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|Title: ||Characterisation of the MIRI spectrometer, an instrument for the James Webb Space Telescope|
|Authors: ||Briggs, Michael|
|Supervisor(s): ||Wright, Gillian|
|Issue Date: ||2010|
|Publisher: ||The University of Edinburgh|
|Abstract: ||The MIRI-MRS is a future space based Medium Resolution Spectrometer and
one of four instruments to be integrated onto The James Webb Space Telescope.
The Medium Resolution Spectrometer is designed to be diffraction limited
across its entire passband of 5 - 28.3 microns. It achieves this through the
spectral filtering of the passband into four channels with each one containing
an integral field unit optimised for minimal diffraction losses. The integral
field unit enables the simultaneous measurement of the spectral data across
the entire field of view.
The design of the Medium Resolution Spectrometer is outlined with particular
reference to the choice of slice widths used for each channel to minimise the
diffraction losses from the slicing mechanism. The slice widths are also used
to derive the extent of the field of view and combined with the along slice
plate scale at the detector the technique required for complete spatial sampling
of the spectrometer is outlined. The operation of the Channel 1 image slicer
component was tested cryogenically at 5 microns for diffraction losses due to
the slicing of the point spread function. This was so that the actual diffraction
losses could be measured and compared with the optical model. From the
resulting analysis I concluded that the operation of the image slicers were well
understood for diffraction losses.
Performance tests were required on the instrument because of its novel design.
This was the first implementation of an integral field unit operating between
5 - 28.3 microns and it was necessary to ensure that the operation of the image
slicer did not induce unacceptable diffraction losses into the instrument.
Tests were required on the assembled instrument to verify the optical design.
A Verification Model of MIRI was built to enable test verification of the optical
design. This testing was carried out in advance of the MIRI Flight Model assembly
so that changes could be made to the Flight Model design if necessary.
This testing phase was also designed to define the calibration process necessary
to prepare the MIRI Flight Model for scientific operations. For the testing
phase it was necessary to create an astronomical source simulator. This MIRI
Telescope Simulator was constructed in Madrid where I spent two months ensuring
the point source movement across the field of view would be sufficient
to investigate the Medium Resolution Spectrometer.
My contribution was to help assemble both the Verification and Flight Models.
I also participated in the Verification Model testing phase from the test design
phase to the test implementation and data analysis. My role in the analysis was
to investigate the field of view of the Medium Resolution Spectrometer Verification
Model and whether the field of view requirements for the spectrometer
were met. During this analysis I also verified that the diffraction effects of the
end-to-end instrument were well understood by the optical model.
The Medium Resolution Spectrometer Verification Model field of view compromised
the field of view requirement for the spectrometer. A similar analysis
for the Flight Model showed that there would be a low probability that the
field of view requirement would be met. As a result of the analysis I defined a
new slit mask design that would align the field of view sampled by Channel 1
to increase the aligned field of view. As a result there is a high probability that
the field of view requirement for the Flight Model will be exceeded.
The test analysis discovered a magnification effect within the spectrometer
which must be properly characterised to enable accurate field of view reconstruction.
I designed a test necessary for the calibration phase of the Flight
Model to enable full spatial alignment of the Medium Resolution Spectrometer.
I also measured an excess flux level in the Channel 1 observations at the
detector and there was a ghost detected in the Channel 1 images. Whilst the
origin of either the excess flux or the ghost could not be completely determined
I investigated the possibility that they will not be present in the Flight Model
due to the slight design differences. If present however they will not increase
the background level of an observation above the requirement outlined for
|Keywords: ||Medium Resolution Spectrometer|
James Webb Space Telescope.
minimal diffraction losses
|Appears in Collections:||Physics thesis and dissertation collection|
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