In addition, standard-star files should specify the system they employ, and the source of the standard values. The system should be placed in a character descriptor called SYSTEM. This may be up to 32 characters long, because of the need to distinguish between such alternatives as 'JOHNSON-MORGAN UBV', 'LANDOLT UBV', and 'KRON-COUSINS UBVRI',
Catalog stars may have not only columns specifying values in the system of observation, but also other systems. However, data from other systems, except for V magnitudes, will be ignored.
There is a potential problem in the use of indices like U-B as column labels that users should be aware of. MIDAS table files do not distinguish between upper and lower case in column labels. Thus, while it is possible to use 'u-b' as a column label for the uvby system, it cannot be distinguished from 'U-B' by programs or MIDAS procedures that read these files. However, the string may be entered in the proper case when the label is created, and will appear correctly on plots, table listings, etc.
Furthermore, because the provision for column arithmetic was built into the table system before the need for color index-names as column labels was apparent, it will be necessary to use the double-quote mark (") around such indices when referring to them as column labels. For example, in a MIDAS command line, the B-V column must be referred to as :"B-V". Although this is inconvenient, it does allow such names to appear on plots, etc. The alternative (which will be automatically applied by MIDAS in the absence of the double-quote marks) is to convert the minus sign to an underscore, so that we would have B_V instead of B-V. This appears to be even more inconvenient for photometrists than to put up with the quotes.