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The calibration of ISO data. I (General)
Definition of ISO calibration standards
Stars
The absolute flux calibration of three of the four ISO instruments (CAM,
SWS, and PHT in the 2.5 to 45 micron range) is
based primarily on results obtained on stars selected from the
ISO Ground-Based Preparatory Programme (GBPP)
and from the Cohen, Walker,
Witteborn et al. (CWW) absolute calibration programme.
The confidence in the absolute flux level of the calibration stars is about
3% in the 2.5 to 35 microns wavelength range and better than 10% at longer
wavelengths up to 300 microns.
Asteroids
Asteroids are used to cover the 35 to 200 microns wavelength range in ISO
calibration since stars are usually too faint to be used in this wavelength
range.
An extensive ground and airborne based calibration programme was carried out
to quantify the fluxes and to monitor the light curves of the prime asteroids
1 Ceres, 2 Pallas, 4 Vesta, and 10 Hygiea, and other asteroids that have
been observed with ISO. For the current calibration the absolute fluxes of
the asteroids were derived using a thermophysical model
(Mueller and Lagerros
1998). The accuracy of the predicted asteroid fluxes is in general 10% in
the wavelength range from 24 to 500 microns.
Planets
The outer planets Neptune and Uranus were used for calibration at the
longer wavelengths in the high flux density range up to 1000
Jy. The model SED's for a given time in the mission were provided by M. Abbas.
Currently, the absolute flux calibration of LWS is
based on an updated Uranus model as primary source provided by
Griffin and Orton (1993).
Comparison with independent model predictions for Uranus showed a consistency
of better than 20% in the wavelength range between 30 and 240 microns.
At shorter wavelengths the models become less certain.
Photometric and spectral data
Results from the ISO Ground-Based Preparatory
Programme (GBPP). They consist of near and mid-IR
photometry of a wide sample of stars observed from
La Palma (P. Hammersley) and ESO-La Silla (N. van der Bliek,
J. Manfroid and P. Bouchet) observatories.
Results from M. Cohen et al., including calibrated
spectra and extrapolations to 300 microns for a few stars.
Models
Models of Uranus and Neptune
Model spectra of stars in the
Ground Based Preparatory Program. The visible magnitudes and near-infrared
photometry
observed in the GBPP have been fitted with a Kurucz model to provide the flux densities at longer wavelengths. The overall spectral
energy distribution is believed to be correct within 5% .
Model spectra assembled from
ground-based and KAO observations from M Cohen et al.
Definition of the system zero-points
The model spectra of Vega
(1-300 microns, R = 4000 throughout, 400 kB) provides the definition of the
system zero points
(by adoption of the ideal [i.e. model] Vega as zero magnitude in every
passband) so that long wave measurements can be quoted in magnitudes or at
least with reference to a common context as measurements at other shorter
wavelengths.
The model spectral energy distribution of
Sirius (1-300 mic, R = 1000 up
to 35 microns) is also available. This star was also considered a primary
reference star for calibration purposes.
The calibration of ISO data. II (per instrument)
Other documents
Cross-calibration
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Pedro Garcia-Lario
(Last update: 19-07-2000)