Setting up of standard ultraviolet passbands with CCD detectors

V. Straižys, R. Lazauskaitė
1995 Open Astronomy  
The possibilities to set up the ultraviolet passbands of the systems WBV, UBV &ná the Vilnius system with CCD detectors are investigated. It is shown that the ultraviolet passbands, set up with the standard filters, in the case of some CCDs are very different from the standard passbands. Instrumental color indices obtained with such CCDs cannot be transformed to the standard systems with sufficient accuracy since the transformation equations are non-linear and dependent on luminosity classes
more » ... interstellar reddenings. For each CCD, the individual ultraviolet filter should by adjusted. Unauthenticated Download Date | 3/12/20 12:12 AM Setting up of standard ultraviolet passbands 89 the air mass (if color indices are not reduced to outside the atmosphere or the reduction is not correct). The complexity of these relations is related to the band-width effect on the effective wavelengths of the ultraviolet passbands which depend in a complicated manner on temperature, luminosity, interstellar and atmospheric extinction. In its turn, this dependence is related with the height and form of the Balmer jump as well as with the steep and variable decline of the transmission curve of the atmosphere. All these complicated transformations might be avoided if the response function of the instrumental system were sufficiently close to the response function of the standard system. In this case the color equation is absent and the observer needs only to adjust the zero-point of the instrumental and the standard color indices. For achieving the identity of the instrumental and the standard systems, detectors with the same form of the sensitivity function and identical filters should be used. Unfortunately, in many cases observers pay too little attention to the response functions of their equipment. As a result, the complicated nonlinear transformations, depending on luminosity, interstellar reddening and air mass, become necessary. Since the luminosities and reddenings of the program stars usually are unknown, the observers are forced to use only approximate (usually linear or parabolic) transformations and the resulting color indices have errors which in some cases exceed the accuracy of the photoelectric measurements by several times. The classical example is the case with the ultraviolet color index U-B of the UBV photometric system. When the system was introduced, at least two wrong steps were done: (1) two ultraviolet systems (the 1950/51 winter system and the 1951 summer system) with different ultraviolet response functions were transformed into one by using a linear equation and (2) the U-B color indices were transformed to outside the atmosphere neglecting the dependence of the extinction coefficient on spectral class, luminosity and interstellar reddening. Due to these errors, the system of U-B color indices is ill-defined and contains systematic errors depending on air mass, declination and right ascension. As a result, a unique U-B system does not exist and all subsequent applications of the system contain its original drawback (Straizys 1973 (Straizys , 1977 (Straizys , 1983 (Straizys , 1992 . During the last decade, a new detector type, the CCD camera, became the usual instrument for stellar photometry. Due to high Unauthenticated Download Date | 3/12/20 12:12 AM
doi:10.1515/astro-1995-0108 fatcat:f7mp4vgvxfftpixc65nuvtx3re