Standard and instrumental systems

When a standard photometric system is first set up the detectors and filters used define its passbands. Also the originators of the system will typically observe and publish a set of standard stars which define the magnitude scale for the system.

Subsequently, instrumentation for observing in the system will be built at other observatories. There are, for example, many observatories with photometers and CCDs capable of observing in the Johnson-Morgan system. However, the original passbands can never be reproduced precisely, even if the original instrumentation is simply copied and similar filters are purchased from the same manufacturers. The system in which the new instrumentation actually observes is called its natural or instrumental system. In this cookbook the standard system to which a given instrumental system approximates is called the target standard system. Usually considerable effort is expended to make the instrumental system match the target standard system as closely as possible⁶.

Figure: The sensitivity or quantum efficiency as a function of wavelength for various types of detectors. The quantum efficiency is simply the fraction of incident photons which are detected. Here it is plotted on a logarithmic scale. Thinned and bulk CCDs are simply different types of CCDs. Photomultiplier tubes are used in photoelectric photometers. Note that CCDs are more sensitive to red light than photomultiplier tubes. Adapted from McLean[56]

However, in order to make reproducible observations one of the calibrations which must be done is to convert instrumental to standard magnitudes. Conceptually this calibration is done be re-observing the standard stars for the system and comparing the instrumental and standard magnitudes. If the instrumental system is a good match to the standard system then it may be possible to compare just the corresponding bands in the two systems. Conversely, if the two systems are less well-matched or high precision is required then the standard magnitude may have to be computed from the corresponding band in the instrumental system with corrections using the colour indices.