| This three-piece Schüler photometric filter set is a popular low-cost option for tri-color astronomical CCD imaging. They can also be used for photometry. The American Association of Variable Star Observers (AAVSO) orders the V (visual) filter for its members to assure a uniformity of variable star brightness measurements from member to member. The filters are parfocal. They are mounted in 1.25" diameter threaded aluminum eyepiece holders that fit most CCD filter wheels, such as those manufactured by Santa Barbara Instrument Group (SBIG). Each filter is supplied in its own protective plastic case. The filter set includes a blue, visual (green), and red filter. Based upon the Johnson-Cousins broad band design, Schüler filters define their passbands by assembling a composite stack of a variety of Schott colored glasses to absorb unwanted portions of the light spectrum, rather than using costly ion-deposited interference (dichroic) coatings to reflect back the unwanted light. The spectral transmission characteristics of peak transmission, center wavelength, bandwidth, and depth and range of blocking, are determined by the absorption properties of the Schott glass materials themselves. Transmission spectral characteristics may vary slightly from lot to lot. However, the Schott standards and tolerances are considered good enough for almost all photometric astronomical observations. A significant advantage to making these filters using Schott colored absorption glasses is that they are usually lower in cost than similar interference-coated filters. Unlike dichroic filters, colored glass filters do not change their spectral shape with the changing slope of the light cone of different telescope focal ratios. You will get the same spectral response with these filters whether you are using the gentle light cone slope of an f/10 scope or the faster converging cone of an f/5 telescope. These photometric colored glass filters do not reach the high peak transmissions above 90% of dichroic interference filters. The blue filter has almost no sensitivity to doubly ionized oxygen (OIII) emissions. As such, any planetary nebula imaged with these filters will result in a nearly pure green central region rather than the correct blue-green teal color. The hydrogen-alpha and sulfur emissions captured by the red photometric filter are at about 60%, whereas they are >90% in most tri-color RGB dichroic filters. In addition, the response of the red photometric filter extends well into the near-infrared. This is not to say that these filters are inefficient. Their passbands are quite broad and the overall efficiency of these filters (V + B + R) is actually higher than many of the dichroic filters available to imagers.
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