Institut für Astronomie und Astrophysik Abteilung Astronomie Sand 1, D-72076 Tübingen, Germany

Table of Contents

• Introduction
• Index of Subroutines

Introduction

The routines in this directory are for reading and writing (disk) FITS binary table extensions. Access to FITS files is direct, rather than passing through intermediate file structures (such as SDAS/Geis). Some routines for reading and writing primary FITS files, and for reading and writing FITS tapes, are included to support writing binary tables. The various routines are:

Primary FITS files -- writing

Primary FITS files -- reading

	FXREAD		Read primary FITS file
	FXPAR		Extract values from a FITS header

Binary tables -- writing

One thing that sometimes confuses first time users is that the logical unit variable UNIT is an output parameter of FXBCREATE, not an input parameter. One does not specify the logical unit number for the file; instead FXBCREATE assigns one via a call to GET_LUN. The returned value is then used by the other routines. A similar situation exists in regard to the column number argument to FXBADDCOL. Column numbers are assigned in the order in which FXBADDCOL is called. See the LaTeX file "fits_bintable.tex" for more information.

Binary tables -- reading

	FXBOPEN		Open FITS binary table extension for reading.
	FXBREAD		Read data from FITS binary table.
	FXBCLOSE	Close FITS binary table.
	FXBTDIM		Parse keywords from binary tables with a TDIM-like
			format.
	FXBHELP		Show information about the binary table columns.
	FXBFIND		Find column keywords in header.
	FXBCOLNUM	Returns the column number of a specified column.
	FXBHEADER	Returns the header of an opened FITS binary table.
	FXBISOPEN	Returns whether or not a FITS binary table is open.
	FXBSTATE	Returns the state of a FITS binary table.
	FXBDIMEN	Returns the dimensions of a binary table column.

Multidimensional Array Facility

In addition to the keywords described in the binary tables extension proposal, several additional keywords are supported by FXBADDCOL with the /VARIABLE keyword. Other than that, support for variable-length arrays is automatic. Some operations, such as reading entire columns, and the multidimensional array facility described above, are not allowed with variable-length arrays.

Ordinarily, the default THEAP value (NAXIS1*NAXIS2) is used to write the variable-length arrays. However, a different THEAP value can be used by using FXADDPAR to insert the desired value into the binary table header before calling FXBCREATE.

IEEE Not-a-Number (NaN) Special Values

At present, there is no support for IEEE +/- Infinity and -0 special values. However, it would be a simple matter to add support similar to IEEE NaN.

Bit, Logical, and Double-precision Complex Arrays

Bit arrays (type "X" in FITS binary tables) are treated in IDL as byte arrays with approximately 1/8 the number of elements. Support for this is automatic when reading binary tables. Columns can be defined as type "X" when writing binary tables if the BIT keyword is passed to FXBADDCOL giving the number of bits, and the data array is of type byte. Dimension information is ignored for bit arrays, since the dimensions apply to the bits, and not to the bytes that IDL processes.

Logical arrays (type "L" in FITS binary tables) are treated in IDL as byte arrays. Support for this is automatic when reading binary tables. Columns can be defined as type "L" when writing binary tables if the /LOGICAL keyword is passed to FXBADDCOL, and the data array is of type byte.

Virtual Columns

To use the virtual column convention, the user must call FXBREAD with the /VIRTUAL keyword, and must also reference the desired information by name rather than by column number. FXBREAD will then look first for a column with that name. If it doesn't find one, it then looks for a keyword with that name in the header.

Index of Subroutines

[Home Page] [Software, Documentation] [IDL Documentation] [Quick Reference] [Feedback]