WFC3
and New Technology
Instrument Configuration
Instrument Performance
Specialized
Descriptions:
WFC3
and New Technology
WFC3 is a
fourth-generation instrument for HST. It is built on a
low-cost paradigm that reuses existing designs and parts
wherever possible. In addition, as new detector technologies
become available, these are being incorporated into the
instrument design to maximize the instrument's scientific
productivity.
The primary
driver for WFC3 is its ability to provide a backup capability
to ACS's and WFPC2's imaging capability. To keep costs
down, the WF/PC (1) SOFA filter
mechanism will be reused. This will provide up to 48 filter
slots. The filters for WFC3 will consist of the most popular
filters from WF/PC (1 & 2) and ACS. The detectors will
be state-of-the-art wide-field CCDs which, with suitable
coatings, are capable of supporting efficient observations
down to the Near-UV
(NUV) ~ 200 nm
by selecting MgF2 coatings for the optical
elements.
The near-UV/visible
channel (UVIS) uses a large format CCD design that provides
a single 4096 x 4096 pixel format. This is similar to
the configuration used by the HST Advanced Camera for
Surveys, deployed in early 2002. WFC3 is currently scheduled
to be deployed in late 2004. The additional time available
for the WFC3 development allows us to pursue advanced
coatings for the backside-illuminated WFC3 CCDs. These
coatings can provide greater than 50% quantum efficiency
(QE) at 200 nm which highlight WFC3's near-UV capabilities.
WFC3 will
utilize a 4K x 4K pixel CCD assembly, a simple 2-reflection
corrector system, and a filter set incorporating the most
popular filters for the ACS and WFPC2. The UVIS channel
will image a 160 x 160 arcsec field which each pixel covering
40 milliarcseconds when projected on the sky.
The near-IR
channel uses state-of-the-art IR focal plane arrays from
the Rockwell Science Center. These detectors are a more
advanced version of the ones on the HST Near-Infrared
Camera and Multi-Object Spectrometer (NICMOS) instrument,
providing a factor of 16 increase in the number of pixels,
and over a factor of 2 increase in quantum efficiency.
The progression of Rockwell IR arrays is shown in the
image below. Another innovation in the IR detectors, is
the tailoring the long-wavelength cutoff to a shorter
wavelength than is usual for HgCdTe devices. This cutoff
(at 1.9 um) allows the detector to operate at relatively
warm temperatures (~ -120C) with acceptable dark current.
This capability allows the instrument to use simple, low-cost,
thermoelectric cooling systems instead of the cryogens
or mechanical cryocoolers that are typical in IR instruments.
Instrument
Configuration
WFC3 is configured
as a two-channel instrument. The incoming beam from the
HST is directed into the instrument using a pickoff mirror.
It is then corrected for spherical aberration in the HST
primary using a two-element optical system to re-image
and correct the pupil plane. The corrected beam is then
sent to either a near-UV/visible UV-VIS channel or a near-IR
channel.
The table
below summarizes the characteristics of these two channels.
Instrument
Performance
The expected
total instrument throughput as a function of wavelength
is shown in the chart below. The wide wavelength coverage
at high efficiency is made possible by the dual-channel
design using two detector technologies.
The optical
purity of the instrument will support diffraction-limited
imaging over most of its range. This allows the instrument
to exploit another unique capability, that of a well-defined
and uniform point-spread-function over the entire field-of-view.
Specialized
Descriptions:
