Instrument file

Instruments are more complicated than star catalogs, but a MIDAS table file is still useful for holding the description of an instrument. The file is partitioned into two sub-tables. The main sub-table contains information about the passbands used, and the second sub-table describes the detector(s). General information about the instrument is stored in descriptors.

The MAKE/PHOTOMETER command will solicit the necessary information (if you need to make up a new table), or display an existing table (so you can check its contents). Because the structure of instrument files is rather complicated, you should use MAKE/PHOTOMETER to build a new file when an instrument changes, rather than trying to edit the file. This will ensure that all the necessary information is included, and that the file contains an internally consistent description of your instrument.

Basic data for an instrument file include the passbands used (and any coding that is used to represent them in the data); the detectors used, and their properties; and general information about temperature control and optical cleanliness. If your data include neutral-density filters or indicate which measuring aperture was used, be sure to include the corresponding information in this file. See the ``File Formats'' Appendix for details.

One property that needs special attention in pulse-counting systems is the type of dead time. There are (in the textbook approximation) two kinds of counters; these are known as paralyzable and non-paralyzable, and the corresponding dead times are described as extending or non-extending. Different analytical expressions relate observed and true counting rates for the two types, so it is important to know which kind you have. The matter is very clearly discussed by Evans [4], which is a standard reference on this subject.

If an instrument contains filters for more than one system, it may be useful to maintain separate files for the different systems used (e.g., UBV and uvby). This should certainly be done if different filter wheels are used for different systems.