Specdre Extension v.1.1

The items of the Extension added with Specdre version 1.1 are described below. All top-level items are optional, but each is either complete or absent.

• .COORD1 and .COORD2 are NDF structures each with data, label and units components. .COORDi.DATA_ARRAY are _REAL or _DOUBLE. Both have the same shape, which is similar to that of <myndf>. The only difference is that in .COORDi the spectroscopic axis is degenerate (has only one pixel). Either both or neither of .COORD1 and .COORD2 must exist. The data values default to a multiple copy of the pixel centres of the main array along the first and second non-spectroscopic axes. For example, if .SPECAXIS is 2, then .COORD1.DATA_ARRAY is a multiple copy of <myndf>.AXIS(1).DATA_ARRAY and .COORD2.DATA_ARRAY is a multiple copy of <myndf>.AXIS(3).DATA_ARRAY. In the same example, if <myndf> has shape nx by ny by nz, then both .COORD i have shape nx by 1 by nz. .COORDi store for each spectrum a two-dimensional position. This position could be used by a plot routine to position spectra according to COORDi on the plot. The values may or may not be sky positions. They could be in any coordinate system and using any units. Labels and units must be stored with both NDF structures. Their default values are copies from the relevant axes of <myndf>, or `unknown' if the relevant axis has no .LABEL or .UNITS. The data components must not contain bad values.

One difference between .SPECVALS/.SPECWIDS and .COORD i is important to note. .SPECVALS and .SPECWIDS are to be used (within Specdre) as replacement of the centres and widths in <myndf>.AXIS(specaxis). But .COORDi are not intended as replacements for the corresponding axis information, these structures are used only in special circumstances. In practice the consequences are as follows.

• If an application requires axis information for any axis, then .SPECVALS/SPECWIDS must override <myndf>.AXIS( specaxis), but .COORDi are ignored completely.

• If an application requires information about the more general position information as provided by .COORDi, then these structures are looked for. Only in their absence are the relevant axes in <myndf> used to generate the same information.