C Effective Wavelengths

Most ASTROM reductions work in mean $\left[\alpha ,\delta \right]$ of date, absorbing various small rotations and distortions of the field into the fit. This normally delivers results of adequate accuracy. However, for slightly improved accuracy, especially at low elevations and with wide fields, ASTROM can optionally be supplied with additional information (an accurate date and time, observatory location, etc.) to enable reduction in observed place. The further possibility is then available of correcting for atmospheric dispersion, by supplying effective colours for the reference and unknown stars; this can sometimes be important.

The following tables estimate the effective wavelength for different spectral types and detector/filter passbands. The figures given are the median wavelength of the appropriate blackbody spectrum within the specified band, and are thus only a very rough guide. Some detectors peak strongly; some filters leak outside their nominal passband; both may roll off gradually at the edges of the passband; some stellar spectra have pronounced absorption or emission features.

The tables cover blue cutoffs from 320 nm and red cutoffs up to 800 nm, in steps of 20 nm and with a minimum passband of 100 nm, for 13 colour temperatures. B$-$V and U$-$V colours and the nearest main-sequence spectral types are also given, as listed in Table 99 of Astrophysical Quantities.

For any given exposure, only one line in the tables will be needed, corresponding to the spectral response of the detector plus filter used. The appropriate effective wavelength for any star in the exposure can then be found by looking in the appropriate column.