Selecting standard stars

You will usually select standards from either the computer-readable or printed versions of catalogues of standard stars (see Section ). If you are observing in the Johnson-Morgan system then Landolt's catalogues are probably the most useful.

You should not use catalogues of standards blindly. Rather, you should read the paper or other documentation accompanying the catalogue; it will contain details of the limitations, applicability and use of the catalogue which you should be aware of in order to use it effectively. Also, standards from different catalogues should not normally be mixed in a given observing programme.

The desirable properties for a set of standard stars include the following:

• a range of zenith distances (and hence air masses) similar to, but slightly larger than, those of the programme objects. Also the range in air mass should be at least 1.0. Since the air mass at the zenith is 1.0, to get a range of 1.0 you need to observe at an air mass of at least 2.0, corresponding to a zenith distance of $60^{\circ}$ (see Table ). A reasonable upper limit to the air mass for observing standards is about 2.5 (though this will depend on the site),

• a range of celestial coordinates similar to those of the programme objects (this criterion is, of course, related to the previous one),

• a range of colours and magnitudes which are similar to (or slightly larger than) those of the programme objects.

These criteria are really merely special cases of the usual requirement that calibrators should occupy a similar volume of parameter space as the things which they are calibrating.

The number of standard stars chosen will vary depending on the aims of your programme. However, for most purposes fifteen to twenty is probably adequate. The advantage of having this number of standards is that a representative range of air masses, magnitudes and colours can be sampled.

Section  gives a recipe for selecting standard stars from a computer-readable catalogue.