Telecentric optics sets are often confused with a barlow lens. Both can be used to increase the focal length. Telecentric assys are designed so that the exit pupil seemingly is positioned at infinity, which means that the center ray from any point in the field appears to come from infinity and is therefore perpendicular to the image plane and parallel to the optic axis. This means that the off axis beam does arrive at the image plane with the same angular geometry as the axial rays. All field elements look as if they where on axis, across the image plane and - unlike to a barlow lens - the edge field rays are not tipped bundles.
Because all the principal rays across the image plane are perpendicular to the image plane,the rays at the edge of the field will pass through an etalon just in front of the focal plane with exactly the same geometry as the rays on axis. So in an f/30 telecentric optical arrangement, the etalon sees the exact same geometry clear across the field, and the spectral bandpass does not shift in wavelength across the entire field of view.
Starting from an aperture ratio of ~f/15 (+2x Telecentric), ~f/10 (+3x Telecentric) or ~f/7.5 (+4x Telecentric), the Baader TZ-systems will create a parallel beam with ~f/30 aperture ratio. In the case of largely deviating telescope focal lengths, the clear objective aperture of the telescope should be reduced in diameter to the point that the final resulting f/ratio is close to f/30 again (we do offer a suitable iris-diaphragm for this purpose).
TZ-system distances to the camera sensor – when measured from their rear lens – range from 200mm for the TZ-2 to 230 mm for the TZ-4 - to 250 mm for the TZ-3. This provides enough room for the H-alpha filter housing and most any accessories, e.g. a Baader 2" mirror diagonal or any conceivable camera device. All TZ-coatings are matched to the different lens glass-substrates and optimized for maximum throughput at 656 nm.
