This extension header is the one that would be used for a structure designed by D. Wells to illustrate an efficient use of a binary table to hold spectra. The spectrometer for this illustration has 1024 channels, and the data are in floating point form. This design is, intentionally, grossly simplified for the sole purpose of illustration. Wells notes that real spectral observations would probably require 50 table columns in each row in addition to the spectra. The precise column labels are not part of the proposal; if such a structure were to be adopted, an ad hoc group would work out the details. Also, the data types used might vary among installations.
In this example, the primary header is not included, but, as with the ASCII table extension example, there must be one to start the data set. Other extensions might be present as well, so this extension isn't necessarily going to be the second header-data unit on the data set. The XTENSION= 'BINTABLE' string at the start of the record indicates that the header is for a binary table extension. The BITPIX, NAXIS, and GCOUNT keywords have the values required for a binary table extension. The value of 0 for PCOUNT shows that there are no heap or other data following the table. To describe one spectrum, 4134 bytes are needed (NAXIS1); this particular table has 20 spectra (NAXIS2). The NAXIS1 value is equal to the sum of the number of bytes required by the different fields. Each row of the table can hold a 1024-channel spectrum and five additional items of information to describe it: scan number, source ID, frequency of reference channel, channel-to-channel difference, and the total number of channels. Frequencies are in Hertz (TUNIT3 and TUNIT4 keywords). There is room in field 6 for 1024 channels; there could be fewer. The title or name of the table is SPECTRUM (EXTNAME keyword); this table could be the second version or perhaps the second spectrum in the FITS file (EXTVER keyword).