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The Lyman Alpha Photometer (LAP), developed for flight on the Mars Orbiter Mission (MOM) spacecraft in 2013, is primarily designed to measure deuterium to hydrogen abundance ratio of the Martian exosphere over a 6-month period from a 263 km  71,358 km elliptical orbit around Mars. A set of ultrapure (99.999%) hydrogen and deuterium gas-filled cells comprising tungsten filaments, a 25 mm diameter collection lens and a solar-blind photomultiplier tube together with an 8 nm bandpass Lyman alpha filter are the principal electro-optical assemblies of the instrument. This article presents scientific objectives of LAP and its performance specifications along with details of instrument design. The ground characterization techniques to assess LAP operational performance are also presented. End-to-end test results and evaluation matrix of LAP were satisfactory, well within the desired specifications. The first LAP onboard operation was carried out during the cruise phase of MOM spacecraft journey to verify its functionality and all recorded on-board health parameters were satisfactory.

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