FLI Kepler KL6060 BI — System Throughput

Detector QE × filter × (optional) reducer × telescope × atmosphere  |  Sloan u′g′r′i′z′ + 3 nm narrowband  |  Figl 1.5 m RC (Al×2) · 900 m

QE: digitized from the FLI KL6060 BI datasheet “Absolute Quantum Efficiency” plot (GPixel GSense6060 BI).   Sloan filters: SVO Filter Profile Service, primed set (SLOAN/SDSS.{u,g,r,i,z}prime_filter).   Narrowband: idealized 3 nm FWHM flat-top, 90% peak, centred on [O III] 500.7, Hα 656.3, [S II] 672.4 nm.   Atmosphere: analytic model — Rayleigh (scaled to 900 m pressure) + ozone Chappuis (300 DU) + aerosol (AOD₅₅₀=0.08, α=1.3), per airmass. Smooth continuum only; discrete telluric bands (O₂ A-band 760 nm, H₂O) are not modelled.   Telescope: Figl 1.5 m RC = two aluminium mirrors, throughput R_Al(λ)² (M1×M2; mirrors aluminized on-site). The M1 and M2 sliders set each mirror's reflectance at 550 nm (max 92% = fresh evaporated aluminium; lower = tarnished/dusty); the telescope curve is their product R_M1·R_M2.   Reducer: the 0.7× focal reducer — cemented triplet (S-FPL51 / S-BAL42 / S-FPL51) + S-FPL51 singlet + S-BSL7 field flattener, 6 AR-coated air-glass surfaces (assumed good multilayer AR, ~0.5 %/surface). T_red = Ohara glass internal transmittance (the 112 mm S-FPL51 path dominates, Ti from the local datasheets) × coatings. It passes ~94–95 % across g′r′i′z′ and ~90 % at 436 nm, but collapses in the near-UV: thick S-FPL51 absorbs hard below ~360 nm, cutting u′ (355 nm) to ~57 % and blue-truncating the u′ band (reducer design band 436–706 nm; u′/z′ are extrapolations). The AR coating quality is the main lever for g′–z′ (±5–6 points).   Dark line = bare detector QE; dashed blue = atmosphere; dash-dot silver = telescope; dashed amber = reducer.