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Using our general method for evaluating the transformability of passbands (Young, A.T. 1994 A\&AS 288, 683), we compute the transformability of instrumental passbands for CCD imaging in four commonly used filter systems at KPNO, CTIO, ESO/La~Silla, and MPG/Calar Alto. We compare Johnson--Cousins UBV(RI)$_C$ instrumental passbands to standard passbands provided by M. Bessell; Washington to the standard photoelectric passbands by Canterna, et al.\ (1979); Str\"{o}mgren to the standard passbands; and Gunn to Palomar PFUEI standard passbands provided by J.A.Biretta. We examine most possible filter-detector combinations. There are considerable variations in transformability universally among photometric systems, among observatories in the same photometric system, and within an observatory in the filter-detector combinations.
The broad-band Johnson--Cousins and Washington systems are far more transformable than the intermediate-band Str\"{o}mgren and Gunn systems. There are particular problems within the Johnson--Cousins and Washington systems, however. Many B filters are hardly transformable, and I is only marginally better, with great variability. At CAO the I filters are not transformable to Cousins I. Washington T$_1$ filters at KPNO and CTIO are not transformable, which is awkward because this is the usual magnitude for this system. The Washington C filters at KPNO and CTIO are far less transformable than the ESO C filter. The poor transformability of C and T$_1$ at KPNO and CTIO is unfortunate in that the C-T$_1$ color index is commonly used at these observatories to estimate the metallicity of globular clusters in GCSs from their integrated light.
We demonstrate the difference in transformability between well-chosen filter-detector combinations and poor ones using synthetic colors. We also provide guidance to observers in the selection of the best filter-detector combinations with which to obtain transformable photometric data.
