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© 2021 by Grant Matthews
Design for CERES instrument replacement Libera
The CERES instruments on the ground and in-flight suffer UV telescope degradation that is un-trackable by onboard calibration once placed on-board the spacecraft. In 2020, the replacement for CERES called Libera was proposed, to perform ERB measurements in the coming decades. It is vital that Libera be designed differently to counter these calibration issues. Here a concept is presented that neglects the use of onboard SW sources or solar diffusers. This is because they have failed to work on any of the 13 different ERB scanning instruments from NIMBUS7, ERBE, CERES & GERB programs since the 1970's.
Left: The CERES instrument with solar diffusers and lamps. Right: Zedika design for Libera without solar diffusers and lamps, replaced with narrowband interference filters.
Instead this, the only peer reviewed concept uses direct views of spectrally filtered sunlight, with narrowband interference filters, as on the right of the instrument diagrams, showing original CERES and Zedika Libera concepts compared.
By far the best orbital SW calibration source for Earth observing instruments is the Sun. However, the challenge to overcome is that the Sun is around 5 orders of magnitude brighter than the Earth. Presented in Figs (a) - (d) is a way to attenuate the Sun spectrally with narrowband filters and precision masks of around 16% throughput (N).
© 2021 by Grant Matthews
(a) Narrowband 410nm filters throughput compared to solar spectrum. (b) Libera filter optical train on orbit 1, giving results V1. (c) Libera filter optical train on orbit 2, giving results V2. (b) Libera filter optical train on orbit 3, giving results V3.
The mathematical theory of this is detailed in an AMS JOAT paper (see publications). The principal is this:
G: is the unknown Libera gain at the filter wavelength and what we want to know
F: is the known solar surface radiance at the filter wavelength (from TIM/TSI)
N: is the known fractional mask throughput measured on the ground (approx 0.16)
V1: is the orbit 1 known Libera signal in setup of Fig.(b) above
V2: is the orbit 2 known Libera signal in setup of Fig.(c) above
V3: is the orbit 3 known Libera signal in setup of Fig.(d) above
G = (V2 x V3)/(V1 x F x N) in Volts per W/m .Sr.um
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