The IDL Astronomy Library FAQ

This document is intended to answer the most Frequently Asked Questions (FAQs) about the IDL Astronomy Library. The FAQ is a dynamic document that will change as information changes. Suggestions for corrections or additional sections are welcome -- please email them to Wayne Landsman.

A. Purpose and Organization of the IDL Astronomy Library

B. IDL Software for Astronomy

B1. What astronomical image display widgets are available?
B2. Are there any IDL widgets for astronomical spectroscopy?
B3. What IDL software is available for image deconvolution?
B4. Will there be any updates to the IDLPHOT photometry package?

Section A.

Purpose and Organization of the IDL Astronomy Library

A1. Where can I find out about stars, planets, black holes, galaxies etc.?

Sorry, you've come to the wrong place. The IDL Astronomy Library is a collection of low-level software in the commercial language IDL , and is mainly of interest to professional astronomers who already have purchased an IDL software license. For general questions about astronomy, one good starting point is the list of Web resources available at the WEBStars page at Goddard Space Flight Center.

A2. I haven't purchased an IDL license -- can I still use the Library? Can I use it with PV-WAVE or GDL?

Sorry, the programs in the IDL Astronomy Library will not run without an IDL license. You are welcome to use any of the programs as metacode and convert them to your favorite language.

The GNU Data Language (GDL) is a free IDL compatible incremental compiler. I have not tested most of the IDL Astronomy Library procedures under GDL but READFITS() and WRITEFITS do work.

PV-WAVE is a commercial programming language with similarities to IDL. In fact, many years ago (1990) it was an identical language to IDL, but has since evolved separately. I do not attempt to make the programs in the IDL Astronomy Library compatible with PV-WAVE, but since the IDL Astronomy Library has little use of widgets or object-oriented programming (see A7 ), much of the code can probably still be converted to PV-WAVE without too much effort.

A3. How do I install the IDL Astronomy Library?

The entire contents of the Library can be loaded from one of two .tar files or from a .zip file at the download site . The astron.dir.tar.gz file maintains the procedures in separate topical directories, whereas the astron.tar.gz file has the same files but with all the procedures in a single directory. The astron.zip file has identical contents to astron.tar.gz but in .zip format.

The procedures in the IDL Astronomy Library need simply be placed in a directory included in the IDL !PATH. This can either be done at startup (using the UNIX environment variable IDL_PATH or the Windows/Macintosh preferences file) or by simply concatenating the directory to the !PATH system variable. Note that the procedures in the IDL Astro Library are not a self-contained "package" but rather a collection of low-level procedures to be incorporated with the user's own software.

For historical reasons, some of the IDL Astronomy Library procedures use the following three non-standard system variables. These must first be defined with the ASTROLIB procedure (or the equivalent DEFSYS definitions).

!PRIV - Used in a couple of the database programs. Needs to be set when modifying a database.
!TEXTOUT - Used in the program TEXTOPEN which is called by the database and several other programs. Programs which use this non-standard system variable also allow a keyword TEXTOUT to be used instead of the system variable
!TEXTUNIT - Logical output unit number used by TEXTOPEN

The procedure ASTROLIB also defines a default ASTRO_DATA data directory. This directory contains a FITS file JPLEPH.405 containing the JPL DE405 ephemeris, which is used by several routines (e.g. JPLEPHINTERP) to compute highly accurate positions and velocities of major solar system objects (including the Earth and the Sun). However, due to its size, the JPLEPH.405 file is not included in the standard .tar and .zip distributions of the IDL Astro Library . Thus, the use the JPL procedures the following two steps are required:

Copy the files in http://idlastro.gsfc.nasa.gov/ftp/data/ to your /data directory
Define the environment variable ASTRO-DATA (and/or edit the ASTROLIB procedure) to point to your /data directory

A4. Who supports the IDL Astronomy Library?

The IDL Astronomy Library has been supported by NASA under a small grant from the Astrophysics Data Program. The first grant was awarded in 1990 to Wayne Landsman for three years, and the grant was renewed in 1993 and 1996. At various times, other people , including Frank Varosi, Krishna Venkatakrishna, and Joseph Lyu, have worked part-time on the Library.

At the end of November 2000, all funding for the IDL Astronomy Library ended. Applications to renew support for the IDL Astronomy Library were made to the NASA Applied Information Systems Research in both September 2000 and September 2001 but were not successful. I am currently looking for other sources of funding.

Many of the programs in the IDL Astronomy Library have been contributed by the user community. In particular, Don Lindler, Craig Markwardt, Tom McGlynn, and Bill Thompson have made significant contributions to the Library.

A5. Which versions and platforms of IDL are supported by IDL Astronomy Library?

The programs in the IDL Astronomy Library should work in V6.1 and all later versions of IDL. Frozen versions of the IDL Astronomy library compatible with older versions of IDL are available in the old directory, but these will be missing more recent updates and bug fixes.

There is also a V6.4 directory which contain modifications of a few IDL Astronomy Library procedures to take advantage of features introduced in newer versions of IDL. The procedures in these directories will not work in earlier versions of IDL.

I currently only have access to IDL on Unix (Solaris & Linux) and Macintosh machines, though I try to maintain Windows compatibility.

A6. Is there on online help file available for the IDL Astronomy Library?

Each procedure contains a documentation header. Unfortunately, there is no easy way in IDL to link the documentation for user procedures with either the hypertext help or the Acrobat .PDF files used for intrinsic procedures. One way to create help files is to use the standard IDL procedure MK_HTML_HELP to create an .html help file, though this help file can be awkward to use, due to its large size and lack of subcategories. An alternative is to simply use the one-line description of procedures, available at Goddard or in Tübingen or to mirror one of these sites locally. Users of Emacs can use the excellent help facilities incorporated into the IDLWAVE mode.

