane is tilted 22° with respect to the incoming beam, thus the field of view as projected onto the sky is rectangular, with an aspect ratio of ~0.90. This distortion affects both the photometric accuracy and astrometric precision of the IR images. For a thorough discussion of WFC3 geometric distortion, we refer the reader to Chapter 4.