Why silver halide?
Much more effective for filtering out every last bit of aberration is a single-mode fiber, in which all higher modes soon end up in the cladding, where they can be absorbed or otherwise gotten rid of. The JPL/TAU group, which is developing modal filters for NASA’s TPF, chose silver halide as a fiber material for its excellent transmission in the 12 to 20 µm wavelength range. (The TPF will operate over the 6 to 20 µm range; the 6 to 12 µm portion of that range can be covered by a chalcogenide-fiber modal filter.)2 Mid-IR was chosen for the TPF for more than one reason. “The motivation to observe at mid-infrared wavelengths is threefold: the star-planet contrast is more favorable than at shorter wavelengths; there are clear biomarkers; and the optical tolerances of the observatory are relaxed,” says Alexander Ksendzov, one of the JPL researchers.
