The photometric filter wheel with custom filters designed for the SBIG STF-402M consists of B, V, and I band filters, and a clear filter. These are high quality, parfocal, AR coated photometric filters.
Photometric Filters for STF-402M
Although priced as an entry level CCD camera, the STF-402M 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. 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 STF-402M now has an enhanced cooling package and photometric filter wheel option, giving performance similar to our larger cameras but at a significant savings. The STF-402M also uses a fast USB 2.0 interface with full frame downloads in less than 1 second. The STF-402M is an excellent choice for making high precision photometric measurements where its extraordinary sensitivity can be exploited with professional results.
In response to requests for an easier and lower cost photometric option, we have designed an internal photometric filter wheel for the STF-402M. The BVIC filter wheel, when installed, the 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. The combined package costs $1000 less than an equivalent system with an external filter wheel.
Based upon the recommendations of Arne Henden of the AAVSO, the photometry set designed for the STF-402M consists of B, V, and I band filters, and a clear filter. 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 STF-402M 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 STF-402M+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: this filter carousel only operates with the STF-402M camera. STF cameras with larger sensors require an external filter wheel.