Enabling New Science with WFC3
Jason Kalirai, jkalirai@stsci.edu, John MacKenty, mackenty@stsci.edu, and the WFC3
Team
The new Wide Field Camera 3 (WFC3) on Hubble is in full scientific operation! Still in
its first year, the instrument is quickly emerging as one of astronomy’s most important
tools, and is likely to answer a wide range of scientific questions about the cosmos. In the
first weeks of science operations, WFC3 captured images of an impact on Jupiter and
detected the highest redshift galaxies ever observed. Over the coming year, about half of
all Hubble observations will use the new instrument. In our local cosmic neighborhood,
WFC3 will perform a wide variety of studies, including characterization of planets,
brown dwarfs, and hydrogen-burning stars. Beyond, it will improve our understanding
the formation of stars and galaxies, map cosmological relationships, and perhaps reveal
the fate of the universe.
WFC3 in Orbit: First Performance Results
The cameras on Hubble have profoundly influenced the course of modern astrophysics.
Until now, the Wide Field Planetary Camera 2 (WFPC2), the Near Infra-Red Camera and
Multi-Object Spectrometer (NICMOS), and the Advanced Camera for Surveys (ACS)
provided the deepest and most sensitive images ever of stellar populations in the
universe. Observing programs like the deep imaging surveys of star clusters and the
Hubble Deep Fields have clarified the basic processes that operate in the formation and
evolution of stars and galaxies. WFC3 is the next leap forward. Its ultraviolet-visible
(UVIS) and the infrared (IR) cameras both offer fields several arcmin wide. Both have
ultra-sensitive detectors: CCD in UVIS and mercury-cadmium-telluride in IR. Both
operate at high resolution: 0.04 arcsec in UVIS and 0.13 arcsec in IR. Together, the two
cameras contain 77 narrow-, medium-, and wide-band filters—and 3 grisms. WFC3’s
combined capabilities allow panchromatic imaging and spectroscopy over a 0.2–1.7
micron wavelength range. It offers substantially higher imaging sensitivity and resolution
than previous instruments in the UV and IR.
Following its installation in Hubble in May 2009 by the crew of STS-125, the WFC3
team conducted a comprehensive checkout and calibration program to measure the onorbit response of the instrument and characterize its performance. Calibration data from
the first 10–12 weeks of operation have been fully analyzed and reported in a series of 30
instrument science reports (ISRs, available at
http://www.stsci.edu/hst/wfc3/documents/ISRs/ ). These calibrations used standard stars
and star fields, as well as internal lamps. This effort established the optical alignment,
pointing corrections, geometric distortion, point-spread function (PSF), and photometric
sensitivity and stability. For the most part, these measurements confirmed our groundbased understanding of the detectors. However, as a pleasant surprise, on-orbit
observations of photometric standard stars indicate that the instrument is 10–20% more
sensitive than expected—across the entire UV–VIS–IR wavelength range.
For specific scientific investigations with the newly refurbished Hubble, users often ask,
“Which instrument should I use?” For many projects, the most important metric is the
limiting magnitude reached in a given exposure time. Figure 1 compares Hubble imaging
instruments in terms of this metric.
Figure 1: The five-sigma, limiting, AB magnitude of a point source reached in 10 hours
of exposure with various Hubble imaging instruments. The calculation assumes an
optimal aperture for extraction. Over almost the entire wavelength range probed by
Hubble instruments, WFC3 offers the highest sensitivity.
