September 19, 2002University of Colorado
The Off-Plane Optionfor the
Reflection Grating Spectrometer
Webster Cash
University of Colorado
September 19, 2002University of Colorado
Chandra Spectra LookLike Traditional GroundSpectra.
Can We Afford to StepBack???
September 19, 2002University of Colorado
Off-plane Mount
sinsinsin
d
n
In-plane Mount
d
n sinsin
September 19, 2002University of Colorado
Radial Groove Gratings
September 19, 2002University of Colorado
Off-plane Resolution
At typical values of off-plane angles and 15” telescope resolutionR ~ several hundred → thousand
Sub-Aperturing improves it further
cos
sinsinsin
BR
September 19, 2002University of Colorado
An Off-plane X-ray Spectrum
September 19, 2002University of Colorado
Off-plane Tradeoffs
• Higher Throughput• Higher Resolution• Better Packing Geometry• Looser Alignment Tolerances
CON• Higher Groove Density
PRO
September 19, 2002University of Colorado
Packing Geometry
+1
0
Central grating must be removed.Half the light goes through.
+1
0
Gratings may be packed optimally
In-plane
Off-plane
September 19, 2002University of Colorado
Throughput•Littrow configuration = = blaze angle - Better Groove Illumination - Maximum efficiency• Constant Graze Angle
September 19, 2002University of Colorado
Holographic Gratings
Last year we reviewed approaches to fabricatinghigh density gratings.
At Jobin-Yvon (outside Paris)Create rulings using interference pattern in resistIon-Etch Master to Create Blaze
Radial Geometry – Type 4 Aberrated BeamsDensity: Up to 5800 g/mm Triangular (<35 deg blaze)
In UV holographic blazed gratings have very low scatterand good efficiency – same in x-ray?
September 19, 2002University of Colorado
Raytracing – Arc of Diffraction
September 19, 2002University of Colorado
Raytrace – 35 & 35.07Å
September 19, 2002University of Colorado
Raytracing of Wavelength Pairs and +.07Å
10Å
90Å80Å70Å60Å
50Å40Å35Å30Å
25Å20Å15Å
September 19, 2002University of Colorado
Internal Structure of Telescope
Blur Favors Dispersion in Off-plane DirectionSpectral line of HeII 304Å
displaying In-plane scatter
Data from a radial grating in the
off-plane mount, Wilkinson
September 19, 2002University of Colorado
Subaperture Effect
September 19, 2002University of Colorado
Grating Modules
R450.0mmInner MirrorsHigh Energy
R151.4mm
Off-plane Grating ModuleLocations on Envelope
R770.0mmOuter MirrorsGrating Area
September 19, 2002University of Colorado
Can Improve Performance
September 19, 2002University of Colorado
Can Improve Performance
September 19, 2002University of Colorado
Raytracing – Arc of Diffraction
September 19, 2002University of Colorado
Raytrace – 35 & 35.028Å
September 19, 2002University of Colorado
Raytracing of Wavelength Pairs and +.028Å
10Å
90Å80Å70Å60Å
50Å40Å35Å30Å
25Å20Å15Å
September 19, 2002University of Colorado
Resolution
0.1 1.0 10.0Energy (keV)
100
1000
10,000
E/
E Calorim
eter –
2eV
I-P n=1
I-P n=2
Primary Response
ASSUMPTIONS:5500g/mm15” SXT2” gratings2” alignment
<35% ResponseExtended CCD
Mission Requirement
Mission Goal
O-P n=1
O-P n=2
O-P n=3
MissionRequirement
September 19, 2002University of Colorado
Effective Area
Energy (keV)
cm2
ASSUMPTIONS:
Coverage 40% of outer envelope
Off-Plane Groove Efficiency 80% of theoretical
85% Structure Transmission
CCD thin Al filter only
0.1 1.0 10.00
5000
off-plane
1000
2000
3000
4000
baseline
Goal
Mission Requirement
September 19, 2002University of Colorado0.1 1.0 10.0
Energy (keV)
15
area
x r
esol
utio
n ÷
106
off-plane
0
5
10
20
in-plane
calorimeter
Figure of Merit
September 19, 2002University of Colorado0.1 1.0 10.0
Energy (keV)
15
area
x r
esol
utio
n/E
(kev
)/ 1
06 off-plane – R~3000
0
5
10
20
in-plane
calorimeter
Figure of Merit with Spectral Weighting
off-plane – R~1500
September 19, 2002University of Colorado
Pros & Cons of Off-plane vs. Baseline Design
● Pro:
– Greater Resolution from Sub-aperturing
– Greater Collecting Area – higher groove efficiency
– Less Sensitivity to Grating Alignment
– Less Sensitivity to Grating Flatness
– Lower scatter in Dispersion Direction
– Fewer Gratings Required
– Thicker Substrates Acceptable
– Smaller Structure Required
● Con:
– Higher groove density required
September 19, 2002University of Colorado
Difficulties of High Resolution (/Δ>1200)
• flatter gratings• tighter alignment• tighter focus• telescope depth of focus adjustment• zero order monitor essential to aspect solution• more difficult calibration• greater astigmatism
– higher background– more source overlap
September 19, 2002University of Colorado
Depth of Field Problem
Solutions for Study:Smaller GratingsCurved GratingsAdjust Telescope Segments
Hope that it ismerely a matter ofmounting existing shellsat different radii
September 19, 2002University of Colorado
Resolution Degradation
1 10 100Grating Resolution (arcsec)
100
1000
10,000
E/
E
September 19, 2002University of Colorado
Off-plane Grating Module
22cmGrating size:10cm x 10cm x 0.2cmGraze angle: 2.7o
GratingsQty. 20
11cm
11cm
Holder
September 19, 2002University of Colorado
Off-plane Grating Resolution Options
~ 1000 ~ 5000
• SXA (Al/SiC) substrates
• Easy tolerances
• Simple mount
• No thermal gradient
• Mass OK
• Glass/Si substrates?
