SPIRE Instrument and Calibration page for Archive Users

 

         

 

Welcome to the Spectral and Photometric Imaging Receiver (SPIRE) information page. In this page you will find information and references to documents detailing the SPIRE instrument operations (observing modes, calibration), data processing and the data products offered in the Herschel Science Archive (HSA).

SPIRE consists of two sub-instruments: an imaging photometer with three broad bands centred on 250, 350 and 500 µm and a two-band imaging Fourier-Transform spectrometer (FTS) covering 194-318 µm and 294-671 µm. When relevant, the information in the different sections is separated per sub-instrument.

SPIRE Documentation

Extensive set of documentation is compiled in the SPIRE section of the Herschel Explanatory Legacy Library. In order to get acquainted with the SPIRE products, and how to best exploit them, the following top-level documents should be read first:

  • SPIRE Quick-Start Guide: provides concise information about SPIRE, the observing modes, the calibration schemes, the data products available in the HSA and some other useful topics. This guide should be used as an entry document for archive users.
  • SPIRE Handbook: this is the main document on SPIRE, covering much broader concepts and providing much more details than the Quick-Start Guide.
  • SPIRE Products Explained: Description of the SPIRE observational context and the available products at different levels.
  • SPIRE Data Reduction Guide: detailed description of the SPIRE data, the standard pipelines, interactive analysis tools and recipes. Read in case you want to analyse your SPIRE data within the Herschel Data Processing Environment (HIPE). Many considerations and recipes in this document, although HIPE-oriented, are applicable in general.

SPIRE Data Products

The SPIRE products can be accessed through the HSA or within HIPE. An overview of the available products is given in the SPIRE Product Explained [link to be created], and can be summarised as follows:

  • Products generated by the standard instrument pipelines (SPG): these have been processed essentially by HIPE 14.1 (see also Data Processing section below), and cover both observations performed during the Routine Science phase, and during the early Performance Validation and Routine Calibration campaigns.
  • User-Provided Data Products (UPDPs): these are products delivered to the Herschel Science Centre by Herschel Key Science Progammes teams. All UPDPs stored in the HSA can also be checked and downloaded from this page.

    Caveats: users should be warned that a fraction of the UPDPs were generated with earlier versions of the data processing pipelines and calibration files. These older products have caveats, which may have been fixed in the archival data.

  • Highly-Processed Data Products (HPDPs): these are products delivered by experts from the Instrument Control Centres, the Herschel Science Centre and the NASA/Herschel Science Centre. They offer quality-enhanced (improved calibration) and/or complementary products to those processed with the standard pipelines. All HPDPs stored in the HSA are listed and can be downloaded from this page.
  • Ancillary Data Products (ADP): these are products not necessarily linked to observations. They, however, provide important information concerning the instrument spatial responses (PSFs), sensitivity curves, health monitoring and trend analysis information etc. Some ADPs are more engineer-oriented products from pre-launch and during the mission. All Ancillary Data Products stored in the HSA can also be checked and downloaded from this page.

The vast majority of the SPIRE products can be considered science-ready. The main residual artefacts from the standard calibration pipelines are summarised in Data Products Known Issues, together with a mention on their severity and suggestions on how to deal with them, if the instrument specialists could provide such an advice.

The recommendation is to use HPDPs, if there are HPDPs available as a replacement for the SPGs. These are usually of the highest science quality. Sometimes the HPDPs are better products for a particular science case, e.g. background subtracted spectra for point-like sources. Therefore the decision to use those should be based on the information provided in the HPDP release notes and a careful examination if the products are optimal for the science case.

On the contrary, using UPDPs instead of the SPGs should be considered carefully, because of the caveats mentioned above. Some UPDPs, however, are better products regardless of being processed with an earlier version of the pipeline and the calibration context. A careful read of the UPDP release note or the science paper, provided by the Key Project team, the sub-instrument What's New pages and the updates to the Calibration Tree should be consulted before taking this decision.

SPIRE Data Processing

The final pipeline products served in the HSA have been processed for their vast majority with the standard product generation pipeline version 14.1, using SPIRE Calibration Tree SPIRE_CAL_14_3. The only exceptions are SPIRE Photometer level-3 mosaic maps, which were processed with version 14.2.

