ISO Calibration


The Absolute Flux Calibration of SWS

Calibration of point sources with SWS (2.4 - 45 microns)

Definition of standards

The absolute flux calibration of SWS relies primarily on a few stellar sources (see Table 1) selected from the ISO Ground-Based Preparatory Programme (GBPP) and from the Cohen, Walker, Witteborn et al. (CWW) absolute calibration programme. The accuracy of these models is estimated to be better than 3-5% in the SWS range. The calibration of every source is tied to Vega as the system zero-point.

Table 2 summarizes the full list of astronomical calibration standards used for the absolute flux calibration of SWS and the determination of the Relative Spectral Response Function for which model SEDs are also available. Note that we do not include Solar System objects in this Table. The asteroids Pallas and Ceres were initially chosen as calibrators for Bands 3 and 4 (lambda > 12 microns) where high flux densities are predicted from standard thermophysical models (Mueller and Lagerros 1998). However, these sources are not ideal for SWS calibration due to errors caused by ISO tracking problems of fast moving Solar System objects. This problem is more serious for calibration observations needed to derive the in-orbit RSRF. The complete list with all the calibration observations performed with SWS during the mission can be found in the document "SWS Photometric Calibrations at End of Mission. I." by P. Morris (January 15, 1999).

NML Cyg replaced Pallas and Ceres in Bands 3 and 4 because of its brightness (exceeding 1000 Jy) and good visibility. This is an enigmatic object, however, with suspected variability, an HII region of unknown extent in the SWS apertures, and a high mass loss rate. The tertiary nature of the available SED (low resolution and photometric uncertainty of 20-30%) warrants caution for using NML Cyg for responsivity measurements. In band, we did use NML Cyg to check for broad-band discrepancies between the laboratory and in-orbit Relative Spectral Response Function (RSRF).

This RSRF was measured for each detector during ground-based testing using blackbody sources with a range of temperatures (Teff = 30-300 K) filling the SWS aperture. Accuracy of the Instrument Level Tests (ILT) responses was expected to be better than 30%, determined largely by the +-1 K uncertainty of the blackbody temperature.

Since launch, the RSRF was measured from special calibration observations of standard stars. The special mode of observations were more efficient than the standard grating scan mode and ensured additional wavelength overlap in each AOT-band at maximum grating resolution.

Comparison with the reference SEDs listed above allowed to inspect for mismatches with special care in the spectral features which can be propagated into the RSRF. The derivation of the RSRF calibration files is described in detail in the document The ISO SWS Relative Spectral Response Calibration" by B. Vandenbussche. Uncertainty in the broad-band shapes is probably not worse than ~10%.

In each of the 12 independent SWS grating and 5 independent SWS Fabry-Perot (F-P) AOT bands (defined by detector block, aperture and spectral order) a wavelength and a bandpass have been chosen optimized to where the RSRF is at its maximum, and where the spectra of the calibration standards are featureless. These so-called "key wavelengths" (see Table 3) were used for determining the scaling constants between signal (microvolts/s) to flux density (Jy). This was performed through grating SWS06 calibration observations of the astronomical standards centered at these "key wavelengths" over the corresponding passbands.

For the F-P mode, sources considered to be continuum in their emission at the 5A-6' key wavelengths were observed. They are also listed in the document "SWS Photometric Calibrations at End of Mission. I." by P. Morris (January 15, 1999).


Relevant documents

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(Last update: 17-May-2004)