Spitzer Documentation & Tools
IRS Instrument Handbook

Chapter 1.    Introduction

1.1  Document Purpose and Scope

The IRS Instrument Handbook is one in a series of four documents that explain the operations of the Spitzer Space Telescope and its three instruments.  The Spitzer Space Telescope Handbook gives an overview of the entire Spitzer mission and explains the operations of the observatory, while the other three handbooks document the operation of, and the data produced by, the individual instruments (IRAC, IRS, and MIPS). 

The IRS Instrument Handbook is intended to provide all the information necessary to understand the IRS standard data products, as processed by version S18.18 of the data reduction pipeline, and which are retrievable from the Spitzer Heritage Archive.  In addition to the detailed pipeline processing steps and data product details, background information is provided about the IRS instrument itself, its observational modes, and all aspects of IRS data calibration.  While an introduction to data analysis is provided in this document, please refer to the additional documentation available on the Data Analysis and Tools section of the website.

For readers familiar with the Spitzer cryogenic mission documentation, this Handbook is based on information compiled from the IRS Data Handbook, the IRS Pipeline Description Document, and the Spitzer Observerís Manual.

This is version 4 of this document.  Changes from previous versions are listed in Appendix A.

1.2  Basic Definitions

This section contains a description of the most commonly used terms in this handbook.  A complete list of acronyms can be found in Appendix B.

The InfraRed Spectrograph (IRS) provided spectroscopic capabilities to the Spitzer Space Telescope.  An Astronomical Observing Template (AOT) is the list of parameters for a distinct Spitzer observing mode.  There are 3 possible IRS AOTs:  Staring Mode, Mapping Mode, and Peak-Up Imaging (PUI).  Each of these is described in Chapter 3.  A fundamental unit of Spitzer observing is the Astronomical Observation Request (AOR; also referred to plainly as an Observation), which is an AOT with all of the relevant parameters fully specified.  We also refer to the data that results from such a request as an AOR.  Each AOR is identified in the Spitzer Heritage Archive by a unique observation identification number known as an AORKEY.  An AOR is composed of several Data Collection Events (DCEs), or single-frame exposures.  Basic Calibrated Data (BCD) products (also known as Level 1 products) are data derived from DCEs pushed through the pipelines. See Chapter 5 for a description of the IRS pipelines.  BCDs are designed to be the most reliable product produced by automated processing.  Post-BCD products (also known as PBCD or Level 2 products) are derived from a full AOR, e.g., a combination of several BCDs.  Automated Post-BCD products do not combine data from more than one AOR; users must perform this task themselves.  Post-BCD data is meant to provide a quick look at the data.  Users should always carefully examine Post-BCD products to determine if the standard reduction was optimal for their particular data set. See Chapter 6 for a more complete description of the BCD and Post-BCD data products.

The IRS has four modules which provided low- and medium-resolution spectroscopy: Short-Low (SL), Long-Low (LL), Short-High (SH), and Long-High (LH). Spectroscopy could be obtained in either Staring Mode or Mapping Mode.  In addition, the IRS provided imaging in two filters centered at 16 and 22 micron.  This imaging capability was used for two purposes:  1) to identify point sources and accurately place them in any of the IRS slits (Peak-Up Acquisition or PUA); and 2) to perform photometry on science sources (Peak-Up Imaging or PUI).  A full description of the IRS can be found in Chapter 2, and a description of the operating modes can be found in Chapter 3.

1.3  IRS Essentials

Most readers of this Handbook are interested in learning how to reduce their IRS data.  Users can download IRS data from the Spitzer Heritage Archive, and they can familiarize themselves with the downloaded data files by reading Chapter 6. As users examine and reduce their data, they should keep in mind the common data features and caveats which are described in Chapter 7. An introduction to the analysis of IRS data is provided in Chapter 8.  The Data Analysis and Tools section of the Spitzer website provides information on the software most commonly used to reduce IRS data, while the Data Analysis Cookbook provides examples of how to reduce specific IRS data sets. Below, we highlight some essentials that users should keep in mind as they approach data reduction.

Staring Mode:  The single most important step in reducing IRS staring mode data is subtracting the background.  This is easily accomplished either by starting with bksub products (see Chapter 6) for low resolution data, or by performing your own 2D background subraction with an offset sky for high-resolution data.  In either case, 2D background subtraction should remove most bad pixels and clearly reveal the source. Many users will be able to proceed to spectral extraction using the tools SPICE or SMART after this step.

In some cases, users may wish to remove bad pixels prior to spectral extraction.  This can happen if there are residual bad pixels remaining after 2D background subtraction, or if 2D background subtraction is problematic.  The latter may occur, for example, if dedicated sky observations are unavailable for either high-resolution observations or for low-resolution observations of extended sources.  IRSCLEAN is a convenient tool for removing bad pixels.

Users dealing with very long integrations or maps of faint sources, particularly in Long-Low, should consider removing latents prior to background subtraction or bad pixel removal.  Latents are described in Chapter 7.

In some cases, users will find that their extracted spectra show fringing. Fringes in high-resolution data can be mitigated by the software tool IRSFRINGE. Scalloping or orders (particularly in LH, but also other modules) can be mitigated using the software tool DARKSETTLE.

Mapping Mode:  If you are creating spectral maps, then CUBISM will be your primary software tool.

Peak-up Imaging:  Most users will reduce imaging data using MOPEX

All modes:  In some cases, expert users will want to run their raw IRS data through the IRS pipeline using non-default parameters.  These users may use the software tool CUPID.

1.4  Standard Acknowledgements for IRS Publications

Please see the Guidelines for Publication Acknowledgements for the appropriate acknowledgment to include in papers based on Spitzer data.  Investigators using IRS data should also provide reference(s) to the seminal papers describing the Observatory, the seminal IRS paper (Houck et al. 2004, ApJS 154, 18), and any additional papers describing the IRS, as appropriate.

1.5  How to Contact Us

A broad collection of information about the IRS (http://irsa.ipac.caltech.edu/data/SPITZER/docs/irs/) and IRS Data Analysis (http://irsa.ipac.caltech.edu/data/SPITZER/docs/dataanalysistools) is available on the Spitzer website.  In addition you may contact the helpdesk at http://irsa.ipac.caltech.edu/data/SPITZER/docs/spitzerhelpdesk.