The application of synthetic spectra to colour system transformations

Synthetic spectra and the photoelectric and various photographic UBV sensitivity functions are used to compute theoretical colours for F and G dwarfs. Similar calculations are made for hotter stars. It is shown that photographic V magnitudes obtained using 103a-G emulsion and a GG II filter will be in better agreement with photoelectric V magnitudes than is the case when the 103a-D emulsion is used. The photographic and photoelectric U and B magnitudes are in good agreement. It is also shown that the use of S20 photocathodes for direct photography with image tubes will introduce large colour corrections terms if V magnitudes are to be measured, unless a BG18 filter is used in addition to the GG II filter.