The Mid-Infrared Instrument (MIRI) is one of four scientific instruments on the James Webb Space Telescope (JWST)
observatory, scheduled for launch in 2018. It will provide unique capabilities to probe the distant or deeply dust-enshrouded
regions of the Universe, investigating the history of star and planet formation from the earliest universe to
the present day. To enable this the instrument optical module must be cooled below 7K, presenting specific challenges
for the environmental testing and calibration activities.
The assembly, integration and verification (AIV) activities for the proto-flight model (pFM) instrument ran from March
2010 to May 2012 at RAL where the instrument has been put through a full suite of environmental and performance tests
with a non-conventional single cryo-test approach.
In this paper we present an overview of the testing conducted on the MIRI pFM including ambient alignment testing,
vibration testing, gravity release testing, cryogenic performance and calibration testing, functional testing at ambient and
operational temperatures, thermal balance tests, and Electro-Magnetic Compatibility (EMC) testing. We discuss how
tests were planned and managed to ensure that the whole AIV process remained on schedule and give an insight into the
lessons learned from this process. We also show how the process of requirement verification for this complex system
was managed and documented. We describe how the risks associated with a single long duration test at operating
temperature were controlled so that the complete suite of environmental tests could be used to build up a full picture of
The high reliability of the mechanisms of any space instrument is one of the most critical and challenging requirements.
This is even more pronounced in the case of cryogenic instruments, such as the Mid-Infrared Instrument (MIRI) to be
flown on the James Webb Space Telescope (JWST) – which will be cooled down to below 7 K. MIRI hosts three wheel
mechanisms for filter, grating and dichroic selection. All of them have an open loop torque drive and thus the precise
characterisation of the mechanisms and their motors is fundamental to achieve minimum heat load and maximum
reliability of the mechanism movements over the lifetime.
In this paper we present the overview of the characterisation and verification of the MIRI wheel mechanisms. Our
method is based on measuring back EMF voltages generated by the two phase cold redundant motors of the wheel
mechanisms after they had been fully integrated into the MIRI optical module. We present the analysis of the data and
the resulting performance increase. We discuss the optimisation of the open loop drive, as well as the verification of the
measurement results and the physical model of the motors and mechanisms.
The Verification Model (VM) of MIRI has recently completed an extensive programme of cryogenic testing, with the
Flight Model (FM) now being assembled and made ready to begin performance testing in the next few months. By
combining those VM test results which relate to MIRI's scientific performance with measurements made on FM
components and sub-assemblies, we have been able to refine and develop the existing model of the instrument's
throughput and sensitivity.
We present the main components of the model, its correlation with the existing test results and its predictions for
MIRI's performance on orbit.