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The TZ-4 Telecentric System offers 4x focal extension for a parallel optical beam, along with plentiful backfocus, in conjunction with any SolarSpectrum H-alpha filter.
As both Telecentric optics and barlow lenses are used for focal length expansion, they are often confused for each other. Telecentric systems are designed to have the exit pupil positioned at apparent infinity, which indicates that the center ray from any point in the field appears to originate from infinity and is therefore perpendicular to the image plane and parallel to the optic axis. This means the off axis beam arrives at the image plane with an angular geometry that is the same as the axial rays. All field elements appear to be on axis across the image plane, and, unlike a barlow lens, edge field rays are not tipped bundles.
Since all the principal rays over the image plane are perpendicular to the image plane, the rays at the edge of the field will move through an etalon just ahead of the focal plane with precisely the same geometry at the on axis rays. So for an f/30 telecentric optical system, the etalon detects identical geometry clear across the field, while the spectral bandpass does not shift in wavelength over the full field of view.
Beginning with an aperture ratio of approximately f/15 (+2x Telecentric), f/10 (+3x Telecentric), or f/7.5 (+4x Telecentric), Baader TZ focal extension systems generate a parallel beam with a ~f/30 aperture ratio. For telescopes with a large difference in focal lengths, the clear objective aperture of the telescope should have its diameter lessened to the point that the end f/ratio is almost f/30 again.
Taken from their rear lens, TZ-system working distance measurements to the camera sensor are 200 mm for the TZ-2, 230 mm for the TZ-4, and 250 mm for the TZ-3. This provides adequate space for the H-alpha filter housing, as well as a wide variety of accessories like a Baader 2" mirror diagonal or any camera device you can dream up. All TZ-coatings are matched to the individual lens glass-substrates and enhanced for maximum 656 nm throughput.