Grazing-incidence design and others L. Poletto Istituto Nazionale per la Fisica della Materia (INFM) Department of Electronics and Informatics - Padova (Italy) EUS Meeting 4 November 2004

Performance evaluation of any spectroscopic design Three optical parameters are calculated in the evaluation of the performance: 1) the spatial resolution in the direction perpendicular to the slit 2) the spatial resolution in the direction parallel to the slit 3) the spectral resolution PARAMETER NO. 1 DEPENDS ONLY ON THE OPTICAL PROPERTIES OF THE TELESCOPE PARAMETERS NO. 2 DEPENDS ON THE PERFORMANCE OF THE WHOLE INSTRUMENT (TELESCOPE + SPECTROMETER) PARAMETERS NO. 3 DEPENDS ON THE PERFORMANCE OF THE SPECTROMETER telescope slit spectrometer detector

The grazing-incidence Wolter telescope Grazing-incidence telescope two concave mirrors (parabola and hyperbole) and a plane mirror rastering: rotation of the plane mirror CHARACTERISTICS nm spectral region 18 arcmin  18 arcmin field-of-view length <1 m

Grazing-incidence design: characteristics TelescopeWolter II Focal length1200 mm Incidence angles72 deg - 74 deg Slit Size6  m  6.3 mm Resolution1 arcsec GratingTVLS Groove density3600 lines/mm Entrance arm300 mm Exit arm900 mm Spectral region nm Detector Pixel size9  m  18  m Format2200  1100 pixel Spectral resolving element 28 mÅ (14 km/s) Spatial resolving element 1 arcsec (150 km at 0.2 AU) Instrument length 1000 mm

Grazing-incidence design: performance

Efficiency at 60 nm Total efficiency at wavelength E TOT ( ) = A [cm 2 ]  E( )  PS [arcsec 2 ] A EF entrance aperture E( )combined efficiency (telescope, spectrometer, detector) at wavelength PSpixel size CDS on SOHO, NIS2 channelE TOT_CDS (60 nm) = Grazing-incidence design at 60 nmA EF = 25 cm 2 = 5 cm × 5 cm Si-Au coated opticsR mirrors = 0.62, 0.67, 0.78 E grating = 0.15 E detector = 0.30 E TOT (60 nm) = 0.36 = 8 TIMES CDS EFFICIENCY Normal-incidence design at 60 nmA EF = 25 cm 2 SiC opticsR mirror = 0.30 (high thermal absorption in the visible and near IR) The same efficiency as the GI design is obtained with A EF = 25 cm 2 = 5 cm × 5 cm Au opticsR mirror = 0.12 (low thermal absorption) The same efficiency as the GI design is obtained with A EF = 65 cm 2 = 8 cm × 8 cm

Thermal load: GI versus NI Grazing-incidence configuration: 5 cm × 5 cm Thermal load85 W Power absorption on the mirrors42 W - 20 W ( solar constants) Power density on the slit plane4 solar constants (f = 1200 mm) Normal-incidence configuration: SiC optics, 5 cm × 5 cm Thermal load85 W Power absorption on the mirror54 W (16 solar constants) Power density on the slit plane16 solar constants (f = 600 mm) Normal-incidence configuration: Au optics, 8 cm × 8 cm Thermal load218 W Power absorption on the mirror35 W (10 solar constants) Power density on the slit plane170 solar constants (f = 600 mm)

Some considerations on the entrance filter As proposed in the Astrium Payload Integration Study, an entrance filter could reduce to zero the thermal load on the optics. A suitable filter for the 60 nm region is a thin Al foil (200 nm, 0.6 transmission) VERY RISKY SOLUTION: single point failure FEASIBLE ? Grazing-incidence configuration The filter is on the entrance aperture Thermal load on the filter 25 solar constants on the Al foil Normal-incidence configuration The filter is inserted at the end of the entrance tube (0.8 m) 20 solar constants on the Al foil NO FILTER AVAILABLE ABOVE 90 NM

Alternative configuration: GI grating + NI design The thermal load on the focusing optics is reduced to zero if the visible and near IR radiation is deflected by a suitable first optical element out of the entrance aperture of the mirror.  THE DIVISION BETWEEN THE EUV RADIATION AND THE VISIBLE RADIATION IS PERFORMED BY A PLANE DIFFRACTION GRATING ON THE ENTRANCE

Alternative configuration: characteristics Entrance aperture Size50 mm × 70 mm Grating Incidence angle75 deg Groove density360 gr/mm CoatingAu Size195 mm × 70 mm Rastering Grating rotation TelescopeParabola Focal length650 mm Incidence angle1.5 deg Size30 mm × 70 mm Slit Size5.2  m  0.9 mm Resolution1 arcsec GratingTVLS Groove density4800 lines/mm Entrance arm150 mm Exit arm930 mm Magnification6.2 Size10 mm  20 mm Dispersion0.16 nm/mm Spectral region nm Detector Pixel size17.5  m  5  m Format1780  1100 pixel Spectral resolving element 28 mÅ (14 km/s) Spatial resolving element 1 arcsec (150 km at 0.2 AU) Instrument length 950 mm

Alternative configuration: performance

Alternative configuration: efficiency at 60 nm Total efficiency at wavelength E TOT ( ) = A [cm 2 ]  E( )  PS [arcsec 2 ] A EF entrance aperture E( )combined efficiency (telescope, spectrometer, detector) at wavelength PSpixel size CDS on SOHO, NIS2 channelE TOT_CDS (60 nm) = Alternative configurationA EF = 35 cm 2 E grating_1 = 0.35 R mirror = 0.30 E grating_2 = 0.15 E detector = 0.30 E TOT (60 nm) = 0.16 = 3.5 TIMES CDS EFFICIENCY

Alternative configuration: thermal load Entrance aperture: 5 cm × 7 cm Thermal load120 W Power absorption on the grating15 W (0.8 solar constant)  105 W can be simply rejected out of the instrument through a suitable aperture  only 15 W absorbed  NO THERMAL LOAD ON THE TELESCOPE

Alternative configuration: experimental observations Two wavelengths from the same spatial region are dispersed by the first grating and imaged by the telescope in different zones on the entrance slit plane.  THE SAME SPATIAL REGION IS OBSERVED AT DIFFERENT WAVELENGTHS IN DIFFERENT IMAGES (SO IN DIFFERENT TIMES)  ON THE SAME IMAGE, EVERY WAVELENGTH COMES FROM A DIFFERENT SPATIAL REGION OF THE SUN

Alternative configuration: conclusions ADVANTAGES No thermal load on the telescope, entrance slit and grating The coatings on the optics can be optimized for any spectral region in the EUV ( nm) Only 15 W have to be dissipated DRAWBACKS The efficiency is lower than the NI or GI configurations, but anyway higher than CDS The same spatial region is observed at different wavelengths in different times This effect can be mitigated by using multiple entrance slits OBSERVATION The requested resources (mass and envelope) can be minimized by a close integration with other instruments