Herschel Calibrator Models

During the Herschel Space Observatory mission, use was made of several different types of celestial flux calibrator. The use of these calibrators for each of the three Herschel instruments and their pipelines is described in the HIFI, PACS and SPIRE handbooks.

Well before the launch of Herschel, several areas of work on improving the accuracy of the flux calibrator models for the far-infrared and sub-millimetre were undertaken. At the time it was realised that Herschel was a pathfinder mission and only limited accurate information existed on flux calibrators. The issues were addressed at dedicated meetings and the decision made that the prime calibrators would be planets (possibly Jovian satellites) and F and G type bright stars - visible to Herschel in the far-infrared. Asteroid models were also constructed which could be verified against prime calibrators but could also be used for such activities as e.g. accurate non-linearity determination for PACS.

These web pages present the information on the calibration sources used at various stages of the mission. The source selection itself did not change, although some stellar sources were removed from the original listing and not used in the ultimate calibrations of the PACS instrument - the reasons are explained below.

In essence, the prime calibration sources for each of the instruments were as follows:

Asteroid models were confirmed to be consistent with stellar and planetary emission models and used in particular for PACS non-linearity measurements.

On this page we present the models used and improved upon during the Herschel mission. ALL the Herschel flux calibration models have been placed in the Herschel Science Archive as Ancillary Data Products - see http://www.cosmos.esa.int/web/herschel/ancillary-data-products for details. The vast majority of improvements were based on feedback from Herschel information (for example, PACS and SPIRE spectroscopic data to constrain the Neptune model, which in turn led to revised photometric fluxes extracted from the updated models), as well as cross-comparison between Herschel instruments and with other missions.

Finally, complemetary information is given concerning workshops dedicated to the definition and development of those primary and secunday calibrators, put in perspective with the calibration strategy of each instrument.

Planetary Models

Mars

This was the main model used for HIFI calibration. It was used to measure and model the beam of HIFI (see Jellema 2015; Ph.D. thesis, U. Groningen, The Netherlands).

Flux calibration is based on the on-line model of Lellouch & Moreno and originally presented for use with Herschel in Feb 2008. Note that distances used in the model are distances from Earth - adjustment would need to be made for Herschel-Mars distances as compared to Earth-Mars distances. The model is available as an on-line tool at.

A description of the model is provided here:

The approximate error on the fluxes for the final model is ~5% absolute (peak-to-peak).

Data used for calibrating the HIFI beams that included predicted Rayleigh-Jeans temperature maps for the Mars observations are provided at the following link, together with a table compiling the details about each observation considered in the data-set:

Uranus

Uranus was the main planet model used for the calibration of the SPIRE spectrometer (FTS), since Neptune was too faint. Several versions of the model were made over the lifetime of the mission as feedback from Herschel observing and cross-comparisons enabled more precise information to be included in the atmospheric model. The ultimate model used by the SPIRE spectrometer team was labelled ESA4 and produced by Glenn Orton (JPL).

  • ESA2 (link to model): Uranus model at launch. No feedback from Herschel observations (bootstrapping) possible before this date (Moreno).
  • ESA3: Unused.
  • ESA4 (link to model): Includes Spitzer data for constraining the continuum, but no lines (Orton; also see Orton et al, 2014, Icarus 243, 471)
  • A model labelled ESA5 was later prepared as a revision of the ESA4 model to account for spillover radiance outside the Spitzer IRS slit. Note however that this model poorly fitted the PACS range and was therefore not used for the PACS calibration (see also Mueller et al. 2016, A&A 588, A109). As such it was not officially endorsed by the Herschel Calibration Group. Please get in touch directly with Glenn Orton (Glenn.S.Orton@jpl.nasa.gov) in case you want to get access to this model.

The approximate error on the fluxes for the final model is ~5% absolute (peak-to-peak).

Later model information was also able to be compared to Planck (a simultaneous mission with Herschel) and WMAP results. Quasi-Contemporaneos Uranus and Mars measurements by HIFI allowed for an accurate comparison to the Mars calibrator used by HIFI which show consistency well within the error bars (~2%).

