STScI Annual Report 2002: A Living Mission

44 achievements
At the workshop, the ACS group described a geometric distortion model. They reported that the
anneal rate of hot pixels on the ACS Wide Field Camera (WFC) is lower than other Hubble CCDs (chargecoupled
devices). The flat-fields provide ~ 1% photometric accuracy over the entire field of view. On-orbit
measurements indicate higher than expected sensitivity for WFC, up to 20% in the blue.
The NICMOS group reported that the cryocooler is providing a very stable temperature, which
facilitated re-calibration. The new operating temperature has increased the instrument’s sensitivity.
Others at the workshop presented new calibrations and insights about existing or retired instruments—STIS,
WFPC2, the Faint Object Spectrograph (FOS), and the Fine Guidance Sensors (FGS).
The Institute previewed calibration plans for the Cosmic Origins Spectrograph (COS) and Wide Field
Camera 3 (WFC3), which will be installed on the next servicing mission.
During 2002, we improved the accuracy of the pipeline flux calibration of STIS MAMA (Multi-Anode
Microchannel Array) spectroscopic modes. From the time of its installation on Hubble, the throughputs
of these modes have varied by wavelength-dependent amounts ranging between 1% and 3% per year.
Now, we have calibrated this time variation, and the STIS pipeline calibrates the fluxes for first order
spectra with an accuracy ~ 1%. Also, there had been a problem with the blaze function shifting on an
echelle spectrum when it was offset on the detector. This produced an incorrect flux calibration, which
was especially severe at the ends of an order. We introduced an algorithm, developed by the STIS instrument
team, that dramatically improves the flux calibration for all primary STIS echelle settings.
We improved STIS background corrections. The transition to the backup side-2 electronics in mid
2001 had left STIS without a working temperature-control circuit for the CCD detector. The resulting
temperature fluctuations cause significant variations in the detector’s dark current and hot-pixel intensities.
Fortunately, we could show that the read out of another temperature sensor on the CCD housing is
well correlated with the CCD dark current, and we now use these readings in the pipeline to scale the
reference dark file to the appropriate temperature to correct each data set.
The phosphorescence of impurities in the MgF 2 faceplate dominates the dark current of the nearultraviolet
MAMA detector. This dark current varies in a complex way depending on the thermal history of
the detector. In mid 2002, we introduced an algorithm in the pipeline that delivers a new dark reference
for each data set based on the tracked record of detector temperatures. This calibration is correct to
within 5% to 10% about 95% of the time, which largely eliminates the bright edges previously visible in
many dark-limited near-ultraviolet MAMA images.
For the first time in four years, the Institute has revised the data handbooks (DHBs), which are the
primary user’s guides to understanding and analyzing Hubble data. Now, each instrument has its own
DHB, describing data acquisition, pipeline processing, archiving, file formats, and software for analyzing
data. Each DHB provides an overview of the instrument, the structure of its data files, calibration, and
data analysis details. The Institute website offers the DHBs in PDF and HTML formats. This new publication
scheme makes it easier to incorporate new information and get it to users quickly.
Stimulate the use of Hubble data
The Institute provides the astronomy community with easy access to the entire suite of Hubble data. After
an observation, we process, calibrate, and archive new observational data promptly. Also, we make all
non-proprietary science data available to the public.
The Treasury and Archive Legacy programs, begun in Cycle 11, will result in new forms of data
products for distribution to the community via the Archive. The Institute has defined standards and procedures
for the winning Treasury and Legacy teams to provide these processed materials to the archive.
The ACS Early Release observations were extremely popular in the community, as first demonstrations
of the capabilities of the new camera. The GOODS survey images, which have no proprietary period,
have also proven to be very popular. To support the interest in these data sets, the archive set up an ftp
site for easy access to these data products.
We upgraded services to archive users through our ongoing collaborations with the Canadian
Astronomy Data Centre (CADC) and Space Telescope European Coordination Facility (ST-ECF). In
November, this partnership announced the availability of over 16,000 WFPC2 ‘associations’, which are
co-added, cosmic-ray rejected images drawn from observations of the same target for different programs.
This provides a quick method for astronomers to get the deepest images possible for a target,
without having to extract and process all the sub-images.