| Different CCD cameras have different spectral response curves. Because of this, it is vital that you use proper photometric filters when observing variable stars with a CCD camera for the American Association of Variable Star Observers (AAVSO), as unfiltered CCD observations are generally not accepted by the AAVSO. Using the appropriate filter on your CCD equalizes its passband to the response of all other similarly-filtered CCD chips. This allows your data to be added to that of other CCD observers without the magnitude scatter typical of visual observations (due to the sensitivity of the human eye varying from individual to individual).
Schüler UVBRI filters are a popular, low-cost choice for a variety of astronomical photometric measurements. This Schüler IR (infrared) filter is an IR pass filter, not an IR blocking filter. It has a spectral response centered in the very near infrared range. The human eye cannot see this portion of the spectrum so this IR filter may look opaque when held up to the light. Using an IR filter alone has limited value, as most CCD magnitude measurements requires the comparing the light flux of comparison stars through two or three filters. However, an IR filter by itself is very useful for timing the secondary minima of Algol-type eclipsing binaries. Since it is the cool star that is being covered during the secondary eclipse, you get a much deeper minimum in IR than in V. For the fast-reacting CCD imager, an IR afterglow is often reported after a GRB (Gamma Ray Burst) event, which this filter may help record.
The AAVSO has a program to monitor a limited number of selected stars in the B (blue), V (visual), R (red), and I (infrared) ranges using filtered CCD observations. The program is designed to let the AAVSO see how well data can be combined from different observers with different telescopes, cameras, and software. The stars for which the AAVSO has BVRI finder charts include S Per, U Ori, VX Gem, DH Dra, VX UMa, W Leo, RU Vir, and RR Boo.
The 5-piece Schüler UVBRI filter set (of which the filter is a part) is used to measure and approximate stellar color temperatures, as well as make the BVRI measurements mentioned above.
Based upon the Johnson-Cousins design, Schuler photometric filters are a composite made from several dyed-in-the-mass Schott glasses. Unlike dichroic (surface coated) filters, mass-colored glass filters do not change their spectral shape as the focal ratio of the telescope changes. You will get the same spectral response through the filter whether you are using an f/10 or f/5 telescope. The filter is supplied in an aluminum cell with standard 1.25" filter threads that fit the 1.25" nosepiece of all CCD cameras. A snap-lid plastic case protects the filter when it is not in use.
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