...)¹

You should be aware - and beware - that different authors define the terms flux density, flux and intensity differently, and they are sometimes used interchangeably!

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... magnitudes²

Some magnitudes : Sirius = -1.5, full Moon = -12.5, Sun = -26.8

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... parsec³

Strictly speaking this is the apparent magnitude which would be observed in the absence of interstellar extinction (see Appendix ).

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...$m_{v}$⁴

The last major catalogues compiled using magnitudes estimated by direct observation are the great Durchmusterungen produced in the late nineteenth and early twentieth centuries: the Bonner Durchmusterung, the Bonner Südliche Durchmusterung and the Cordoba Durchmusterung. However, the visual system is still in use, particularly for variable-star work. The various extensive archives of variable-star data consist largely of visual observations, mostly contributed by amateurs.

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... magnitude⁵

En passant, for stars $B - V$ is primarily related to temperature (and hence spectral class) while $U - B$ is a more complex function of both luminosity and temperature.

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... possible⁶

Clearly, instrumentation will be designed so that the combination of detector and filters matches the target system as closely as possible. However, there are a number of potential pitfalls. One is that most of the older photometric systems were originally set up using photoelectric detectors. Modern CCDs are usually relatively more sensitive in the red and less in the blue than photoelectric detectors (see Figure ). Thus, a CCD detector will usually use a different or additional set of filters to match a given system than a photoelectric detector. Another potential problem is that some filters are prone to `leakage'. Here the filter correctly blocks light at wavelengths surrounding the required passband but becomes transparent again at very different wavelengths (so, for example, a filter which correctly defined the $R$ band might also leak light at much shorter wavelengths, perhaps corresponding to the $U$ or $B$ bands). If leakage occurs it is necessary to use an additional filter, a so-called blocking filter, to remove the extraneous light. The choice of filters to match photometric systems is far beyond the scope of this cookbook and as an observer using CCD detectors neither will it normally concern you. However, it is useful to be aware of some of the potential problems.

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... aperture)⁷

The name of the technique comes from an earlier generation of astronomical instrumentation when photometry was carried out with a single-element photoelectric photometer. A circular aperture was placed in front of the photometer to limit its view of the sky. Using software to define a circular region in a CCD frame mimics the effect of a physical aperture limiting the field of view of a single-element photometer.

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... average⁸

Though it is simplest to think of determining the average sky background level, in practice a straightforward mean is often not a good measure of the sky background, because of contamination of the chosen region by faint stars. Instead techniques are often used which minimise the effects of outlying values in the sky background histogram, such as computing the median instead of the mean.

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... star⁹

Strictly speaking it is not necessary to use a standard star; any star can be used as long as it is not variable on the time-scale of a night's observing.

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... night¹⁰

Obviously the atmospheric extinction depends on the prevailing atmospheric conditions. An unusual example of variation caused by atmospheric conditions is the disruption following the eruption of Mount Pinatubo, as described by Forbes et al.[27].

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... ideal¹¹

Strictly speaking you must use display software which accesses the Starlink graphics database (see SUN/48[21]). However, you will not normally be aware of the graphics database and certainly do not need to know anything about it. It is simply a mechanism which allows different applications to co-operate in using the same plot.

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... table¹²

An image displayed with the lutneg colour table mimics the appearance of a conventional astronomical photographic plate: stars appear as dark spots on a light background. Various other colour tables are available in KAPPA. For example, lutgrey sets up a positive grey-scale (light stars against a dark background) and lutheat sets up a pseudo-heat sequence.

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... galaxies¹³

The intensity profiles of the images of extended objects usually fall off more slowly with increasing radius than those of stars and hence when working with extended objects it is necessary to be careful to choose an aperture sufficiently large to include the required fraction of the total light from the object.

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...PHOTOM_RECIP)¹⁴

Indeed, technically GAIA is acting as a `front-end' to the PHOTOM application autophotom. However, as a user you will not normally be concerned with these details.

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... catalogues¹⁵

In this recipe, and more generally in CURSA, the terms `catalogue' and `table' are usually used interchangeably.

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... absorption¹⁶

Total absorption in, say, the V-band would usually be written as A_Vand known as the visual extinction.

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... keywords¹⁷

In this context, a keyword is simply the name of each datum or item of information. For example, the keyword for the air mass might be `AIRMASS'.

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... FITS¹⁸

The original FITS format was proposed by Wells et al.[79] in 1981. However, it has been developed and enhanced over the years. The FITS standard is now maintained and documented by the FITS Support Office of the Astrophysics Data Facility at the NASA Goddard Space Flight Center (see URL: http://fits.gsfc.nasa.gov/fits_home.html ). Though FITS is basically an astronomical format it is sometimes mentioned in books about standard image formats. See, for example, Graphics File Formats by Kay and Levine[47].

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