3 Data reduction

Initial data reduction to remove instrumental effects such as flat fielding and cosmic-ray removal, and mapping between the two-dimensional detector co-ordinates and the data cube, is highly instrument dependent. The IFU instruments currently in use, with obvious exceptions, tend not to be common user but instead fast-track or in-house instruments. This has had a strong influence on the available data-reduction software.

3.1 Reduction paradigms

There are two paradigms for IFS data reduction. First, the ‘traditional’ method, adapted from multi-object spectroscopy (MOS), where the output from each fibre is extracted by tracing the spectrum and accounting for wavelength-dependent distortion (normally referred to as the MOS paradigm). More recently, with the arrival of TEIFU where the fibre outputs are under-sampled by the detector, an alternative paradigm has arisen (usually referred to as the longslit paradigm). Although the independence of the spatial samples is lost due to the under-sampling of the point-spread function (PSF) by the detector, it can be shown that this is irrelevant so long as the target is critically sampled by the IFU; see Allington-Smith & Content (1998). Here the methods adapted from MOS cannot be used and the resulting dataset bears more resemblance to traditional longslit spectroscopy than to MOS data.

3.2 INTEGRAL data

INTEGRAL is an integral-field spectroscopic facility deployed at the Nasmyth focus of the WHT, and channels the light into WYFFOS fibre spectrograph. Up to six fibre bundles are available, although only three bundles are used in normal operation, with field sizes ranging from 10 to 40 arcseconds and different fibre core sizes. These options allow observers to make the most efficient use of the prevailing seeing conditions. More information can be found at http://www.iac.es/proyect/integral/.

Data-reduction facilities for the instrument are provided by the INTEGRAL IRAFpackage which contains some standard tasks from other IRAF packages (such as ONEDSPEC, SPECRED, and CCDRED) and some custom tasks designed to deal with INTEGRAL data. The package can be downloaded from

http://andromeda.roque.ing.iac.es/~astrosw/InstSoft/integral/integral-0.3.tar.gz along with the package user manual, which contains installation instructions and a description of the available reduction software (see Section 7). It should be noted that to compile the INTEGRAL package requires the Starlink public-domain algorithms (PDA) package to be present on your machine (see SUN/194).

3.3 OASIS data

OASIS is an integral field spectrograph for use with AOB/PUEO although it can also be used at the direct f/8 Cassegrain focus as a backup mode and for science programs necessitating IFS but with a coarser spatial sampling defined by the natural seeing. OASIS can currently be used in two modes. The imagery mode is used primary for accurate pointing on objects. Image quality has been optimized so that high spatial resolution (0.$\text{′′}$1) images can be obtained. The spectroscopic mode offers low-to-medium spectral resolution with a wide range of spatial samplings performed by an array of hexagonal micro-lenses. Depending of the configuration employed, the spectrographic field diameter varies from 1.5 arcsec to 10 arcsec. More information can be found at http://www.cfht.hawaii.edu/Instruments/Spectroscopy/OASIS/.

Data-reduction facilities is provided by the XOASIS software and detailed installation and ‘cookbook style’ usage instructions are available online.

3.4 SAURON data

SAURON is another IFS based on the TIGRE/OASIS concept of a micro-lens array built for the WHT. It has an array of 1500 square lens, and a wide field, either 10 or 35 arcsec${}^2$. Data reduction is via pipeline software (XSAURON and PALANTIR) especially developed for the instrument which was modelled after the XOASIS package. More information about the instrument can be found at http://www.strw.leidenuniv.nl/sauron/.

3.5 GMOS data

The two Gemini Multi-Object Spectrographs (GMOS), one for each Gemini telescope, provides facilities for two-dimensional spectroscopy over a contiguous field of $\sim$50 arcsec${}^2$ with 0.2-arcsec sampling. A small background field will be available at fixed separation ($>1$ arcmin) from the main object field for accurate background subtraction. Details of the IFU itself can be found at http://www.gemini.edu/sciops/instruments/gmos/gmosIFU.html.

Data-reduction facilities for the instrument are provided in the Gemini IRAF package.

3.6 CIRPASS data

CIRPASS is a near-infrared spectrograph with a 499-element, lens and fibre, integral-field unit to collect the light from the target object. CIRPASS is available as a visitor instrument on the Gemini telescopes. Users of CIRPASS are strongly encouraged to collaborate fully with the instrument team in Cambridge to get the most out of their Gemini time.

