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

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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

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F=6

P F

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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]

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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

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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-?)