9 Selecting and Observing Standard Stars

This section describes some common practices for selecting and observing standard stars. To recapitulate: the purpose of observing standard stars is to allow the instrumental magnitudes measured for programme objects to be converted into calibrated magnitudes in the target standard photometric system. The effects which the calibration must remove are atmospheric extinction and the mismatch between the instrumental and standard systems.

Though the procedures described here are often good practice they are not always appropriate. Clearly you should tailor your choice of standards and observing practices to the aims of your programme. Indeed, there are some sorts of photometric programme for which it is un-necessary to observe standard stars at all.

9.1 Selecting standard stars

You will usually select standards from either the computer-readable or printed versions of catalogues of standard stars (see Section 7.3). 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 $6{0}^{\circ }$ (see Table 3). 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 13 gives a recipe for selecting standard stars from a computer-readable catalogue.

9.2 Observing standard stars

Because transient variations in atmospheric conditions can cause unpredictable variations in the atmospheric extinction it is necessary to regularly monitor the standard stars throughout a night’s observing. When making photometric observations care and caution pay dividends. A typical strategy might be to start the night’s observing with a series of observations of standard stars, covering a range of zenith distances. These observations can be used to make a preliminary estimate of the atmospheric extinction. Then as the night progresses observations of standards are regularly interspersed amongst the observations of programme objects.

Often modern observatories will have software which allows approximate ‘rough and ready’ reductions to be carried out in near real time, thus allowing instrumental magnitudes to be computed for the standard stars pari passu the continuing observations. This facility is extremely useful because it allows the atmospheric extinction to be monitored as the night progresses. However, reductions carried out during the observing session are only approximate. They are useful for monitoring the atmospheric extinction, but are not the most accurate results obtainable from the observations.

You should always reduce your data after the observing session has finished, starting from the full set of observations.

Passing clouds and light mist will obviously affect the atmospheric extinction. Furthermore, they can be difficult to detect by eye, even if you regularly look outside the telescope dome (it is difficult to see light cloud at night with eyes that are not dark-adapted). However, it is easy to spot any deterioration in the observing conditions if the atmospheric extinction is being monitored regularly. Observations of programme objects can be suspended until good conditions return. Using these techniques, and with good conditions and modern instrumentation, it is perfectly feasible to carry out photometry to an accuracy of 0.01 magnitude without resorting to any special tricks.

It is prudent to keep a note of the Right Ascension, Declination and UT or sidereal time of each observation (for both standard stars and programme objects) as it is made. Usually (though not always) the air mass or zenith distance will be calculated automatically and added to the header information for each observation as it is written. However, if you have kept your own notes you can calculate these quantities yourself if necessary, either as a check or because they are not otherwise available. (See Appendix B for further details of calculating the zenith distance.)