Measuring aperture

The wings of a telescopic image are due to surface scattering caused by microroughness of polished optical surfaces, and to (usually) a much smaller extent to scattering by dirt on the optics. Contrary to popular mythology, atmospheric effects are quite negligible. These wings contain some tens of per cent of the total starlight, for typical surface quality. If the measuring aperture varies, a varying amount of starlight will be excluded. Furthermore, the excluded fraction is wavelength-dependent; so the transformation from instrumental to standard system changes. Therefore, the field stop, physical or synthesized, within which the measurements were made, should always be constant.

Unfortunately, sometimes it is necessary to combine measurements made with different field stops. Observers should realize that this really means different instruments; the differences are usually several per cent. Calibration data should be taken (i.e., several stars of different colors observed with all the apertures used) to determine the transformations between them.

Usually, the actual aperture sizes are only approximately known. In any case, the actual variation of the excluded energy fraction with radius is not accurately predictable. Therefore, it suffices to retain codes for apertures, rather than try to deal with them quantitatively. The code should be put in a column labelled DIAPHRAGM (the common term for the field stop).