extras -- Information about the extra bands in 3D output
When one dimensional spectra are extracted by the tasks in the apextract package the user may specify that additional extra associated information be extracted at the same time. This information is produced when the extras parameter is "yes".
The associated information is recorded as additional "bands" (the IRAF term for the third dimension of a three dimensional image) of the output extracted spectral image. Extracted spectra are currently stored as IRAF images with dispersion information given in the image header. The image axes for such images are:
1 (columns) - dispersion axis 2 (lines) - spectrum axis (each line is a separate spectrum) 3 (bands) - extras axis (each band is associated data)
The lengths of the second and third axes, that is the number of lines and bands, may be one or more. If there is only one band the image will be two dimensional and if there is only one line and one band the image will be one dimensional. Note that the format parameter controls whether multiple apertures are written to separate images or to a single image. Thus, if the format is "onedspec" this means that the second dimension will always be of length one and, if the extras parameter is no, the output images will be one dimensional.
The associated data in the image bands depends on which extraction options are performed. The various types of data are:
The primary spectrum flux values. Simple aperture sum if variance weighting or cleaning was done. Background spectrum if background subtraction was done. Sigma spectrum if variance weighting or cleaning was done.
The primary spectrum is always the first band and will be the cleaned and/or variance weighted and/or background subtracted spectrum. The simple aperture sum spectrum allows comparing against the results of the variance weighting or pixel rejection options. When background subtraction is performed the subtracted background is recorded in one of the bands. When variance weighting or pixel rejection is performed the software generates an estimate of the uncertainty in the extracted flux as a sigma.
The identity of the various bands is given by the image header keywords BANDIDn (where n is the band number). This also serves to document which extraction options were used.
For more information get help under the topic "apextract.package".