Chapter 1
Introduction

The Far Ultraviolet Spectroscopic Explorer (FUSE) was a NASA mission designed, built, and operated by the Johns Hopkins University Department of Physics and Astronomy in Baltimore, MD. FUSE operated between 24 June 1999 and 18 October 2007. During that time, it acquired spectra over the wavelength range 905 λ 1187 Å, with a spectral resolving power of 15000-20000. FUSE routinely obtained high quality spectra of point sources with continuum flux levels between a few ×10^-14 and 5 × 10^-10 erg cm ^-2s^-1Å^-1. The upper limit was set by safety considerations for the detectors. The lower limit is a rough estimate for normal observations and, with effort and attention to detail, usable spectra can be extracted for sources up to ten times fainter.

The primary focus of this handbook is to document the FUSE data products. However, in order to understand them and work with them, we have included a brief description of the FUSE instrument and the FUSE science data processing pipeline system (CalFUSE) used to produce them. More complete descriptions of these can be found in the Instrument Handbook (2009), and in Dixon et al. (2007): "CalFUSE Version 3: A Data Reduction Pipeline for the Far Ultraviolet Spectroscopic Explorer".

The contents of the remaining chapters of this handbook are:

Chapter 2	provides a basic description of the FUSE satellite and science instrumentation.
Chapter 3	gives an overview of the data processing pipeline used to produce the data.
Chapter 4	describes the naming and contents of the science data files and the preview files. It also lists some issues that can affect the fidelity of the different types of data products, and indicates which files are best suited for which applications.
Chapter 5	describes the naming and contents of ancillary data from the satellite that can be used to supplement the primary science data products.
Chapter 6	details the contents of the FITS headers of the science and ancillary data files.
Chapter 7	lists the various features and instrumental effects that can compromise the quality of FUSE data as well as methods to check for their occurrence.
Chapter 8	gives an overview of methods to retrieve, read, display and analyze FUSE data. It also provides a list of the various stellar and interstellar line atlases that can help familiarize users with the FUSE spectral range. References to atomic and molecular line lists relevant to analyze the FUSE spectra are also given.
Chapter 9	lists examples of types of observations that required non-standard analysis techniques. Frequently asked questions are also reported in this chapter.

It is expected that this handbook will be used by investigators with three broad levels of interest, and that not all of the information will be needed by everyone. Consequently, the following roadmap is provided:

Casual users who simply wish to assess whether FUSE data are relevant to their goals. Their major need is to quickly examine the data. These users may not be too concerned about the details, accuracy or idiosyncrasies of the data at this stage.

Intermediate users who wish to understand FUSE data well enough to publish scientifically meaningful results based upon them. These users should examine the exposure-level calibrated spectrum files in order to verify the integrity of the data. They should also be fully aware of the different systematics that can affect the data. A prior knowledge of the software developed to analyze FUSE data and the tools available to determine if the target of interest is in a crowded field would be useful.

Advanced users who have scientific objectives that require data products beyond the archived spectra, and must perform additional processing to achieve their goals. Such users may, for example, wish to extract information from spectra of very faint objects, or use FUSE to analyze times series data. These users should be acquainted with most if not all of the material in this handbook. Their major focus will probably be on using the intermediate data files (IDFs). Knowledge of the suite of IDL routines or FUSE Tools in C, and in some cases of the CalFUSE pipeline itself, is strongly recommended.

Table 1.1 gives a list of the chapters and appendices that the users need to become familiar with based on the expertise level they seek to acquire prior to using FUSE data. This table is aimed at helping the user navigate through this document (and the FUSE Instrument handbook 2009 to some extent) according to their scientific needs. It is by no means exclusive.

Table 1.1: FUSE Data Information According to Expertise Level

Expertise Level	Chapter	Appendix
Casual	1, 4, 7, 8	A
Intermediate	1, 2, 3, 4, 7, 8	A, C, F
Advanced	1, 2, 3, 4, 5, 6, 7, 8, 9	A, B, C, D, E, F

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