The data-reduction and analysis software for CIRPASS runs under IRAF. There is a cookbook available at http://www.ast.cam.ac.uk/~optics/cirpass/datared/cookbook_sn1987a.php.

The final data product for the science data is an $x$,$y$,$\lambda$ data cube (see Section 4.3), which can be visualised as a cube where the $z$-axis is wavelength and each plane is a picture of what the IFU observed at that wavelength.

The first version of the data-reduction package will deal separately with the different types of data (e.g. dome flats, sky flats, arc lamps, flux standards and target observations). For each type of data there are one or more pipeline scripts to reduce the data, with each pipeline script running a series of IRAF tasks.

Links to more information on the ongoing development of the reduction software can be found on the web at http://www.ast.cam.ac.uk/~optics/cirpass/docs.html.

3.7 SMIRFS data

SMIRFS was constructed by the Durham group as a technology demonstrator for the more ambitious integral-field units which Durham has producing and continues to develop for the WHT and Gemini (e.g. GMOS). The IFU works with CGS4 on UKIRT to provide IFS for the near infrared (1–2 $\mu$m in the $J$ and $H$ bands). The SMIRFS IFU is available for use in collaboration with the SMIRFS-IFU team, please contact Jeremy Allington-Smith (J.R.Allington-Smith@durham.ac.uk).

More information on the technical specifications of the SMIRFS instrument, and the science that can be done with it, can be found at http://star-www.dur.ac.uk/~jra/ukirt_ifu.html.

3.8 TEIFU data

TEIFU is a system for integral-field spectroscopy using adaptively corrected images produced by the ELECTRA and NAOMI AO systems on the WHT. It is also able to operate in a stand-alone mode without an adaptive-optics system.

Data-reduction facilities will be provided by the IMSPEC IRAF package which is under development at Durham. No detailed information is available at this time.

3.9 UIST data

UIST is a general-purpose imager and spectrometer operating in the 1–5 $\mu$m range at UKIRT, It was commissioned in 2002 October, replacing all spectroscopy functions of CGS4 except for echelle spectroscopy, all imaging functions of IRCAM/TUFTI and all imaging functions of UFTI except the Fabry-Perot filter. It also includes a deployable image slicing IFU mounted in the slit wheel. Slicing mirrors are used to reformat a $6.0\times 3.3$-arcsec region of the sky into fourteen slices (the IFU contains eighteen slicing mirrors but four are currently not usable), each fifty pixels long, offset from one another along their length. This produces a staggered column on slitlets (as shown in Figure 1) which is used as the input for the spectrometer in place of the long slit.

Figure 1: The UIST staggered slitlets.

Data acquisition, reduction and control software is provided by the JAC ORAC system. The data-reduction part of the system, ORAC-DR, is provided by JAC and distributed by Starlink, and consists of a fully automated perl-based pipelining software (Economouet al. 1999) sitting on top of the Starlink software collection. A general introduction to the ORAC-DR system can be found in SUN/230. The data-reduction recipes are documented in SUN/246 and at the UKIRT web site. An arc spectrum (Ar or Kr from the UIST calibration unit) is used to straighten the staggered slitlets (which correspond to a wavelength displacement from one slice to another) and apply a wavelength calibration to the image. The individual slice images will then be copied to form $y$-$\lambda$ planes of an $x$,$y$,$\lambda$ data cube. Recipes are also provided to carry out tasks such as flat-fielding and flux-calibration. Many of the recipes have specific requirements in terms of, for instance, darks and flats fields which must be acquired before a target observation is obtained and reduced on-line.

3.10 VIMOS data

VIMOS has been developed in fast track under ESO contract by the VIRMOS consortium, headed by the Laboratoire d’Astrophysique de Marseille. In IFU mode the field of view is between 13$\times$13 arcsec${}^2$ and 54$\times$54 arcsec${}^2$ at 0.33 or 0.67 arcsec fibre${}^{-1}$.

Data-reduction software (DRS) will be made available by ESO, with the procedures available as standalone packages, or under the VIMOS pipeline-reduction software.

http://www.oamp.fr/virmos/virmos_publications.htm contains links to various publications. Two papers are scheduled for the 2005 November Astronomical Journal.

3.11 Other IFU instruments

Since the list of instruments was compiled for the original version of this cookbook, many new Integral Field Units and area-spectroscopy instruments have come online, such as SINFONI and FLAMES at the VLT, GNIRS-IFU at Gemini-S, FISICA, SNIFS; and several are being designed, for instance KMOS-1, MUSE, and FRIDA.