SETUP is used to view the current default observing site longitude and latitude that VISTA is using and to change these values if desired. The Latitude and Longitude of the observing site are used to help compute the necessary airmass (atmospheric extinction) corrections used for spectroscopy and photometry. This is very important, because the default behavior of xvista is to recompute airmasses from the observation position and time, rather than use values which may be included in the header by the telescope (see below).
On startup, the VISTA searches for the environment variables V_LONGITUDE and V_LATITUDE. If these are found, the values are loaded into the VISTA internal variables. The units for both longitude and latitude are assumed to be degrees.
If you issue SETUP with no arguments, the current values will be printed. You can enter new values using the LONG= and LAT= keywords, or else you will be prompted for them by the program. Coordinates for 9 different observatories are preloaded into xvista, and can be selected at the prompt or by keywords (MMT, KPNO, CTIO, LOWELL, APO, LICK, AAT (or SS0), ESO, CFHT).
The arguments CHECKAIR, COMPUTEAIR, and USEAIR can be used if one wishes to consider the value of the AIRMASS card within an image's FITS header. If you issue SETUP CHECKAIR, then the programs which compute airmasses will check them against the value in the FITS header; if the latter exists, the user will be warned if the computed airmass differs from the header airmass by more than 0.03 airmasses. If SETUP COMPUTEAIR has been issued (this is the default startup configuration), the program will just use the computed value regardless (and you'd better have the longitude and latitude right!). If SETUP USEAIR has been issued, the program will use the airmass from the AIRMASS or AM card in the FITS header if it exists, otherwise it will use the computed value. Using any of CHECKAIR, COMPUTEAIR, or USEAIR will change the default mode of execution until a subsequent SETUP command is issued or until you exit xvista. Upon starting up xvista, the default behavior is to use the computed airmass without warning whether it differs from the header value. If you wish to change this default behavior, put a command in your xvista startup file (we highly recommend using SETUP CHECKAIR, but didn't make this the default to maintain compatability with earlier releases).