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Grazing-incidence vs. normal-incidence design
L. PolettoCNR - National Institute for the Physics of MatterDepartment of Information Engineering - Padova (Italy)
EUS MeetingMarch, 3rd 2006
Normal-incidence vs. grazing-incidence design (1/2)
EUS BLOCK DIAGRAM
telescope slit spectrometer detector
TELESCOPE: two options 1) normal-incidence, single mirror
2) grazing-incidence, three mirrors
SLIT
SPECTROMETER normal-incidence VLS concave grating
DETECTOR APS
Normal-incidence vs. grazing-incidence design (2/2)
THE MAIN DIFFERENCE BETWEEN THE TWO CONFIGURATIONS IS
THE TELESCOPE DESIGN
THE SPATIAL AND SPECTRAL RESOLUTIONS OF THE NI
CONFIGURATION ARE HIGHER THAN THE GI ONE
THE TWO DESIGNS MAY OFFER THE SAME SPECTRAL COVERAGE
(NI NEEDS MULTILAYER FOR WAVELENGTHS BELOW 35 NM)
The grazing-incidence Wolter telescope
Grazing-incidence telescope
two concave mirrors and a plane mirror
rastering: rotation of the plane mirror
CHARACTERISTICS
114-126 nm spectral region (I order)
57-63 nm spectral region (II order)
18 arcmin 18 arcmin field-of-view
length <1 m
Plane mirror forrastering
Parabolic mirror
Hyperbolic mirror
Entrance slit
Detector
TVLS grating
From the Sun
Grazing-incidence design: characteristics
Telescope Wolter II Focal length 1200 mm Incidence angles 73.5 deg - 79 deg
Mirror for rastering Incidence angles 84.4 deg - 85 deg
Slit Size 6 m 6.3 mm Resolution 1 arcsec
Grating TVLS Groove density 2400 lines/mm Entrance arm 260 mm Exit arm 680 mm
Spectral region 114-126 nm (I order) 57-63 nm (II order)
Detector Pixel size 10 m 15 m Format 2150 1120 pixel Area 21.5 mm 16.8 mm
Spectral resolving element56 mÅ I order (14 km/s)28 mÅ II order (14 km/s)
Spatial resolving element1 arcsec (150 km at 0.2 AU)
Instrument length 1 m
Grazing-incidence design: performance
Resolution perpendicular to the slit (50% encircled energy)
0.0
0.5
1.0
1.5
2.0
0.0 3.0 6.0 9.0
off-axis angle (arcmin)
sp
ati
al r
es
olu
tio
n (
arc
se
c)
on-axis
4.5'
9'
slit
Resolution parallel to the slit (50% encircled energy)
0.0
1.0
2.0
0.0 3.0 6.0 9.0
off-axis angle (arcmin)
spat
ial
reso
luti
on
(ar
csec
)
on-axis
4.5'
9'
detector
Spectral resolution (I order)
0
20
40
60
80
100
1140 1160 1180 1200 1220 1240 1260
wavelength (Å)
sp
ec
tal
res
olu
tio
n (
mA
)
pixel (14 km/s)
slit image (22 km/s)
Grazing-incidence design: layout
NI coatings (1/2)
0.0
0.1
0.2
0.3
0.4
10 20 30 40 50 60 70 80 90 100 110
wavelength (nm)
refl
ecti
vity
Mo-Si multilayer
0
0.05
0.1
0.15
0.2
10 20 30 40 50 60 70 80 90 100 110
wavelength (nm)
refl
ecti
vity
Au
0
0.1
0.2
0.3
0.4
0.5
10 20 30 40 50 60 70 80 90 100 110
wavelength (nm)
refl
ecti
vity
SiC
NI coatings (2/2)
Mo-Si mulilayer good reflectivity (0.3) at 20, 60, 100 nm
low reflectivity (0.38) in the visible HIGH ABSORBED POWER
Au no reflectivity at 20 nm
low reflectivity (0.15) at 60, 100 nm
high reflectivity (0.80) in the visible LOW ABSORBED POWER