A7. Why are there so few widget and object-oriented procedures in the IDL Astronomy Library?

The emphasis of the IDL Astronomy Library is on low-level algorithms for astronomy (including FITS I/O). The are many excellent Web sites for other aspects of IDL programming. (Probably the three most widely used libraries for astronomers are those of David Fanning, Craig Markwardt, and Ray Sterner . ) Historically, widget procedures are also more difficult to maintain over different IDL platforms and versions.

A8. Are there known incompatibilities between the IDL Astronomy Library and other IDL Libraries?

The procedures BREAK_PATH() , CONCAT_DIR() , FIND_ALL_DIR() , FIND_WITH_DEF() , LIST_WITH_PATH() , lack the VMS support which exists in procedures with the same name in the Solarsoft library

A9. Where can I find IDL software for a particular instrument?

In general, the IDL Astronomy Library does not contain any instrument-specific software. Check the Links to other IDL Astronomy Sites for IDL software for a particular instrument. Please inform Wayne Landsman of any additions or corrections to this page.

The one exception to the "no instrument specific" software for the IDL Astronomy Library has been for the WFPC2 (Wide Field/Planetary Camera) instrument on the Hubble Space Telescope (HST). The exception is made because WFPC2 is so widely used, and there is no other site supporting IDL software for it.

A10. How is configuration control (version numbers) maintained for the IDL Astronomy Library?

Unfortunately, I do not have the facilities to maintain strict configuration control. Procedures in the download site are updated as soon as a bug fix or enhancement has been implemented and tested for backwards compatibility. The tar and zip files are updated roughly every month. (Therefore, the ASCII file version of a procedure may be up to a month newer than what is found in the tar file.) Every change is recorded in the "Revision History" section of the documentation header, and is also recorded in the news file.

A11. How should the IDL Astronomy Library be cited or acknowledged in publication?

If extensive use is made of the IDL Astronomy Library then either the Web home page should be cited, or the following reference given:
Landsman, W. B 1993 in Astronomical Data Analysis Software and Systems II, A.S.P. Conference Series, Vol. 52, ed. R. J. Hanisch, R. J. V. Brissenden, and Jeannette Barnes, p. 246.

A12. Are there any mirror sites for the IDL Astronomy Library?

The Astronomical Institute of Tübingen used to mirror the IDL Astronomy Library nightly, but the latest version is from 2005. I do not know of any other mirrors.

A13. Is there a E-mail distribution list for the IDL Astronomy Library?

I have discontinued the previous mailing list because notices were so infrequent. I will post news about major updates to the comp.lang.idl-pvwave newsgroup

A14. What is the licensing for the IDL Astronomy Library?

The IDL Astronomy Library procedures are in the public domain. If included in a package with other procedures, I suggest that the Astronomy Library procedures be kept in a separate directory. Please contact me if you wish to make changes to a procedures, as there is a good chance that I would be able to include the changes in the official version.

Section B

IDL Software in Astronomy

B1. What astronomical image display widgets are available within IDL?

I know of three sophisticated image display widgets written in IDL, that work with FITS files and recognize the world coordinate systems for astronomy.

The ATV Image Display Tool is an IDL program written by Aaron Barth (UC Irvine) modeled after the popular SAOImage image display utility.
The object-oriented programs XCTV, XCTV2, XCTVN, XINSPECT, XINSPECT2 in the bhill contrib directory of IDL Astronomy Library
The program IDP3 (Image Display Paradigm 3) originally in the NICMOS library

Other non-astronomical image display widgets might also be useful. The LOOK widget by Fred Knight is easily extensible for including user-supplied code. The IMDISP utility by Liam Gumley has been especially recommended for formatting images that look good both on the display and on the printer.

B2. Are there any IDL widgets for astronomical spectroscopy?

I recommend the IDL utilities available at the FUSE (Far-Ultraviolet Spectroscopic Explorer) software site. The package contains widgets for continuum normalization, multiple Gaussian or Lorentzian fittings, and equivalent width measurements.

B3. What IDL software is available for image deconvolution?

The IDL astro programs max_likelihood and max_entropy have no method to determine when to stop the iterations, The NICMOS library includes an interactive tool LUCY for Richardson-Lucy deconvolution. Jongchul Chae used IDL maximum entropy code to correct for stray light in solar filter-based magnetograph images.

B4. Will there be any updates to the IDLPHOT photometry package?

The IDLPHOT library was written in 1990 and was based on the 1987 version of the DAOPHOT photometry package. Thus the code for PSF fitting is awkward to use by modern standards, and for PSF-fitting I would recommend one of the modern non-IDL packages such as DAOPHOT II or HSTPhot or Sextractor . However, some of the procedures in IDLPHOT such as mmm.pro to estimate a sky background, find.pro to locate point sources, and aper.pro to perform aperture photometry remain useful and easily cannibilized. While some updates to aper.pro are planned (e.g. to recognize NAN values as bad pixels, and accept an error array) there are no immediate plans to improve the PSF fitting software.

Other available IDL photometry packages include the following:

STARFINDER - A PSF-fitting photometry program from Bologna for the analysis of well-sampled crowded stellar fields
HIIPHOT - A software package by D. Thilker (NMSU, 2000 AJ, 120, 3070 ) designed for accurate photometric characterization of H II regions.
CCDPHOT - Marc Buie's CCD Photometry program originally designed for solar system objects.
OIS - Marc Buie's optimal image subtraction program based on Miller et al. 2008

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Curator: Wayne Landsman
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Last Updated: Jan 2010