At UV and VIS wavelengths, WFC3 offers both advantages and disadvantages compared
to ACS. For example, WFC3 offers 62 UVIS filters, so some studies may be possible
only on WFC3. Nevertheless, the WFC3 field of view is 40% smaller than that of the
ACS wide-field camera (WFC). For this reason, survey-type programs, which must cover
large regions of the sky, may prefer ACS. In terms of VIS sensitivity, although the
throughput of the WFC3/UVIS camera is somewhat lower than ACS/WFC, the new
instrument offers (1) better sampling of the PSF (20% smaller pixels and on-axis
alignment within the telescope); (2) 50% lower read noise; (3) much lower dark current;
and (4) negligible charge transfer efficiency (in the near term). Taking all of these factors
into account, the point-source limiting magnitude reached by WFC3 and ACS in the V
band is similar, and ACS slightly outperforms WFC3 in the I band. This comparison
illustrates Hubble’s unprecedented capability for simultaneous ultra-sensitive, highresolution, wide-field imaging of astrophysical sources using both instruments.
New WFC3 observations of the star clusters 47 Tuc and Omega Cen have provided
geometric distortion solutions of sufficient quality to support the Institute’s
MULTIDRIZZLE image-combination software. Starting in late January 2010, this software
has been at work in the OPUS pipeline. Re-processed data will include these calibrations.
Users should be aware that both these calibrations and the software—which by their
nature could not be fully exercised prior to flight—are still undergoing improvements and
bug fixes. Please consult the Institute’s WFC3 (http://www.stsci.edu/hst/wfc3/) and
MULTIDRIZZLE (http://www.stsci.edu/hst/wfc3/tools/MultiDrizzle/) websites for
additional information.
For the CCD detector, in-flight bias files are available, and since the ground calibrations,
the flat fields appear stable. The IR darks and flats show some variations and improved
in-flight darks are now available, with flats coming in summer 2010. Telescope
backgrounds and detector radiation effects are close to our pre-flight expectations, as
documented in the Cycle 18 version of the WFC3 Instrument Handbook.
The WFC3 team has supported the astronomical community and Hubble users by making
available the latest calibration files, such as biases, flat fields, and distortion solutions.
They can be found on our web site. After some additional validation, they will be
implemented in the automated reduction pipeline. The team made appropriate
modifications to the throughput tables that characterize the sensitivity. Also, they updated
the image headers and the exposure time calculator, and published the new photometric
zero points—both in ISRs and on the instrument web page (WFC3 ISR 2009-30; WFC3
ISR 2009-31; http://www.stsci.edu/hst/wfc3/phot zp lbn/).
The WFC3 team is contacting Cycle 17 general observers who have acquired data from
the instrument. We are particularly interested in feedback to identify new issues and to
define future calibration and characterization activities to maximize the instrument’s
scientific return. We are in the process of executing a more detailed calibration for Cycle
17.
A Bright Future for WFC3
WFC3 observations comprise half of all Hubble data being obtained in Cycle 17. The
first on-orbit measurements of the performance of the instrument have exceeded
expectations, and the community has responded favorably. All three of the successful
Multi-Cycle Treasury Programs use WFC3 as the primary instrument for over 2,200
Hubble orbits in the next few years. The WFC3 demand among Cycle 18 proposers was
more than a factor of two higher than any other Hubble instrument. These new scientific
investigations will provide unprecedented constraints on our understanding of a wide
range of astrophysical problems, from detailed studies of the initial mass function and its
dependencies on age, metallicity, and environment, to global galaxy structure and
evolution investigations, to the study of dark matter, dark energy, and cosmology with
type Ia supernovae. Our confidence in WFC3’s ability to accomplish the scientific goals
of such research projects is tied intimately to our understanding of its performance and
the quality of its calibration, which we expect will improve over the coming year.
Figure 2. Snapshots of four recent WFC3 observations spanning ultraviolet, optical, and
infrared wavelengths at high resolution and over large fields of view. Top-left: The
“butterfly” nebula illustrates the process of stellar death in an intermediate-mass star.
Top-right: Stephan’s Quintet showcases interacting galaxies in the distant Universe.
Bottom-left: Omega Cen is a dense collection of millions of stars that can be individually
resolved to study stellar evolution and star cluster formation. Bottom-right: A stellar jet
in the nearby Carina Nebula.