Introduction

Good photometry requires careful attention to calibrations. If there are no calibration observations of standard and extinction stars, the program-star observations cannot be transformed to a standard system. At the opposite extreme, if there are only observations of standard and extinction stars, there is no time for program-star observations. In either case, the data produce no useful information. Obviously, somewhere in between these extremes lies an optimum distribution of program and standard stars.

The choices of which standards to observe, and when to observe them, involve not only balancing the gain in information by adding a calibration observation against the loss of a program-star observation, but also assumptions about the stability and linearity of the equipment used. A good distribution of standards in each color index is also needed to determine accurate transformations. When all these details had to be attended to by hand, it was easy for observers to neglect some essential item. Much experience was required to distribute observations effectively.

Fortunately, the computer can be told to keep track of all these details, and will not forget them. The planning program helps you approach the optimal distribution of standards, which gives the most accurate and precise results possible for a given amount of observing time.