Abstract
In this thesis the performance of the GOMOS instrument
for the ENVISAT Earth observing satellite of the European
Space Agency has been studied. The emphasis has been on
the characterization of the critical subsystem consisting
of the main telescope, the two monochromator options, and
the CCD-detector. Three test systems have been designed,
tested, and used for the performance analysis of GOMOS: a
Spectro-A Bench Model, a CCD characterization system, and
a measurement system for a holographic imaging grating.
On the basis of the Bench Model and the CCD detector, a
ground-based star spectra measurement system called
Startrek was designed and built.
The first setup, a Bench Model of one channel of the
GOMOS instrument, was used to measure the imaging quality
of this medium resolution spectrometer. However, the
performance of the Bench Model as implemented here was
not yet sufficient for GOMOS because of the difficulties
in producing the mirrors of the instrument. The
theoretical performance is quite good and should be
attainable with more careful production of the mirrors.
The measured spectral resolution and transmittance were
adequate for use as a ground-based star spectrometer.
The characterization of the GOMOS back-up CCD-chip has
been carried out for the linearity, charge spill-over,
quantum efficiency, and sensitivity variation within one
pixel. The sensitivity variations within one pixel were
measured by a 2-dimensional scan, where the light spot
and the scan step were small compared to the pixel size.
The results showed large sensitivity variations within
each pixel. An analytical, 2-dimensional, spatial pixel
sensitivity function for the modeling of these variations
has been formulated for the first time for this type of
CCD on the basis of the measurement results. This
function can be used for correction of the modern
CCD-spectrometer data for instrument effects.
A system like the UVIS Spectrometer Breadboard developed
in this work has not previously been used for imaging
quality measurements of concave, holographic gratings. It
delivers ten times higher mechanical accuracy for the
integration of the grating into the spectrometer than the
geometrical data provided by the manufacturer. It was
used to measure spectral efficiencies and imaging
properties of two different gratings. The results of
imaging measurements were used in the evaluation of the
GOMOS instrument performance. According to these results
the spectral resolution of neither grating is adequate
for the instrument and further optimization of the
grating is necessary. The measured spectral efficiency of
the triangular profile grating meets the instrument
requirements at the UVIS-band.
The Startrek visible imaging spectrometer can be used for
an end-to-end concept validation of the star occultation
principle. It demonstrates that the pointing system of
the instrument keeps the image of a star inside the slit
and that the spectrometer measures the star spectra
successfully. The measurement data of bright stars has
been used for the validation of the data analysis
procedures of the satellite instrument and for total
ozone column measurements. The ozone retrieval using two
spectra measured at different altitudes has been
performed and the total ozone columns calculated from the
Startrek spectra were in agreement with the results of
TOMS satellite and the Russian Filter Instrument M-124
measurements made on the same day close to the Startrek
measurement site.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
|
Award date | 20 Dec 1995 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-4790-X |
Publication status | Published - 1995 |
MoE publication type | G4 Doctoral dissertation (monograph) |
Keywords
- optics
- detectors
- monochromators
- ozone
- stratosphere
- stellar occultation
- imaging spectrometers
- CCD-detectors
- remote sensing
- holographic gratings