CFW-402-BVI

The photometric filter wheel with custom filters designed for the ST-402ME consists of B, V, and I band filters, and a clear filter.  The filters are high quality, parfocal, AR coated photometric filters made by Custom Scientific to SBIG\'s specifications.

Photometric Filters for ST-402ME

Although priced as an entry level CCD camera, the ST-402ME is an exceptionally sensitive and capable instrument. The KAF-0402ME CCD is a scientific grade full frame sensor with the same quality, quantum efficiency, and performance of sensors used in cameras costing many thousands of dollars more. Except for its size (field of view), the CCD characteristics are essentially the same as the main sensors used in the STT-1603ME. It has very high quantum efficiency, high linear dynamic range, full well capacity of 100,000e-, dark current of 1e- or less at 0C, and typical read noise less than 15 e-. The ST-402ME now has an enhanced cooling package and photometric filter wheel option, giving performance similar to our larger cameras but at a significant savings. The ST-402ME also uses a fast USB 2.0 interface with full frame downloads in less than 1 second. The ST-402ME is an excellent choice for making high precision photometric measurements where its extraordinary sensitivity can be exploited with professional results.The ST-402ME has an internal filter wheel that was originally designed with custom RGBC filters for color imaging. When installed, the filter carousel replaces the shutter wheel and acts as both filter wheel and shutter. Due to the filter/shutter wheel design, the filters must be very thin, about 1mm, and only 85mm in diameter. They are also permanently mounted to the carousel. This means it is not possible to add \"off-the-shelf\" photometric filters to the camera without buying an external filter wheel because most photometric filters are about 5 mm thick and made for 1.25\" filter cells. There is no provision for adding standard 1.25\" filters inside the camera, although if one is using only one filter, such as a V filter, it can be screwed into the nosepiece for single color measurements.

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In response to requests for an easier and lower cost photometric option, we have designed a photometric filter set permanently mounted on a replacement internal carousel for the ST-402ME. By saving on the cost of an external filter wheel and larger filters we can offer the ST-402ME Photometric camera for over $1000 less than the same or similar system with external filter wheel. In the 1950\'s Harold Johnson (Yerkes and Macdonald Observatories) established three photometric bands, the U, B, and V based on the sensitivity of the photomultiplier tube that he used at the time. Later on he established red and infrared (R and I) bands using a PMT with enhanced red sensitivity. In the mid-1970\'s, A.W.J. Cousins and John Menzies (South African Astronomical Observatory) used different filters, that when used with a newer, better detector, would reproduce the Johnson bands. Then, in the 1980\'s CCD detectors were beginning to replace the photomultipliers so a new set of filters was required that, when used with the CCD\'s would give the same results as the older filters when used with the PMT. Bessell (Mt. Stromlo and Siding Spring Observatories in Australia) did this in 1990 (PASP, 102, 1990, 1181). It is Bessell\'s filter definitions that are the industry standard today when using CCD\'s.

Based upon the recommendations of Arne Henden, Director of AAVSO, the photometry set designed for the ST-402ME consists of B, V, and I band filters, and a clear filter. The filters are high quality, parfocal, AR coated photometric filters made by Custom Scientific to SBIG\'s specifications. Because the typical photometric filter is made from thick glass that absorb the out of band light, a custom curve is necessary for the I filter in order to make this filter set only 1 mm thick for use in the ST-402ME filter wheel and still match the performance of an equivalent CCD and Bessell filter. The B and V filters are identical to the Bessell filter curves. If one compares the shape of the curve of the custom I filter to the standard Bessell I filter, the custom filter has a sharper cutoff in the near IR whereas the Bessell filter continues to pass light well beyond 1000nm. However, when the QE curve of the camera is taken into account, there is virtually no difference in spectral response between the custom I filter and the Bessell filter. This assures that data collected with the ST-402ME+BVI filters will be consistent with other systems using the Bessell standard filter definitions.

The clear filter is AR coated and unblocked. The BVI carousel can be easily installed by the user and exchanged with a plain shutter or an RGBC filter carousel at any time.

[Note: The filter carousel is designed to work only with the ST-402 version of this camera, for the ST-1603 and ST-3200 an external filter wheel is required due to the size of the sensors.]