Data Formats

GAIA can access images in various different data formats. The ones that you are mostly likely to encounter are FITS images and NDF files, though there are numerous other possibilities, including the IRAF format and old Figaro DST files.

In GAIA (and to anticipate the recipes of Part II) you can check which formats are currently available by clicking on the File Type: button in the file-picker window (see the recipe in Section ). Be careful that you do not inadvertently set the button to specify just one of the formats; it is usually best left set to `any'.

The basic GAIA image display facilities read FITS files. The more astronomical functions usually require files in Starlink's NDF format. However, any necessary format conversions are performed automatically and invisibly `on-the-fly' by applications in the CONVERT package (see SUN/55[4]), and you will not normally be aware that they are happening. It is possible to configure the range of formats currently available to CONVERT. You are unlikely to need to make such a configuration, but if you choose to do so then SSN/20[22] gives the requisite details. However, you should ensure that FITS continues to be one of the formats available (because the basic image display functions read FITS files directly).

A full description of the NDF and FITS formats is beyond the scope of this cookbook. The NDF ($n$-dimensional Data Format; see SUN/33[19]) is the native Starlink format. The FITS¹ (Flexible Image Transport System) format is in widespread use in astronomy. There is a brief introduction to the FITS format in SC/5[7].

In both the NDF and FITS formats, and indeed other common astronomical formats, the information stored in the file includes more than just the array of values for the two-dimensional image. Various other header or auxiliary information describing and annotating the image is also included. Typical auxiliary information might include: the instrument and telescope used, the date and time of observation, details of the instrumental set up etc. In the jargon of computer science such header information is often called `metadata', though this term is rarely used in astronomy. Sometimes you may wish to examine this auxiliary information.

In GAIA (and again anticipating Part II) you can display the headers of a FITS image by clicking on the View menu (on the menu-bar along the top of the main window) and choosing the Fits Header... item (see the recipe in Section ). Some alternative ways of listing the header information for the NDF, FITS and a couple of other formats are given in Appendix B.2 of SC/6[16].

Data files usually contain just a single image. However, both the FITS and NDF formats allow files to contain more than one image, and occasionally you might encounter such a file. Section  includes some notes on how to access a given image inside a file in this case.

The format of a data file is often indicated by specifying a `file-type' at the end of the file-name. NDF files have file-type `.sdf'. When accessing NDF files with GAIA the file-type may optionally be omitted. FITS files usually have a file-type of `.fit' or `.fits'.