SiC no reflectivity at 20 nm
high reflectivity (0.40) at 60, 100 nm
low reflectivity (0.20) in the visible HIGH ABSORBED POWER
THE NI TELESCOPE IS EFFICIENT BELOW 40 NM ONLY WITH MULTILAYER
GI coatings (1/2)
Au or Si-Au
60 80 100 120 140 1600.6
0.7
0.8
0.984 incidence angle
refl
ecti
vity
wavelength (nm) c)
60 80 100 120 140 1600.5
0.6
0.7
0.880 incidence angle
wavelength (nm) b)60 80 100 120 140 160
0.2
0.4
0.6
0.870 incidence angle
Au Si(100 Å)-Au Si(200 Å)-Au Si(400 Å)-Au
refl
ecti
vity
wavelength (nm) a)
0
0.2
0.4
0.6
0.8
10 20 30 40 50 60 70 80 90 100 110
wavelength (nm)
refl
ecti
vity
Au at 80 deg
GI coatings (2/2)
Au constant reflectivity at 20, 60, 100 nm
high reflectivity (> 0.80) in the visible LOW ABSORBED POWER
Si-Au constant reflectivity at 20, 60, 100 nm (higher than Au)
high reflectivity (> 0.60) in the visible LOW ABSORBED POWER
THE GI TELESCOPE IS EFFICIENT AT ANY WAVELENGTH ABOVE 10 NM
Efficiency
Total efficiency at wavelength ETOT() = A [cm2] E() PS [arcsec2]
AEF entrance aperture
E() combined efficiency (telescope, spectrometer, detector) at wavelength PS pixel size
CDS on SOHO, NIS2 channel ETOT_CDS(60 nm) = 0.046
Efficiency at 20 nm
GI design AEF = 25 cm2
Egrating = 0.15
Edetector = 0.30
Si-Au coated optics Rmirrors = 0.55, 0.65, 0.75
ETOT(20 nm) = 0.30 = EFFICIENCY @60nm
Au coated optics Rmirrors = 0.40, 0.52, 0.70
ETOT(20 nm) = 0.16 = EFFICIENCY @60nm
NI design AEF = 25 cm2
Egrating = 0.15
Edetector = 0.30
ML coated optics Rmirrors = 0.30
ETOT(20 nm) = 0.34 = EFFICIENCY @60nm
Efficiency at 60 nm
Grazing-incidence design at 60 nm AEF = 25 cm2
Egrating = 0.15
Edetector = 0.30
Si-Au coated optics Rmirrors = 0.55, 0.65, 0.75
ETOT(60 nm) = 0.30 = 6.6 CDS EFFICIENCY
Au coated optics Rmirrors = 0.40, 0.52, 0.70
ETOT(60 nm) = 0.16 = 3.5 CDS EFFICIENCY
Normal-incidence design at 60 nm AEF = 25 cm2
Egrating = 0.15
Edetector = 0.30
SiC (ML) coated optics Rmirrors = 0.32
ETOT(60 nm) = 0.36 = 7.8 CDS EFFICIENCY
Au coated optics Rmirrors = 0.13
ETOT(60 nm) = 0.15 = 3.2 CDS EFFICIENCY
Efficiency at 120 nm
Grazing-incidence design at 120 nm AEF = 25 cm2
Egrating = 0.15
Edetector = 0.30
Si-Au coated optics Rmirrors = 0.55, 0.65, 0.75
ETOT(120 nm) = 0.30 = EFFICIENCY @60nm
Au coated optics Rmirrors = 0.40, 0.52, 0.70
ETOT(120 nm) = 0.16 = EFFICIENCY @60nm
Normal-incidence design at 120 nm AEF = 25 cm2
Egrating = 0.15
Edetector = 0.30
SiC coated optics Rmirrors = 0.48
ETOT(120 nm) = 0.54 = 1.5 EFFICIENCY @60nm
Au coated optics Rmirrors = 0.16
ETOT(120 nm) = 0.18 = 1.2 EFFICIENCY @60nm
Optics degradation at 20 nm
Multilayer coating
A change of the ML properties (e.g. interdiffusion between adjacent layers, change of period due to
thermal expansion) may alter the reflectivity down to 0.
THE ML IS A “SINGLE POINT FAILURE” FOR OBSERVATIONS AT 20 NM.
THE STABILITY OF ML AT THE EXTREME THERMAL CONDITIONS OF SOLO HAS TO BE PROVED BY
STUDIES AND TESTS, IN VIEW OF THE AO.
