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Blocking filters
BTW the blocking filters used in the PST and the SM series are nothing more than a narrow bandwidth Ha filter.
The nature of the etalon forms a multiple image of the sun in the Ha and the "resonant" images and slightly varying wavelengths ( at either side of the primary image) must be "blocked" from getting through to the eyepiece/ camera. The narrowbandwidth of the BF effectively does this.
The size of the BF is directly related to the focal length of the scope and the final image size of the solar disk, ie a BF5 is designed for a 500mm focal length, the BF10 for a 1000mm FL etc. Coincidently the size of the full solar image is about 5mm in a 500mm FL!!
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