The telecentric is often confused with a barlow. Both can be used to magnify the focal length. “Telecentric” is not a synonym for parallel beam. Telecentrics are designed so that the exit pupil lies 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 arrive at the image plane with the same angular geometry as the axial rays. All field elements look as if they where like they are on axis, across the image plane and unlike a barlow, 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 a f/30 telecentric refractor the etalon sees the exact same 2.5degree geometry clear across the field, and the spectral bandpass does not shift the wavelength across the field.[Download not found]