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W. B. Landsman
Hughes STX Co., Code 681, NASA/GSFC, Greenbelt, MD 20771
I summarize recent developments in the IDL language of particular interest to astronomers. I then mention some additions to the IDL Astronomy Library since the previous report of Landsman (1993). Finally, I critically examine possible drawbacks to the use of IDL for astronomical data analysis.
IDL (Interactive Data Language)
is a commercial plotting, image processing, programming, and graphical
user interface (GUI) development language. It is a language designed to
allow much more rapid scientific data analysis than is possible using
FORTRAN or C. Features of IDL that promote this ``hands-on'' approach to
data analysis include an interpreted, vectorized compiler, an expressive
syntax, and numerous built-in spectral and image processing functions.
IDL is widely used in astronomy, especially in the analysis of space-based data, and in the fields of solar and planetary astronomy. Recent examples of astronomical software packages based on IDL include Bloch et al. (1993), Brekke (1993), McGlynn et al. (1993), Ewing et al. (1993), and Hall et al. (1994). In this paper I summarize recent additions to the native IDL language and to the IDL Astronomy User's Library, and consider possible drawbacks to the use of IDL for astronomical data analysis.
IDL has undergone substantial evolution since its initial release in 1982. Recent enhancements to IDL of particular interest to astronomers include the following:
IDL is a general software package, used in such fields as remote sensing and medical imaging, in addition to astronomy. As such, it does not contain any procedures specific to astronomy. In 1990, I created the IDL Astronomy User's Library (IAUL), which is a collection of astronomy-related procedures written in the IDL language, available via anonymous FTP from idlastro.gsfc.nasa.gov (Landsman 1993). Astronomy-related IDL software contributed from the community is checked for appropriateness, accuracy, and programming standards. Thus, the site is intermediate between an unmoderated bulletin board and a unified data analysis package. The IAUL does not contain any instrument specific software, although it does contain pointers to other anonymous FTP sites containing instrument specific IDL software. (Anonymous FTP sites exist containing IDL reduction software for the IUE, HST/GHRS, COBE, SOHO, and ROSAT instruments.)
An important addition to the IAUL is the support for the 25 astronomical coordinate systems discussed by Greisen & Calabretta (1995). The coordinate conversion software was written by Rick Balsano and the more complicated transformations were verified by Imannuel Freedman. Additional procedures exist that recognize three ways that the world coordinates may be stored in FITS keywords: (1) the original FITS/AIPS system with a reference pixel, rotation, CROTA2, and pixel scale CDELTi, (2) the IRAF/ST ScI system with a reference pixel and a coordinate description matrix CDi_j, and (3) the Greisen & Calabretta proposal which includes both a pixel scale and a rotation and skew matrix. The software also recognizes headers from the ST ScI Digitized Sky Survey and will apply the appropriate nonlinear transformation to the pixel coordinates.
A recent contribution from Tom McGlynn is a generalized FITS reader that supports both variable-length binary tables, and random groups. An especially important feature of this software is that binary table columns are directly mapped into the tags of IDL structure arrays. The full IDL data analysis capabilities (e.g., plotting, sorting, and subscripting) can then be applied to the structure variable.
Other recent additions to the IAUL include mathematics and statistics code to complement the intrinsic ``Numerical Recipes'' routines. Examples include code for principal components analysis (Murtagh & Heck 1987), Kolmogorov-Smirnov statistics, and cubic-spline smoothing.
Every language has its strengths and weaknesses. In this final section I examine five possible drawbacks a hypothetical astronomer might make to the use of IDL for his data analysis.
The IDL Astronomy Library is funded under NASA grant NAS5-32583 to Hughes STX.
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