Overview
Programmatic
Information
Endorsements of the WFC3 IR
Channel
Overview
In June 1997,
NASA made the decision to extend the HST mission from
2005 until 2010. As a result, the age and condition of
the instruments on board the HST became a consideration
that had to be revisited.
After careful
study, NASA decided also to ensure the imaging capabilities
of the HST by replacing the Wide Field Planetary Camera
2 (WFPC2), with a low-cost, "facility" instrument, the
Wide Field Camera 3 (WFC3).
As a facility
instrument, Wide Field Camera 3 represents a new approach
to the development of HST scientific instruments. The
teams of Government, Academia, and Industry who worked
to build the Wide Field Planetary Camera 1, WF/PC (1),
WFPC2, and several other HST instruments are working together
on WFC3, leveraging their experience to provide a superior
instrument at modest cost.
The WFC3 project
is also designed to take advantage of much of the hardware
and software of the previous instruments. WF/PC (1) was
returned following the First Servicing Mission (FSM),
and many of its assemblies are available for reuse. Flight
designs, and in some cases, spare flight hardware, are
available and can be used in WFC3 for minimal cost.
Another feature
of a facility class instrument is that the WFC3 developers
do not get guaranteed observation time on HST. The work
is done as a service to the astronomical community. While
this concept is new to HST and the Space Sciences community,
it has been successfully used by both the Planetary and
Earth Sciences communities in their instrument development
projects.
In response
to overwhelming support from scientific community, NASA
tentatively approved the addition of an infrared (IR)
channel to the WFC3 instrument in April 1999. The IR detectors
on board WFC3 will be a more advanced version of the ones
used on the HST Near-Infrared Camera and Multi-Object
Spectrometer (NICMOS) instrument. An innovative design
will allow the instrument to use simple, low-cost thermoelectric
cooling systems instead of the cryogens or mechanical
cryocoolers that are typical in other IR instruments.
Programmatic
Information
1997
The 2002 Instrument
Selection Review Panel met in June 1997 and selected the
Cosmic Origins Spectrograph (COS) as the instrument to
develop for Servicing Mission 4 in 2003. Servicing Mission
5 (2005) would be the final, "deorbit" mission. Shortly
after this meeting, NASA made the programmatic decision
to continue operation of HST at least until 2010. The
last Servicing Mission for upgrading HST was then changed
from late 2002 to early 2003, with the deorbit mission
deferred until after 2010. The current plan is to cancel
the 2005 Servicing Mission.
As a result
of the decision to extend the HST mission, the Project
initiated a number of studies to address the issues of
resource availability. Specifically, to understand how
to maximize HST's scientific productivity and competitiveness
until 2010, using available resources. These issues range
from the optimal approach to building state-of the art
spacecraft hardware, to the scientific strategy for HST
in its second decade of operation. The WFC3 project is
the result of one such study, chartered by NASA Headquarters,
which addresses the imaging capability of the HST through
2010.
The study
charter recognized that in the period 2002 to 2010, an
adequate imaging capability on HST may not be assured.
In 2010, HST's primary imager, the Advanced Camera for
Surveys (ACS), will be 11 years old. The current imager,
the Wide Field and Planetary Camera 2 (WFPC2) will be
17 years old. The Space Telescope Imaging Spectrograph
(STIS), which has some limited imaging capability, will
be 13 years old. The study addressed the feasibility and
cost of a backup wide-field camera.
1998
In January
1998, the results of the study were presented to the same
2002 Instrument Selection Review Panel that selected the
COS instrument. The detailed results of the study were
presented as two options. Option I is to provide a backup
for the wide-field imaging capability only. Option II
retains all the capabilities of Option I and adds the
CODEX optimized coronagraph. The study report consists
of an introductory narrative and the accompanying graphics.
The panel
recommended proceeding with the basic imager (Option I).
After a favorable review by the Origins Subcommittee in
March 1998, the HST Project was directed to begin WFC3
development.
WFC3
Near-IR Imaging Capability: Interim Report
In addition,
in October of 1998 the Space Telescope Institute Committee
(STIC) formally resolved that "the STIC is convinced that
significant advantages would accrue from the inclusion
of a near-IR (J & H) channel in WFC3," and encouraged
"STScI and NASA ......to identify funding sources, perhaps
even within the Hubble Project and the HST User community.
- In November,
1998 the Cycle 8 Telescope Allocation Committee (TAC)
asked to be briefed on the WFC3/NIR concept. The TAC
made the following formal recommendation:
- "The
TAC believes that the potential scientific return
from adding this channel is enormous," and "....is
unanimous in its view that addition of this capability
will greatly enhance the scientific output of HST
during the period 2003-2010."
- "We
strongly encourage NASA to (1) proceed with design
and costing of the IR channel over the next 6-8
months; and (2) to select a design that achieves
an appropriate balance between UV performance and
IR capability,"
- "
...urges STScI and NASA to explore a wide range
of additional creative approaches to funding WFC3/IR,
including the possibility of reducing the funds
available to GO's by 5% over the next 3 years, if
no other options are viable."
1999
In March 1999,
the NASA Origins Subcommittee Report strongly endorsed
the scientific case for a near-IR channel for WFC3, based
on a preliminary concept from the project. The Subcommittee
Report stated that the "...inclusion of an IR channel...would
allow HST to address a significantly expanded range of
important problems, including the history of star formation
both within the Galaxy and extending to galaxies at high
redshift."
The WFC3 uses
a more advanced version of the detectors used on the HST
Near-Infrared Camera and Multi-Object Spectrometer (NICMOS)
instrument. These WFC3 detectors provide a factor of 16
increase in the number of pixels, and over a factor of
two increase in quantum efficiency.
The progression
of Rockwell IR detector arrays is shown in the image below.
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