06 Oct 05Space Science & Technology Dept1 Solar Orbiter Consortium Meeting 03 Mar 06 Optical Design Of Solar Orbiter Normal Incidence Spectrometer KF Middleton Space Science & Technology Dept Rutherford Appleton Laboratory 03/03/06

06 Oct 05Space Science & Technology Dept2 Acknowledgements The optical design work presented here is a summary and analysis of a design developed by Roger Thomas (GSFC/NASA)

06 Oct 05Space Science & Technology Dept3 Concept Normal incidence telescope feeding torroidal variable line space (TVLS) grating spectrometer The TVLS grating provides additional degrees of freedom, allowing correction of spectrometer aberrations away from the unit magnification

06 Oct 05Space Science & Technology Dept4 Design Goals Ø70mm entrance aperture 800mm (optical dimension) instrument length <1 arc sec spatial resolution 50mÅ – 100mÅ spectral resolution (or better), with better resolution towards shorter wavelengths FOV 20 arc min square – scan to build up perpendicular to slit 10µm detector pixel 2k in spatial direction 2k+ in spectral direction (assume ‘stitched’ detector is possible) Choice of wavebands: –170Å – 210Å‘Short’ –580Å – 630Å‘Medium’ –970Å – 1040Å‘Long’

06 Oct 05Space Science & Technology Dept5 Waveband & Grating Order Options Option #Detector #1Detector #2 1Short n=3 Medium n=1 Long n=1 2Medium n=1Long n=1 3Short n=1Medium n=1 Notes: All 3 wavebands in n=1 is not possible as the grating aberrations cannot be corrected over the correspondingly large range of angles For option 1: 510Å – 630Å is possible in first order if 170Å – 210Å is observed in 3 rd order on the same detector Concentrate on option 1 in this presentation, as it allows all 3 wavebands to be observed

06 Oct 05Space Science & Technology Dept6 Calculation of Resolution Spectral and Spatial Resolution are calculated as follows: Where: w = slit width, mag = grating magnification, l pixel = pixel size, Δl = geometric spot diameter, l dl = diffraction limited spot diameter, D = dispersion (Å/mm)

06 Oct 05Space Science & Technology Dept7 Mirror Scanning The mirror needs to pivot about the slit, so that the slit always remains on the axis of the primary mirror, otherwise off-axis aberrations degrade the imaging quality unacceptably

06 Oct 05Space Science & Technology Dept8 Optical Performance – Option 1 Short n=3, Medium n=1 / Long n=1

06 Oct 05Space Science & Technology Dept9 Optical Performance – Option 1 Short n=3, Medium n=1 / Long n=1 Notes: Stitched detector 2096 x um pixels assumed n=3 order in short waveband  dispersion in n=3 is 1/3 of dispersion in n=1  spectral resolution in n=3 is improved by factor ~3 relative to n=1

06 Oct 05Space Science & Technology Dept10 Related Technology Detectors –This design assumes APS detector (not visible-blind) plus aluminium filter (to suppress the visible) for the short and medium wavelengths MCP (visible-blind) for the long wavelength (aluminium cannot be used as a visible filter at the long wavelength) Coatings –The use of a mulitlayer which provides reflectivity at the short waveband and at the medium and long wavelengths is assumed. Such coatings are possible in theory (e.g. see B Kent’s presentation at the 1 st consortium meeting) but need testing in practice

06 Oct 05Space Science & Technology Dept11 Conclusions A three waveband design is possible in principle, as long as: –Problems with confusion between spectral lines of different orders can be overcome –Appropriate detector technology is available to allow sensing across all three wavebands –A multilayer coating can be proven to withstand the thermal and radiation environment