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Microlens SpectrographMichiel van Noort
Nagaraju Krishnappa
Joerg Bischoff
Observing the Sun
Observing the Sun through the Earth atmosphere
Observing the Sun in detail
Evolving on a timescale of 10s.
Understanding=Spectra
Evolution
How to observe a 3D data cube with a 2D detector?!
I Slice: Use time as the 3rd dimension (scan)I Narrow band imagerI Slit spectrograph
of Resolution, Signal to Noise and Cadence
I High spatial resolution → many slit-spectra
I High spectral resolution → many ”images”
I High Signal to Noise → long exposures
I Rapidly evolving → available time is limited (1-10s).
−→ A good compromise is difficult
I By eliminating the need to scan, 1-2 orders of magnitude canbe gained
I Problem: How to detect a 3D data cube with a 2D detector?
Mapping 3D −→ 2D
Making space for the 3rd dimension
Door number 1
I Make space for spectral dimension by shrinking pixels
I Disperse at a small rotation angle to the pixel grid
I Truncate using a narrow prefilter to avoid overlap
I 3D cube recorded in a single exposure
I De-magnification factor N: N2 spectral ”pixels”
Targets
To be useful we need:
I Critical sampling in image space
I High throughput (∼50%)
I Spectral resolution ∼200000
I Spectral range ∼ 4A. (∼ 350 pixels incl. prefilter, N ≈ 18)
I At least 100x100 image elements
I High frame rate: small image elements (fast CCD)
Instrument Concept
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Microlens ArrayReimager Spectrograph
1. re-imaging optics
2. image reformatter
3. high resolution spectrograph
The image reformatter is the key experimental part of the system,the rest is ”standard”.
Proof of concept
I Instrument uses array of “dots” instead of slit
I Dots can also be created with pinhole mask
I Test of concept with pinhole array...
Test setup:
I No re-imaging
I Pixels ”shrunk” with mask with 22x22 pinholes of 25µm
I Prefilter 4.4A FWHM @ 6302A.
I ”Ordinary” spectrograph (SST/TRIPPEL)
Pinhole array masks almost all light −→ very inefficient ( 0.25%)!
Spectrograph test: Pinholes
0 500 1000 1500 20000
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rcse
c]
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wavelength [pixel]
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ty[H
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Microlens Assembly
Single microlens array was tested in 2000 (Suematsu et al.):
I Array of 50x50 600x600µm microlenses
I 10A FWHM bandpass filter
I 1536 x 1024 CCD
They found:
I Too much straylight → mask needed
I ”Very hard” to align microlenses and pinholes
Project abandoned (built Hinode instead)...
Single lens solution
I Single microlens with [pinhole] mask: pixels imagedon the grating
P1F1 F2
I Image constrast is large → spectralresponse is scene dependent
I Image constrast is wavelength dependent → spectralresponse wavelength dependent
Dual lens solution
Dual microlens array design to:
I Image pupil on the grating
I provide two planes to mask straylight
P1 F2F1 P2
Microlens array 1 images ”pupil” on pupil mask P1P1 imaged on the colimator plane P2.Primary pinhole mask inserted at F2
Modeling
I Critical sampling of the image: focal ratio degradation of afactor 2
I Diffraction effects dominate the microlens assembly
I Mainstream optical design packages do not work (Fresnelcondition violated)
I Propagation calculated by numerical evaluation of
E (x , y , z) =z
iλ
∫ ∫E (x ′, y ′, 0)
1
r2e
−i2πrλ dx ′dy ′
Dual Microlens Assembly
6
0.3
F=0.3
F=6
P F
0.017
0.017
L1 L2
Properties
I Sensitivity to image contrast
Low contrast High contrast
I High transparancy (70-80%)
I Low parasitic light (∼0.03%)
0 20 40 60 80 100Grating size at L=500 [mm]
0.0
0.2
0.4
0.6
0.8
1.0
Cont
aine
d lig
ht fr
actio
n
I Low sensitivity to surface errors
Properties Cont’d
I High sensitivity to angle of incidence: Pupil motion on thegrating ∼ 100mm/deg
I Incoming beam [almost] perfectly telecentric (<0.1 deg.)!
I High sensitivity to microlens co-alignmentI Displacement amplified by Lspec/FL2 (∼5000)!
Alignment crucial −→ Monolithic design
Coupling between exit beam speed and image element size
I Critical sampling: F/=2
I Pupil “apodization”: F/=2
Total beam speed-up: F/=4Small CCD pixels −→ Additional scaling speedup
Prototype
Monolithic design:
n=1.45709
Mask
Rear viewFront view
mµR=2039 mµ
R=
87
mµ6690 mµ196
mµ
70
mµ325
mµ325
I Thick substrate (6.5mm)I Maximum feasible sag ∼ 15µm
I Quality of second lenslet array must be highI Secondary mask in spectrograph focus
I Alignment error < 1µm (array 42× 42mm → 0.01”)
Prototype manufacture
High precision −→ Fraunhofer Institute for Applied Optics Jena
I Front: Lithography + reactive ion beam etching
I Back: Reflow lenslets
I Mask: black chromium
Prototype layoutPrototype layout
Prototype testing
I Delivery in November 2014I To be tested
I Front-back ML alignmentI pupil co-alignmentI transparencyI Contamination (crosstalk + straylight)I pupil sensitivity to constrast
Lab setup:
42x42mm
500mm 310mm 310mm
25x25mm
500mm
3750mm
Spectrograph properties (TBD)
I ”Normal” spectrograph can be used
I Projected grating size ≥ 50x50mm
I Smaller FOV −→ Faster spectrograph
I F-ratio may need to be as low as 5 (possible?)I Fast beam: short spectrograph at high order
I Small FSR allows up to order 1500I Large blaze angle gratingI Increased sensitivity to angle of incidence
I Effects of the non-uniform illumination?I Closely packed multiple identical modules (compact)
I Transmission spectrograph possible?
Field splitter + re-imaging optics
I Larger FOV → multiple modules
I Multiple modules → Field splitter?
I Control of beam telecentricity?
I Large magnification → Low light levels (stray-light problems)
F
x 4
PF
x 5
F
x 4
PF
x 5
Still to come
I Lab tests (Q1 2015)
I Re-imaging optics (Q1-Q3 2015)
I Telescope test (Q3 2015)
I Field splitter (2015-?)
I Spectrograph (2015-?)