Units in Data Files

In the DESI data model, wherever possible we describe the units carried by values in the data files being described. In some cases the values are actually dimensionless, or a simple quantity such as wavelength (Angstrom). In other cases the units are more complex such as spectral flux density (10^-17 erg cm^-2 s^-1 Angstrom^-1).

The majority of the files in the DESI data set are FITS files. The FITS standard supports header keywords that describe the units of the values stored in each HDU. A BUNIT keyword describes the units for an image HDU, while TUNITn keywords describe the units of column n in a table HDU. In the DESI data model, we specify units that follow FITS standards as closely as possible. See Section 4.3 of the FITS Standard (Version 4.0) for more details.

However, even though units are described in this data model, it is not always the case that units are specified in the FITS files themselves. In other words, BUNIT or TUNITn header keywords are not always present. If you routinely read FITS files with QTable, for example, the described units might not appear automatically.