To determine the extinction accurately, you must observe some ``extinction stars'' at both high and low altitudes. While it is possible to get a rough estimate of the extinction by observing different standard stars at high and low airmasses, the relatively large conformity errors in most photometric systems, together with the low accuracy of some standards, make this method very inefficient (see Young , p.178). The practical problem is that the extinction coefficient can change considerably in the few hours needed for an extinction star to move from low to high airmass (or vice versa). To minimize this problem, the program selects stars that traverse a large range in airmass in the shortest possible time; these are stars that pass near your zenith. Furthermore, it asks you to observe extinction stars that are both rising and setting, to avoid a correlation of airmass with time.
However, the airmass changes slowly when stars are near the zenith. But to separate extinction drift from instrumental drift, you must observe a wide range in air masses in a short period of time. Therefore, the program selects times when the extinction stars cross an almucantar a little removed from the zenith, rather than when they are on the meridian. This almucantar typically corresponds to about 1.1 airmasses. The times of these crossings are denoted by 'EAST' and 'WEST' in the output of the planner. To optimize the precision of the extinction determination, the low-altitude observations are placed at about altitude, near 2.36 airmasses. Those scheduled observations are denoted as 'RISING' and 'SETTING'.
Furthermore, to track changes in the extinction accurately, you need an extinction measurement (i.e., an observation of an extinction star at large airmass) two or three times per hour. This means that the stars used must be in the right places in the sky to be at large airmasses when you need them. In particular, although the Cousins E-region standards are excellent secondary standards for transformation purposes, Southern-Hemisphere observers should augment them with extinction stars more evenly distributed on the sky.
Obviously, standard stars used for the transformation from instrumental to standard system can also be used for the transformation from inside to outside the atmosphere (traditionally called ``extinction correction''). To minimize the number of calibration observations, the planning program makes standard stars do double duty as extinction stars. These stars should be bright enough that their photon noise is negligible; the proper magnitude range depends on telescope size and the bandwidth of the filter system used. On large telescopes, bright stars are too bright, especially if you are doing pulse counting.
Finally, the standard stars must have a good distribution in each of the color indices of the system you are using. Because transformations are generally non-linear (, ), a wide range of each color should be covered rather uniformly; it is not enough to observe a few very red and a few very blue stars. All these requirements impose constraints on the selection of standard stars.
Notice that, in using standard stars to measure extinction, we need not use the standard values transformed to the instrumental system (though this is possible). Instead, we use the actual observed instrumental values for these stars, which are considerably more accurate than standard values transformed to the instrumental system (cf. p.184 of ), because of conformity errors . We observe standards at both large and small airmasses, and determine the extinction directly from these observations. This matter is discussed more fully in connection with the reduction program (see below).