4 Comparison of the ALMA and Herschel - ESO

5 Common Calibration of ALMA and
Herschel data
By common calibration we mean measurements of the same sources using ALMA and Herschel.
These sources must have known continuum spectral indices since the bandwidths of
PACS and SPIRE are much larger than those of ALMA receivers. In many cases one must
rely on accurate models of sources. In addition, one may have to account for luminosity
variations and the presence of narrowband features in the spectra of calibration sources. The
Herschel spacecraft will not be permitted to point close to the Sun. The PACS and SPIRE
detectors will saturate when measuring intense sources and these will be confusion limited
when measuring faint sources. Thus, for the faint flux density ranges, calibration sources
might have to be asteroids (e.g., Ceres, Pallas, Vesta, Hygiea), outer planets (Pluto/Charon
system) and/or moons of the giant planets. If higher flux densities are needed, Mars and the
giant planets must be used in both lines and continuum. The brightness temperature in all
the observed bands must be established from model fitting and observations of frequency
ranges free from atmospheric lines (water and CO for Mars) and broad spectral features (for
giant planets). The surface emissivity and expected solid-state features must be accurately
known.
For Mars, in addition to the rocky surface, one must model the influence of dust storms, the
ice caps and the seasonal variation of the brightness temperature. For giant planets, brightness,
temperature and density of the atmosphere, and the abundance profiles and opacities
in lines and continuum must be carefully analysed. The calibrations must be based on a
comprehensive comparison between models and observations, clearly defining the model
assumptions. This will require careful planning of observations, both from ground-based
and from space facilities (eg, the ASTRO-F and Spitzer satellites) before Herschel and
ALMA measurements can begin.
For spectral line calibrations, those transitions common to both instruments are listed in
Table 7. The Herschel beamsize is larger, so for extended sources ALMA must be used to
map the Herschel beam.
More about the proposed strategies for calibrating the Herschel instruments may be found
in http://www.rssd.esa.int/Herschel/hcal_wkshop.shtml
5.1 Data Taking Procedures
These procedures are rather different for Herschel and ALMA. Herschel broadband data will
be taken, employing chopping to balance the emission from the passively cooled tele scope.
In contrast, ALMA data taking involves the correlation of outputs of different antennas, supplemented
by measurements of the source total flux densities with four 12 m single dishes. To
take broadband data with these single dishes, ALMA must use wobbler switching to balance
the emission from the Earth’s atmosphere. Thus the responses of both instruments are biased
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