For example, the telescope aperture, which affects both photon and scintillation noise, must be known, both to select good extinction stars, and to weight the observations properly according to stellar magnitudes. Furthermore, the dome or telescope enclosure determines the sky area from which the Moon may shine on the telescope mirror, raising the apparent ``sky'' brightness. This information is needed both for planning observations and reducing data. The telescope's coordinates are required to determine when the Moon is likely to influence observations, as well as in determining the airmass of a star as a function of time.
Similarly, information is needed about the instrument. One must avoid stars that are too bright for a given instrument on a particular telescope. Many instrumental details influence the methods that must be used in reducing photometric observations, because the data-reduction process must accurately model the instrument's performance. Because instrumental configurations tend to change from one observing run to the next, it is appropriate to separate the instrumental data from the more permanent data that refer to the telescope alone.
Some of these data are already available at the NTT, and will become available at other telescopes, as part of the ESO archiving system. Unfortunately, the archiving system is designed around individual image frames, which do not correspond to the natural elements of photometric data. Therefore, it is necessary to extract the required information from the archive, and re-package it in a form more suited to photometry. So far as possible, individual data formats will be similar to those laid out in the ESO Archive Data Interface Requirements . For example, column labels for the table files described here will (when possible) match the FITS-header keywords used for the same information in archiving.
The data needed in photometry can be grouped into MIDAS table files, each of which contains a natural set of data that belong together. Each table will be described in detail in the following sections. However, here is an overview of the types of tables needed for photometry: