Description

The CURSA photometric calibration functions, in common with most photometric calibration methods, use standard stars. In essence, as well as observing instrumental magnitudes for the programme objects that you are studying you also observe instrumental magnitudes for selected standard stars. These standard stars have a known brightness in your target photometric system. Numerous catalogues of photometric standard stars are available (see SC/6[22] for a brief discussion). You then define the transformation between the instrumental and standard system for the standard stars and apply this transformation to calibrate the instrumental magnitudes of the programme objects into the standard system.

In addition the observed brightness of a star varies throughout a night because of atmospheric extinction or the dimming of starlight by the terrestrial atmosphere. The longer the path length the starlight traverses through the atmosphere the more that it is dimmed. Thus, a star close to the horizon will be dimmed more than one close to the zenith. The path length through the atmosphere is known as the air mass. The air mass can be calculated from the zenith distance. In order to calibrate photometry air masses must be available for both the programme and standard stars.

Thus, a basic set of photometric data consists of:

• a set of standard stars with measured instrumental magnitudes, known `catalogue' magnitudes (so-called because they are obtained from catalogues of photometric standards) in the target photometric system and air masses,

• a set of programme objects with measured instrumental magnitudes and air masses.

The standards are invariably stars; the programme objects can be any sort of astronomical object. Photometric calibration is a two-stage process:

1. define the transformation between the instrumental and catalogue magnitudes for the standard stars, typically by some sort of least squares fitting,

2. apply the transformation to convert the instrumental magnitudes into calibrated magnitudes for the programme objects.

In CURSA the relation between instrumental and catalogue magnitudes is assumed to be of the form:

$$m_{\rm catalogue} = m_{\rm inst} - A + Z + \kappa X$$ (9)

where:

$m_{\rm catalogue}$

is the calibrated magnitude,

$m_{\rm inst}$

is the instrumental magnitude,

$A$

is an arbitrary constant which is often added to the instrumental constants,

$Z$

is the photometric zero point between the standard and instrumental systems,

$\kappa$

is the atmospheric extinction correction,

$X$

is the air mass.

See SC/6 for further discussion of the arbitrary constant $A$. This equation is a particularly simple form for the relation between instrumental and catalogue magnitudes. In particular, it omits any `colour corrections' caused by the instrumental and standard systems being sensitive to different wavelengths. Thus, the CURSA photometric calibration functions should only be used when the instrumental photometric system is well-matched to the target photometric system. Though this may seem a serious limitation, in practice with modern instrumentation the instrumental system is often a good match to the standard system. For the same reason the CURSA applications are not suitable for very high precision work, where even small discrepancies between the instrumental and standard systems must be allowed for.

The basic reason why colour corrections are ignored is because by doing so the functions are much more general. They do not impose constraints on the photometric system that you are using (other than that the instrumental and standard systems should be well-matched) and they do not require you to make observations in any given colours.

Fitting the instrumental and standard magnitudes for the standard stars is usually an `iterative', interactive process. Typically, you will start by fitting all the standard stars, examine the residuals, reject the stars with large residuals, fit the remaining stars and continue until you have a satisfactory solution. (Aberrant results for individual stars can be caused by various effects, including passing clouds.)

For completeness, the subroutine used by the CURSA photometric calibration applications to fit the instrumental and catalogue magnitudes for the standard stars is PDA_DBOLS. This subroutine is described in SUN/194[19].