• More difficult tolerances
• More difficult mount
• Probable thermal gradient issues
• Mass constraint more difficult to meet
September 19, 2002University of Colorado
Off-plane Grating Estimated Tolerances
Error type Zero-order Allowable Tolerances
Equation = 15 arcsec = 2 arcsec
Surface error 36.5m 4.9m
x 36.5m 4.9m
y 1mm 1mm
z 775m 103m
x 11.5° 11.5°
y 0.75 arcsec 0.1 arcsec
z 31.8 arcsec 4.2 arcsec
20
s
cos20
sx
10
wy
sin20
sz
h
w
5sin
20
sin10
September 19, 2002University of Colorado
Off-plane Grating Module Estimated Mass
Materials Gratings (Kg)
Holder (Kg)
Light-weight
One Module
(Kg)
Qty Modules
Total mass
(Kg)
SXA/SXA 1.16 1.20 none 2.36 32 75.65
SXA/SXA 1.16 1.20 25% 2.17 32 69.53
SXA/6061 1.16 1.11 none 2.27 32 72.73
FS/Invar/Ti 0.88 1.568 70% 2.45 32 78.36
FS/Titanium 0.88 1.488 30% 2.37 32 75.82
FS/GrEp/Invar 0.88 1.687 none 2.57 32 82.17
September 19, 2002University of Colorado
Wavefront Error: Resolution 1000
Total allowable error 21 m
Spare 8.054(rss)
Fabrication2.87m
(rss)
Mount 10 um (WAG)
Test(/50)
.013 m
Stability6.23m
(rss)
Alignment10 m
(estimate)
1g Sag.09 m (calc)
Temp (bulk)±2.5°C
2.02 m (rss)
Substrate Figure(3)
1.9m (requirement)
ReplicationEpoxy cure strain
1.0 m (calc)
Replicate Separation Strain
1.9 m(WAG/3)
Creep0.5 m (WAG) Water absorption
(assume 0.3%) 1.6m(calc)Thermal gradient
(0.5°C) 6 m (calc) Jitter (on orbit)
.0003 m (WAG/calc)
Mount 2 m(WAG)
Reflective coatingbimetallic effect
0.3 m (calc)
Replication epoxybimetallic effect
0.0005 m(calc)
Constellation X Off-plane Grating Mount rms Wavefront Error Budget (15 arcsec max)All errors are presented as rms wavefront error
September 19, 2002University of Colorado
Wavefront Error: Resolution 5000
Total allowable error 2.77 mm
Spare 0.754 m
(rss)
Fabrication1.16 m
(rss)
Mount 1.5 m (WAG)
Test(50)
.013 mm
Stability0.62 m
(rss)
Alignment1.75 m (estimate)
1g Sag.11 m (calc)
Temp (bulk)±2.5°C
0.21 m (rss)
Substrate Figure(1.5)
0.95 m (requirement)
ReplicationEpoxy cure strain
0.23 m (calc)
Replicate Separation Strain
0.6 m(WAG/1)
Creep0.1 m (WAG) Water absorption
(assume 0.3%) 0.35 m
(calc)Thermal gradient(0.1°C) 0.5 m
(WAG/calc) Jitter (on orbit).0003 m
(WAG/calc)
Mount 0.2m(WAG)
Reflective coatingbimetallic effect
.07 m (calc)
Replication epoxybimetallic effect
0.008 m(calc)
Constellation X Off-plane Grating Mount rms Wavefront Error Budget (2 arcsec max)All errors are presented as rms wavefront error
September 19, 2002University of Colorado
Off-plane Grating Prototype: steps and schedule
Phase Task Leadtime
1 Preliminary feasiblility study of type 4 aberration corrected grating distribution to approximate radial distribution
4-5 mos.
(Jun ‘02 to ~Oct ‘02)
2 Preliminary study of blaze process using existing masks (30o profile goal).
(work done in parallel with step 1)
4-5 mos.
(Jun ‘02 to ~Oct ‘02)
3 Contingent upon step 1&2 positive result.
Deliverable: 58x58x10mm parallel groove sample with 30o blaze angle.
4 mos.
(Oct ‘02 to ~Feb ‘03)
4 Contingent upon positive test of sample.
Deliverable: 58x58x10mm radial groove distribution with blazed profile.
3 mos.
(Mar ‘03 to ~Jun ‘03)
5 Ray-tracing to optimize recording configuration
Deliverable: 120mm square radial distribuation with blazed profile and flight groove density.
TBD
September 19, 2002University of Colorado
In Conclusion, Off-plane Can:● Match RGS to Calorimeter Scientifically
– R~1500 – greatly eased tolerances
● or Significantly Enhance Con-X Science– R~3000– tolerances at currently expected levels
Study funded by the Con-X project. First results in January.