Optics, detectors and ground-based star spectra measurements of the GOMOS Spectro-A Bench Model: Dissertation

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.