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Figure 15.1: FITS File Parts.
15.2.10
diagnostic
2dfdr produces various files used to diagnose particular operations on the data.
15.3
File Parts
Each FITS format file contains one or more Header Data Units (HDU). HDUs outside the
primary HDU are known as “extensions”. There are three types of HDU: image, ASCII table
and binary table. 2dfdr uses image HDUs for spectra and variance data, and binary tables for
fibre information.
The FITS standard requires each HDU to have a header (keyword) section and data section
(see Figure 15.1). Each header section contains any number of keyword name, value and comment 80-character records. The purpose of the keyword section is to provide information about
the connected HDU data section.
For 2dfdr purposes keywords in the primary IMAGE HDU provide a description of the
observation. The fibre table BINARY HDU header keywords have information relevant for
determining the fibre positioning, such as air temperature and humidity.
2dfdr output files have a history section (within the primary header) which describes the
processing steps used in producing the file. This is contained in FITS keywords with the name
“HISTORY”. This information is available within 2dfdr by selecting a file and pushing the
History button.
What follows are descriptions of the individual HDUs used in 2dfdr output files. Whether
a particular HDU appears in a 2dfdr output file mostly depends on the file type. Each section
below has a Where Used attempting to document the correspondence.
All 2dfdr output files have at least the first three HDUs (marked 0, 1 and 2 below). The
IMAGE, VARIANCE and FIBRE TABLE also ALWAYS appear in this order in reduced files,
but in raw files the order may be different, however IMAGE will still be first.
15.3.1
[0] Primary
A 2-dimension image array holding the raw CCD image ( raw files), processed CCD image (
im files), or spectral data ( ex and red files). Images are the same size as the CCD. Raw
images are slightly larger as they contain the overscan bias region. Spectral data is dimensioned
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