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.