The final version of HIPE is 15.0.1, but this version was only optimised for interactive analysis, and it was not used in product generation for the archive. What's New in 15.x series page provides a high level list of the changes with respect to HIPE 14.x series.

In case you wish to perform data reduction or analysis with HIPE, the latest version can be downloaded at the following page:

Reprocessing products with HIPE 15.0.1 should in principle yield the same outcome as those stored in the HSA. Slight differences are expected in cases where two or more Photometer maps are combined into a Level 2.5, unless the reprocessing include all constituent maps (see the SPIRE Data Reduction Guide for more details).

There are some known issues with HIPE, and these are compiled in the following page:

SPIRE Performance and Calibration

SPIRE Photometer

The primary calibrator for the SPIRE Photometer is Neptune and the detailed calibration framework and accuracy is explained in

Calibration uncertainty: 5.5% (4% absolute and 1.5% relative calibration).

  • The absolute calibration is based on Neptune ESA-4 model (Moreno, private communication) and dominates the error budget; it is correlated across the three SPIRE Photometer bands.
  • Additional sources of uncertainty:
    • photometric, based on source measurements errors (astrometry, confusion noise, source blending etc.)
    • extended source calibration, in addition to the overall uncertainty, 1% from the uncertainty on the SPIRE beam (Schulz et al, in preparation) and 10% on the absolute zero-level derived from Planck-HFI cross-calibration.

SPIRE Spectrometer

The primary calibrator for the SPIRE Spectrometer is Uranus and the detailed calibration framework and accuracy is explained in

Calibration uncertainties:

  • Extended-source calibration, overall ~4%:
    • Systematic from the telescope model: 0.06%
    • Statistical repeatability: 1%
    • Frequency dependent additive continuum offset: consult The SPIRE Quick-Start Guide, the continuum offset ancillary product is available here.
    • Far-field feedhorn efficiency correction: 3%
    • Comparison with the SPIRE photometer for fully extended sources: ~5%.
  • Point-source calibration, overall ~6%, based on repeatability of point-like calibration targets:

Notes:

  • Pointing offset: the pointing uncertainty only results in a reduction in flux and is therefore not a true statistical uncertainty on the recovered flux level. A large pointing offset also results in a significant distortion of the SSW spectrum of a point source and a mismatch between the SLW and SSW spectra.
  • Semi-extended sources: when the source extent is larger than the beam size, but not fully extended, or if there is structure inside the beam, then the uncertainties are dominated by the source-beam coupling (Wu et al., 2013) and are significantly greater than 4%.
  • Spectral resolution: The quoted uncertainties are applicable for High Resolution (HR) spectra observed on the central detectors of the two SPIRE Spectrometer arrays: SSW (191-318 µm) and SLW (294-671 µm). For Low Resolution (LR) the quoted uncertainties are applicable for SSW, while for SLW these are larger due to an empirical correction, which is currently uncharacterised (Marchili et al. 2017).
  • Spectral maps: the overall repeatability is measured at 7% (Benielli et al. 2014), the calibration of the off-axis detectors is more uncertain, especially those that are partially vignetted.

SPIRE Calibration Trees

The current SPIRE_CAL_14_3 and some earlier versions of the SPIRE Calibration Trees are available in the Herschel legacy area. The calibration trees are provided as Java Archive files (JAR) to be used in HIPE. The individual calibration files, stored as FITS files in the JAR file, can be extracted if needed with jar xvf file.jar. The names of the FITS files correspond to the Java classes used in HIPE and are not human-readable, the mapping of the names and their description are provided in the SPIRE Calibration Products page.

 

Calibration tree version SPIRE_CAL_14_3 is the last one and can be used with any version of HIPE more recent than 14.1, including series 15.x.

 

The last three SPIRE calibration trees used in the archive are listed in the following table, the hyperlinks provide the release notes for each version:

SPIRE Cal Tree Release Date Applicable HIPE version
SPIRE_CAL_14_3 Mar 2016 HIPE v14.1 and above
SPIRE_CAL_13_1 Apr 2015 HIPE v13.0
SPIRE_CAL_12_3 May 2014 HIPE v12.1