Optics degradation at 100 nm
Simulation of a C over-coating
GI reflectivity (80 deg)
Au 0.55
Au + 20 Å C 0.53 -3%
Au + 40 Å C 0.52 -5%
NI reflectivity
SiC 0.45
SiC + 20 Å C 0.31 -30%
SiC + 40 Å C 0.23 -50%
LARGE DECREASES FOR NI COATINGS
Optics degradation in the visible
Simulation of a C over-coating
GI reflectivity at 600 nm (80 deg)
Au 0.92
Au + 20 Å C 0.90 -2%
Au + 40 Å C 0.88 -4%
NI reflectivity at 600 nm
SiC 0.20
SiC + 20 Å C 0.21 +5%
SiC + 40 Å C 0.22 +10%
SMALL CHANGES
Thermal load: GI (1/2)
Grazing-incidence configuration: 5 cm × 5 cm entrance area Input thermal load 85 W
Au optics Thermal load on 1st mirror 85 W 6 solar constants Absorption on 1st mirror 17 W 1.2 solar constants Thermal load on 2nd mirror 61 W 16 solar constants Absorption on 2nd mirror 10 W 2.6 solar constants Thermal load on 3rd mirror 19 W 5 solar constants Absorption on 3rd mirror 2 W 0.5 solar constants
Power density on the slit plane 17 W on 21 mm 30 mm area (f = 1200 mm)20 solar constants
Comments 29 W absorbed by the optics (two of them have to be cooled) 39 W absorbed by suitable buffling 17 W on the slit plane, to be absorbed by buffles
Thermal load: GI (2/2)
Grazing-incidence configuration: 5 cm × 5 cm entrance area Input thermal load 85 W
Si-Au optics Thermal load on 1st mirror 85 W 6 solar constants Absorption on 1st mirror 34 W 2.4 solar constants Thermal load on 2nd mirror 46 W 12 solar constants Absorption on 2nd mirror 18 W 5 solar constants Thermal load on 3rd mirror 10 W 2.7 solar constants Absorption on 3rd mirror 4 W 1 solar constant
Power density on the slit plane 6 W on 21 mm 30 mm area (f = 1200 mm)7 solar constants
Comments 56 W absorbed by the optics (all are cooled) 23 W absorbed by suitable buffling 6 W on the slit plane, to be absorbed by buffles
Thermal load: NI (1/2)
Normal-incidence configuration: 5 cm × 5 cm entrance area, 1 m input boom, 5 cm × 5.6 cm mirror Input thermal load 85 W Thermal load on the buffle 22 W Thermal load on the mirror 63 W 16 solar constants
SiC optics Absorption on the mirror 50 W 13 solar constants Power density on the slit plane 13 W on 33 mm diameter (f = 700 mm)
11 solar constantsComments 50 W absorbed by the mirror 22 W absorbed by the entrance buffle 13 W on the slit plane, to be absorbed by buffles
Au optics Absorption on the mirror 13 W 3.4 solar constants Power density on the slit plane 50 W on 33 mm diameter (f = 700 mm)
43 solar constantsComments 13 W absorbed by the mirror 22 W absorbed by the entrance buffle 50 W on the slit plane, to be absorbed by buffles
Thermal load: NI (2/2)
ML coated optics Absorption on the mirror 40 W 13 solar constants Power density on the slit plane 23 W on 33 mm diameter (f = 700 mm)
20 solar constantsComments 40 W absorbed by the mirror 22 W absorbed by the entrance buffle 23 W on the slit plane, to be absorbed by buffles
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
Conclusions
NI DESIGN The NI configuration is more compact and has better optical performance than the GI one. A multilayer coated mirror is required for observations below 40 nm.
GI DESIGN No multilayer coated mirrors are required
AT PRESENT, NI CONFIGURATION IS THE FIRST CHOICE (GI AS A BACKUP SOLUTION).
GIVEN THE EXTREME THERMAL CONDITIONS ON SOLO (34 kW/m2), TESTS AND STUDIES ON
COATING DEGRADATION AT NORMAL-INCIDENCE (BOTH CONVENTIONAL AND MULTILAYERS)
HAVE TO BE PERFORMED IN VIEW OF THE AO.