The UK data-cube format

The MOS-style MEF format, which is the end product of the GMOS and CIRPASS data-reduction software, is not particularly natural way of handling IFS data. Indeed, under the $longslit$ paradigm (used to reduce TEIFU data) these files cannot be generated. The TEIFU-style data cube format has therefore been adopted as the standard UK IFS file format and will be used in the analysis stages for both CIRPASS and UIST. This adoption allows the use of many of the generic applications within the SSC, which due to the adoption of NDF as the standard file interchange format for Starlink applications, has many tasks that can process $N$-dimensional data.

A conversion program for GMOS and CIRPASS data to a more easily analysed data cube, which will involve re-binning the input spectra on to a rectangular array, is therefore desirable.

Table: TEIFU data-cube format

No.	Type	Name	Format	BITPIX	Comment
0	ifs_data.fits
1	IMAGE	SCI	$x \times y\times \lambda$	$-$32	3-D science array
2	IMAGE	VAR	$x \times y\times \lambda$	$-$32	3-D variance array
3	IMAGE	DQ	$x \times y\times \lambda$	16	3-D data-quality array

In the case of this format the IFU geometry information is no longer needed, as the input spectra have already been rebinned, but it is likely that (in the finalised file format) such information will be included as a FITS binary table.

An example of the FITS header block from a TEIFU data cube is shown below.

SIMPLE  =                    T /  file does conform to FITS standard
BITPIX  =                   16 /  number of bits per data pixel
NAXIS   =                    3 /  number of data axes
NAXIS1  =                   59 /  length of data axis 1
NAXIS2  =                  110 /  length of data axis 2
NAXIS3  =                  961 /  length of data axis 3
EXTEND  =                    T /  FITS dataset may contain extensions
OBJECT  = 'T/S MOVING field  ' /  Title of the dataset
DATE    = '2000-10-05T17:50:52'/  file creation date (YYYY-MM-DDThh:mm:ss UTC)
BSCALE  =         3.126180E-02 /  True_value = BSCALE * FITS_value + BZERO
BZERO   =         6.089249E+02 /  True_value = BSCALE * FITS_value + BZERO
BLANK   =               -32768 /  Bad value
CD1_1   =               0.0625 / Axis rotation and scaling matrix
CD2_2   =               0.0625 / Axis rotation and scaling matrix
CD3_3   = 5.795317000000068219 / Axis rotation and scaling matrix
CRVAL1  =             -0.03125 / Axis 1 reference value
CRVAL2  =             -0.03125 / Axis 2 reference value
CRVAL3  = 7302.291864499999065 / Axis 3 reference value
CRPIX1  =                 29.5 / Axis 1 pixel value
CRPIX2  =                 55.0 / Axis 2 pixel value
CRPIX3  =                480.5 / Axis 3 pixel value
WCSDIM  =                    3   
CTYPE1  = 'LINEAR  '           / Quantity represented by axis 1
CTYPE2  = 'LINEAR  '           / Quantity represented by axis 2
CTYPE3  = 'LAMBDA  '           / Quantity represented by axis 3
CD1_2   =                  0.0 / Axis rotation and scaling matrix
CD1_3   =                  0.0 / Axis rotation and scaling matrix
CD2_1   =                  0.0 / Axis rotation and scaling matrix
CD2_3   =                  0.0 / Axis rotation and scaling matrix
CD3_1   =                  0.0 / Axis rotation and scaling matrix
CD3_2   =                  0.0 / Axis rotation and scaling matrix
LTV3    =                -39.0   
LTM1_1  =                  1.0   
LTM2_2  =                  1.0  
LTM3_3  =                  1.0  
WAT0_001= 'system=image'
END

Here the the number and size of the cube dimensions is specified by the NAXIS keywords, and as with the IRAF spectral format the CD keywords define the conversion between world co-ordinates and pixels on the detector, along with the tilt of the dispersion path relative to the detector pixels. While the CRVAL keywords defines the central value of each axis in world co-ordinates, e.g. in the case the spectral axis is centred on $\sim$7302 Å (CRVAL3).

A full dictionary defining FITS header keywords which can be generated by the data-acquisition system is provided on the web by the National Optical Astronomy Observatories (NOAO).