Transformability of Imaging Filters in Four Photometric Systems at KPNO, CTIO, ESO, and CAO

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Session 3 -- Instrumentation and Techniques I: Ground Based
Display presentation, Monday, June 12, 1995, 9:20am - 6:30pm

[3.07] Transformability of Imaging Filters in Four Photometric Systems at KPNO, CTIO, ESO, and CAO

A.T.Young (UCSD), W.J.Roberts (CAS), E.K.Grebel (AI\,U\,Bonn), W.Brandner (AI W\"urzburg)

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.

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