The new Alan Gee II telecompressor is suitable for all SC telescopes. With its variable reduction factor of f3.5 to f 5.9, it provides major advantages for CCD systems.
The wish to have 'two telescopes in one' is a very old one. But, due to the laws of optics, all approaches to this are necessarily a compromise. So a telecompressor needs to illuminate the 24x36 (35mm) format as well as possible (= low vignetting) and keep image errors - especially the inevitable astigmatism - to a minimum. An additional requirement is that eyepiece-side accessories with long optical path lengths, such as star diagonals and bino-viewers, can also be used. Standard telecompressors from the big manufacturers are designed for the much smaller distance to the film plane and so are not suitable for use with bino-viewers.
Due to its design, the Alan Gee II telecompressor allows the lens system to match the accessory used. For this, it is necessary, as in the photographic application, to insert the multi-coated achromat into the baffle tube of the telescope, or - just as with the visual use of bino-viewers - to mount the lens cell along with the lens holder immediately in front of the entrance pupil of the bino-viewers.
The mechanical design takes into account the need to take full advantage of the centre hole through the primary mirror with the baffle tube - but to still hold the optics securely. In spite of the thin wall thicknesses resulting from this, the individual barrels are securely connected via fine threads.
According to the recommendations of Roland Christen (AstroPhysics) to Alan Gee, the new telecompressor - specifically designed for Schmidt-Cassegrain telescopes - has an air-seperated doublet, flat image field and 259mm focal length; it should be positioned, according to the theory, maximumally at half the focal length, in front of the focal point of the eyepiece or the image plane of the camera in order to achieve the best level of correction. Christen indicates that the ideal image distance from the camera-side lens surface to the focal point (field stop / film plane or similar) is 121mm. The individual barrels are therefore differently combined, according to the accessories used (or also partly omitted).