FITS tables of the models prepared for each PACS and SPIRE photometer and spectrometer observations can be fecthed at the following link:

The following release notes give further details on how to use the models.

Neptune

Neptune was the main model used in the SPIRE photometer. Also checked/used in HIFI calibration (see cross-calibration section below). The initial atmospheric model was gradually updated as Herschel data (predominantly spectroscopic lines) pin down more precise atmospheric conditions over the course of the mission. The final model used by SPIRE was ESA4. ESA5 is a slightly better fit but fluxes, compared to the ESA4 model, have no noticeable differences within the SPIRE spectral range.

  • ESA2 (link to model): This is the launch version of the Neptune atmospheric model from Moreno based on original flyby and ground-based data.
  • ESA3 (link to model): This version is the initial update following early Herschel observations and evaluation of pre-existing information. Early incorporation of PACS spectroscopic information, e.g. HD line and mid-IR measurements which constrained the thermal structure of Neptune and improved model output flux accuracy by ~1-2% (also see Feuchtgruber et al., 2013, A&A, 551, A126).
  • ESA4 (link to model): Second update based on improved models incorporating (especially) CO emission line measurements from SPIRE
  • ESA5 (link to model): Final model with reduced errors due to improved Herschel and Planck information.

The approximate error on the fluxes for the final model is ~5% absolute (peak-to-peak).

FITS tables of the models prepared for each PACS and SPIRE photometer and spectrometer observations can be fecthed at the following link:

The following release notes give further details on how to use the models.

Stellar Models

 

Prior to the launch of Herschel, in collaboration with Leen Decin (KU Leuven), a set of MARCS stellar atmosphere models were obtained for some of the brightest infrared stars in the sky. Several of these had previously been used as calibrators for the Spitzer Space Telescope instruments.

The following is a list of the stars for which stellar model atmospheres were calculated and for which MARCS model outputs were determined. The full description of this is contained within Dehaes et al (2011; A&A, 533, A107).

Details about the models prepared for each star are given in these release notes. These notes also provide tabulated fluxes for the standard stars in the PACS wavebands.

It should be noted that the spectra were normalised to accurate K band fluxes as measured by Selby et al (1988; A&AS, 74, 127). For one case, Beta UMi, it was shown that the original normalisation was incorrect and this was dropped as a prime calibrator during the Herschel mission - although it was possible to find the correction factor during operations.

Asteroid Models

 

Observations of asteriods were taken by PACS and SPIRE as secondary calibrators, and by the three Herschel instruments in the framework of science programmes. These model files have been provided by Thomas Mueller (MPE), in the framework of an EU-funded project (SBNAF: Small Bodies Near and Far). More details can be found in these release notes.

For all observations made of asteroids, specific model fluxes were calculated for each date and time an observation was made of an asteroid, taking into account shapes and rotation rates as well as distance and date of the observation. Calculations were made for the Herschel-centric reference frame and for observation mid-time. These models are provided in separate files for the four main asteroids computed with the so-called model 2 (Ceres, Pallas, Vesta, Lutetia), and for the rest of the concerned objects, prepared with so-called model 1. The files can be fetched at the following link:

On top of that, TPM predictions at 10 reference frequencies between 30 and 1000 GHz (ALMA, IRAM, APEX, LMT, etc.) have been prepared for Ceres, Pallas, Vesta and Lutetia, in the period 2014-2020. The model files can be fetched at the following links:

Herschel Calibration Workshops

 

Before and during the mission, several internal calibration workshops took place for Herschel.

Prior to launch these provided a means to determine

  • The necessary preliminary ground-based observations
  • The potential improvements to the physical knowledge of planetary atmospheres to enable better flux calibration of the models

Following launch, feedback from observations also enabled

  • Calibration model updates based on feedback from Herschel models.
  • A better understanding of errors in each instrument observing mode.
  • Cross comparisons between instruments and cross-calibration.

All these areas were discussed within the workshops. The following provides links to the set of presentations made at each workshop.