Press Release

Greater accuracy deepens understanding — Hubble’s Faint Object Spectrograph re-calibrated

By SpaceRef Editor
September 11, 2001
Filed under , ,

The dedicated team effort to understand and correct systematic effects in
observations from Hubble’s Faint Object Spectrograph has now been concluded.
In future astronomers who use the observations from this instrument will be
able to measure the exact velocity of interstellar clouds, as well as the
motions of individual parts of nebulae and galaxies. This will for instance
lead to better determinations of black hole masses.

The Faint Object Spectrograph (FOS) has been one of the key instruments
during the early life of the NASA/ESA Hubble Space Telescope. It was designed
to reveal the chemical composition of both faint and remote targets, as well
as to help unravel the mystery of black holes at the centres of galaxies by
measuring the unusually large motions of gas and stars.

The observations collected by the Faint Object Spectrograph during its
six-year lifetime from August 1990 to December 1996 have been carefully
stored in the Hubble Science Archive — one of the finest and most valuable
astronomical archives in existence. For example, many of the spectra taken
by FOS have shown extraordinarily high mass black holes at the centres of
nearby galaxies — a fact of great significance in the understanding of the
formation and evolution of galaxies. The Hubble Science Archive has been one
of the driving forces behind the increasing awareness in the astronomical
community of the need for careful archiving of high-quality astronomical
observations and calibrations.

The ‘blue part’ of the FOS data have now been revived with a new calibration,
adding substantially to their scientific value. A four-person team based at
the Space Telescope-European Coordinating Facility (ST-ECF) in Garching,
Germany, has carried out this ‘re-calibration’ with support from scientists
at the Space Telescope Science Institute and the Goddard Space Flight Center.
ST-ECF’s ‘Instrument Physical Modelling Group’ has expended ten man-years
of effort in understanding the intricate details of the instrument and in
developing a novel physical model of its operation. This has allowed them
to develop routines to correct for unwanted instrumental and environmental
effects in the measurements of stars and galaxies. Group leader Michael
Rosa explains: “We have shown that our concept of looking at the detailed
physics of a scientific instrument first and then developing new methods for
the calibration is a sound one. By using a physical model we have been able
to remove complex effects that distorted valuable observations.” He adds:
“Only a full overview of the complete archive enables one to gain such
detailed insight. That is why it was impossible to develop the physical
model and improved calibration during the operational part of FOS’ life.”

The group has clarified the exact way in which the instrument operated and
has modified the calibration of the data so as to include the effects of
the influence of the Earth’s magnetic field and of temperature changes.
The accuracy with which velocities can now be derived from the new
calibration-enhanced Faint Object Spectrograph data has been increased
five-fold.

Experienced FOS user Tim de Zeeuw explains the importance of the recent
re-calibration for the scientific value of the FOS archive data: “Using
this new repository of enhanced observations will seem like receiving data
from an entirely new instrument. The re-calibrated data can answer questions
about the exact velocity of interstellar clouds, as well as about the
motions of building blocks of nebulae and galaxies.”

During the Second Hubble Servicing Mission the Faint Object Spectrograph
was replaced by the more powerful Space Telescope Imaging Spectrograph
(STIS) and was brought back to Earth. It is now on permanent display at
the Smithsonian National Air & Space Museum in Washington DC, USA.

The final products of the exacting re-calibration effort will be released
on 11 September 2001. The success of the FOS project has confirmed that
this approach has enormous potential for dealing with the much larger data
sets typical of the newer Hubble instruments. The team has already begun
the next task — the re-calibration of data from the Space Telescope
Imaging Spectrograph — an increase of a factor of ten in data set size
and complexity.

Notes for editors:

The Hubble Space Telescope is a project of international co-operation
between ESA and NASA.

This re-calibration work is part of the continuing ESA/NASA collaboration
on Hubble, and guarantees the continued access of European scientists to
this unique research facility.

Members of the Instrument Physical Modelling Group are: Michael Rosa,
Florian Kerber, Anastasia Alexov and Paul Bristow (all ST-ECF/ESA/ESO).

USEFUL LINKS FOR THIS STORY

* News release PDF

http://sci2.esa.int/hubble/docs/heic0112.pdf

* News release textfile

http://sci2.esa.int/hubble/docs/heic0112.txt

* The Instrument Physical Modelling Group Web pages

http://www.stecf.org/poa/

* The FOS Web pages

http://www.stecf.org/poa/FOS/index.html

* Space Telescope-European Coordinating Facility (ST-ECF)

http://www.stecf.org/

* The Hubble Science Archive

http://archive.eso.org/

* Space Telescope Science Institute

http://www.stsci.edu/

IMAGE CAPTION:
[http://sci.esa.int/hubble/gallery/new_searchresult.cfm?ooid=28294&imgid=12373]
This illustration shows the NASA/ESA Hubble Space Telescope in orbit above
the Earth as it looked before the Second Servicing Mission in 1997. The
Faint Object Spectrograph (FOS) can be seen (marked in yellow) in Hubble’s
instrument bay at the back of the observatory. During the Second Servicing
Mission the two first generation instruments, FOS and the Goddard High
Resolution Spectrograph (GHRS), were replaced by the second generation
instruments, NICMOS (Near Infrared Camera and Multi-Object Spectrometer)
and STIS (Space Telescope Imaging Spectrograph).

A dedicated team effort to understand and correct systematic effects in
observations from FOS has now been concluded and the results are released on
11 September 2001. A four-person team based at the Space Telescope-European
Coordinating Facility (ST-ECF) in Garching, Germany, has carried out this
re-calibration with support from scientists at the Space Telescope Science
Institute and the Goddard Space Flight Center. ST-ECF’s ‘Instrument Physical
Modelling Group’ has expended ten man-years of effort in understanding the
intricate details of the instrument and in developing a novel physical model
of its operation. This has allowed them to develop routines to correct for
unwanted instrumental and environmental effects in the measurements of stars
and galaxies.

Credit: ESA & the Space Telescope-European Coordinating Facility (ST-ECF)

Contacts

Lars Lindberg Christensen

Hubble European Space Agency Information Centre, Garching, Germany

Phone: +49-(0)89-3200-6306

Cellular (24 hr): +49-(0)173-38-72-621

E-mail: lars@eso.org

Dr. Michael Rosa

Space Telescope-European Coordinating Facility (ST-ECF), Garching, Germany

Phone: +49-(0)89-3200-6241

E-mail: mrosa@eso.org

SpaceRef staff editor.