9.2 First Look at ISOCAM Data

As explained in Section 5.3.1, ISOCAM data products were designed to conform as much as possible to FITS standards. They are made up of a primary header plus a binary table extension containing the data. Despite that, some standard software tools for viewing and editing FITS format images, like fv (see: http://heasarc.gsfc.nasa.gov/ Software fv for information on this interactive FITS file editor) or SAOImage DS9 (an astronomical data visualisation application available at http://tdc-www.harvard.edu/ SAOimage) are not able to read ISOCAM images produced by the OLP. On the other hand, IDL can easily read the OLP product files (with the mrdfits command), but an extra modification is needed in order to convert the files from 1024-element arrays to 3232-element images (using the reform command).

As an example, to read a CMAP file:

IDL> structure = mrdfits('cmap########.fits',1,header)
IDL> map = reform(structure.array,32,32,n)}

where n is the third dimension of the cube image (found when executing the mrdfits command).

For CMOS:

IDL> structure = mrdfits('cmos########.fits',1,header)
IDL> mosaic = 
reform(structure.array(*,*,0),structure.naxis1,structure.naxis2)

where structure.naxis1 and structure.naxis2 are the dimensions (in #pixels) of the mosaic image.

For a user who wants to work directly with fully reduced data, the recommended files to start with are:

• CMAP: the ARRAY entry contains the images at different wavelengths or raster positions, calibrated in astronomical flux units and referred to celestial coordinates. Following FITS standards, the images have been recorded using the BZERO and BSCALE keywords, so that: IMAGE=BSCALE*ARRAY+BZERO. The BLANK entry indicates the NaN value for each row (i.e. the pixels with these values have to be ignored when analysing the images). Information about the coordinates, wavelengths, projection parameters, units, etc is also available under different entries. As explained in 7.3.1, the ARRAY entry is composed of 3-row images containing: data (FLUX), errors (FLUX_ERR) and exposure times (EXPOSURE) for each slice.

• CMOS: contains a combination of CMAP calibrated images, if observations are composed of several individual pointings (raster or beam-switch observing modes). As in CMAP, the ARRAY tag has to be scaled: MOSAIC=BSCALE*ARRAY+BZERO.

• CPSL: if the user is only interested in the photometry of a particular source, the file CPSL lists all the point sources detected in the image, and their associated flux (FLUX tag). Note that the sources are detected at map level (not in the mosaic), so that the same sky source can appear in the table as many times as it is detected in different maps making up the mosaic (e.g. images recorded at different raster positions). The tag MAPINDEX identifies the individual map where the sources are found, and the Y, Z, RA and DEC tags identify their coordinates on the detector and in the sky, respectively.

• CSSP: If the image contains data corresponding to more than one wavelength (CVF mode), then the file CSSP groups all the detections stored in CPSL at all wavelengths for each sky target and builds the spectra (FLUX versus WAVELENG tags) of all the point sources detected. The tag PSLINDEX relates each CSSP value (flux of a source at a particular wavelength) with its identification in CPSL (NSRC).

Of course, tools to visualise and analyse scientifically these files are also available in CIA.

Users retrieving the so-called `browse products' (CPS files) should bear in mind that these products are essentially similar to the CMAP or CMOS (where relevant) files, with the difference that they only store the images in their extensions. The information about the image (coordinates, pfov, etc) is available in the header. However, errors are not stored. In CAM CVF observations a second CPS file contains the wavelengths for each of the planes of the cube image. The main advantage of the browse products is that they can be directly read by FTOOLS ( http://heasarc.gsfc.nasa.gov/ Software FTOOLS), but the recommendation is not to use them for science purposes.