EXTINCT will correct a wavelength-calibrated spectrum 'source' for atmospheric extinction. The zenith angle is measured from the header information, and the air mass calculated with a polynomial in secant(z) to account for the finite thickness of the atmosphere. A set of extinction values is calculated at several wavelengths, and a spline drawn through them. The spectrum is multiplied by the spline to correct it for extinction.
The extinction correction which is used is the average extinction for Lick Observatory. The keyword CTIO will have EXTINCT use values appropriate for Cerro Tololo; KPNO uses the Kitt Peak extinction coefficients. These are very similar. Note that the set of coefficients used is remembered between successive calls to EXTINCT; if you specify CTIO on one call to EXTINCT, the CTIO coefficients will be used on all successive calls unless the keyword LICK or KPNO is explicitly used on the command line.
The longitude and latitude of the observatory at which the data were acquired must be defined either before running VISTA by defining the UNIX environment variables V_LONGITUDE and V_LATITUDE, or by using the VISTA SETUP command. You will need to do this to obtain correct determinations of airmass or extinction when reducing CCD images from other observatories. The longitude and latitude must be in decimal degrees: For example
SETENV V_LONGITUDE 121.64554 SETENV V_LATITUDE 37.43029Is the longitude and latitude of the Lick Observatory (Mount Hamilton, CA).
If these variables are not defined, defaults set at compile time will be used. You can also redefine the longitude with the SETUP command inside of VISTA.