Download GI design - RAL Solar Orbiter

EUS Meeting
4 November 2004
Grazing-incidence design and others
L. Poletto
Istituto Nazionale per la Fisica della Materia (INFM)
Department of Electronics and Informatics - Padova (Italy)
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
58-63 nm spectral region
18 arcmin  18 arcmin field-of-view
length <1 m
Plane mirror for
rastering
Entrance slit
Hyperbolic mirror
From the Sun

Parabolic mirror

Detector
TVLS grating
Grazing-incidence design: characteristics
Telescope
Focal length
Incidence angles
Wolter II
1200 mm
72 deg - 74 deg
Slit
Size
Resolution
6 mm  6.3 mm
1 arcsec
Grating
Groove density
Entrance arm
Exit arm
TVLS
3600 lines/mm
300 mm
900 mm
Spectral region
58-63 nm
Detector
Pixel size
Format
9 mm  18 mm
2200  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
Resolution perpendicular to the slit
Resolution parallel to the slit
on-axis
2.0
on-axis
4.5'
2.0
4.5'
9'
detector
spatial resolution (arcsec)
slit
1.5
1.0
0.5
0.0
-9.0
-6.0
-3.0
0.0
3.0
6.0
1.5
1.0
0.5
0.0
-9.0
9.0
-6.0
-3.0
0.0
off-axis angle (arcmin)
off-axis angle (arcmin)
Spectral resolution
pixel (14 km/s)
slit image (28 km/s)
80
spectal resolution (mA)
spatial resolution (arcsec)
9'
60
40
20
0
58.0
60.5
wavelength (nm)
63.0
3.0
6.0
9.0
Efficiency at 60 nm
Total efficiency at wavelength 
ETOT() = A [cm2]  E()  PS [arcsec2]
AEF
E()
PS
entrance aperture
combined efficiency (telescope, spectrometer, detector) at wavelength 
pixel size
CDS on SOHO, NIS2 channel
ETOT_CDS(60 nm) = 0.046
Grazing-incidence design at 60 nm
AEF = 25 cm2 = 5 cm × 5 cm
Si-Au coated optics
Rmirrors = 0.62, 0.67, 0.78
Egrating = 0.15
Edetector = 0.30
ETOT(60 nm) = 0.36 = 8 TIMES CDS EFFICIENCY
Normal-incidence design at 60 nm
AEF = 25 cm2
SiC optics
Rmirror = 0.30 (high thermal absorption in the visible and near IR)
The same efficiency as the GI design is obtained with A EF = 25 cm2 = 5 cm × 5 cm
Au optics
Rmirror = 0.12 (low thermal absorption)
The same efficiency as the GI design is obtained with A EF = 65 cm2 = 8 cm × 8 cm
Thermal load: GI versus NI
Grazing-incidence configuration: 5 cm × 5 cm
Thermal load
85 W
Power absorption on the mirrors
42 W - 20 W (3.3 - 3.7 solar constants)
Power density on the slit plane
4 solar constants (f = 1200 mm)
Normal-incidence configuration: SiC optics, 5 cm × 5 cm
Thermal load
85 W
Power absorption on the mirror
54 W (16 solar constants)
Power density on the slit plane
16 solar constants (f = 600 mm)
Normal-incidence configuration: Au optics, 8 cm × 8 cm
Thermal load
218 W
Power absorption on the mirror
35 W (10 solar constants)
Power density on the slit plane
170 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
Zero order
Parabola
Detector
Plane GI
grating
TVLS
grating
Alternative configuration: characteristics
Entrance aperture
Size
50 mm × 70 mm
Grating
Incidence angle
Groove density
Coating
Size
75 deg
360 gr/mm
Au
195 mm × 70 mm
Grating
Groove density
Entrance arm
Exit arm
Magnification
Size
Dispersion
TVLS
4800 lines/mm
150 mm
930 mm
6.2
10 mm  20 mm
0.16 nm/mm
Spectral region
58-63 nm
Rastering
Grating rotation
Telescope
Focal length
Incidence angle
Size
Parabola
650 mm
1.5 deg
30 mm × 70 mm
Slit
Size
Resolution
5.2 mm  0.9 mm
1 arcsec
Detector
Pixel size
Format
17.5 mm  5 mm
1780  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
Resolution perpendicular to the slit
Resolution parallel to the slit
aberrations
2.0
2.0
aberrations
detector
slit
spatial resolution (arcsec)
1.5
1.0
0.5
-7.0
-5.0
-3.0
-1.0
1.0
3.0
5.0
7.0
1.0
0.5
0.0
-9.0
9.0
-6.0
-3.0
0.0
3.0
off-axis angle (arcmin)
Spectral resolution
pixel (14 km/s)
slit image (36 km/s)
80
spectal resolution (mA)
0.0
-9.0
1.5
60
40
20
0
58.0
60.5
wavelength (nm)
63.0
6.0
9.0
Alternative configuration: efficiency at 60 nm
Total efficiency at wavelength 
ETOT() = A [cm2]  E()  PS [arcsec2]
AEF
E()
PS
entrance aperture
combined efficiency (telescope, spectrometer, detector) at wavelength 
pixel size
CDS on SOHO, NIS2 channel
ETOT_CDS(60 nm) = 0.046
Alternative configuration
AEF = 35 cm2
Egrating_1 = 0.35
Rmirror = 0.30
Egrating_2 = 0.15
Edetector = 0.30
ETOT(60 nm) = 0.16 = 3.5 TIMES CDS EFFICIENCY
Alternative configuration: thermal load
Entrance aperture: 5 cm × 7 cm
Thermal load
Power absorption on the grating
120 W
15 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
0.3 deg
2
1
0.3 deg
58.6 nm
60.5 nm
62.4 nm
Entrance slit
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 (15-150 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