MEMS-based
Spectrographs
Recent Advances
on their
Optical Design
P. Spanò
INAF
STScI, Baltimore
Osservatorio Astronomico di Brera, ITALY
June 25, 2010
INAF & O.A.Brera

Since 2003 O.A.Brera is one
of the 19 institutes currently
part of the Instituto
Nazionale di Astrofisica
(INAF)

12 “Observatories” and 7
Institutes former belonging
to the National Research
Council (CNR)
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MEMS-based Spectrographs:
Advances on their Optical Design
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Merate Observatory
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Advances on their Optical Design
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From ground to SPACE
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We, GOLEM (Gruppo Ottiche e LEnti Merate) are a small team of
astronomers and engineers located in Merate
We works mainly for ground-based telescopes (e.g., the GRB shooter REM
in La Silla, Chile) and focal plane instrumentation (e.g., X-shooter on ESO
VLT in Paranal, Chile, again) for optical-to-NIR wavelengths
More recently, we were involved in space-based projects, like SPACE (now
EUCLID)
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MEMS-based Spectrographs:
Advances on their Optical Design
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SPACE
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Advances on their Optical Design
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
ESA led a study to merge together the
two Dark Energy missions, SPACE and
DUNE into EUCLID
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Weak-lensing and BAOs as probes for DE

1.2m shared telescope
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VIS + NIR photometry

NIR spectroscopy
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MEMS-based Spectrographs:
Advances on their Optical Design
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Digital Micromirror Devices
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MEMS-based Spectrographs:
Advances on their Optical Design
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DMD principles - I
Incident ray
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ON: +12°
OFF: -12°
DMD micromirror
DMD surface
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June 25, 2010
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DMD micromirror can tilt (along
its diagonal) by +/- 12 deg (in
the Cinema DLP)
They correspond to two
different states: On and Off
A third state (power off) exists,
with a 0° angle
MEMS-based Spectrographs:
Advances on their Optical Design
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DMD principles - II
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Advances on their Optical Design
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DMD principles - III
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Tilt happens along the
diagonal of the
micromirror, so a 45
deg rotation of the
device is required to
keep chief rays within a
plane perpendicular to
the DMD surface
MEMS-based Spectrographs:
Advances on their Optical Design
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DMD illumination geometry - I
Rotation axis of the
DMD mirror
0°
Reflected ray
24°
(angle w.r.t. DMD normal)
DMD normal
45°
45°
DMD area
DMD micromirror
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MEMS-based Spectrographs:
Advances on their Optical Design
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DMD illumination geometry - II
28º
16.73º
24º
16.73º
4º
20º
4º
Minimum and
maximum
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17.9º
17.9º
23º
23º
Equal amplitude
MEMS-based Spectrographs:
Advances on their Optical Design
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One perpendicular
to field
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RITMOS
(Meyer et al. 2004)
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F/8 beam
0.8” micromirrors
(@Mees 0.6m tel.)
11’x8’ FoV
(@Mees 0.6m tel.)
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0.39-0.49 um
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R=6000
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MEMS-based Spectrographs:
Advances on their Optical Design
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IRMOS
(MacKenty et al. 2004)
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F/5.6 beam
17um micromirrors
(=0.2”@KPNO 4m)
3’x2’ FoV
(@KPNO 4m)
0.85-2.5 um
(ZJHK)
R=300-3000
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Advances on their Optical Design
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SPACE DMD-baseline
(Content et al., Durham Univ.)
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F/2.2 beam
14um micromirrors
(=1” @ 1.2m)
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49’x34’ FoV
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0.9-1.7 um
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R=400
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Very complex mirrors,
large & heavy,
complex mechanisms
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MEMS-based Spectrographs:
Advances on their Optical Design
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SPACE DMD altenative I
(by Grange et al., Lab. Astroph. Marseille)
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F/2.7 beam
14um micromirrors
(=0.9” @ 1.2m)
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26’x14’ FoV
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1.0-1.7 um
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R=400
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Simpler, smaller,
reduced performances
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MEMS-based Spectrographs:
Advances on their Optical Design
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SPACE DMD altenative II
(by Spanò et al., INAF)
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F/4 beam
14um micromirrors
(=0.6” @ 1.2m)
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20’x11’ FoV
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0.9-1.75 um

R=400

50cm
Very compact,
small field
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Advances on their Optical Design
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TIR prisms
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Advances on their Optical Design
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TIR on beamers
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MEMS-based Spectrographs:
Advances on their Optical Design
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Beamer Requirements
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Advances on their Optical Design
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(Bowron, et al. SPIE 5186,2003)
Example of TIR image quality
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MEMS-based Spectrographs:
Advances on their Optical Design
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DMD @ Galileo Telescope
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Advances on their Optical Design
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A simplified approach
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Modular design, simple layout, cheap optics
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Low resolution
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“Large” field of view
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High efficiency
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“Off-the-shelf” optics
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Advances on their Optical Design
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The starting idea
(Zamkotsian et al., ASP Conf. 207, 2000)
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F/7 beam
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Proposed for NIRMOS
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Two spherical mirrors
and a convex spherical
grating
1:1 Offner-like design
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Advances on their Optical Design
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The design
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Two arms: Spectro & Imaging
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Wavelengths: 400-800 nm
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Spectral resolution: 250
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Focal ratio: F/4
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FoV: 4.5’x7’
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Detectors: 2kx2k (spect.), 1kx1k (imaging)
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MEMS-based Spectrographs:
Advances on their Optical Design
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Optical Design
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MEMS-based Spectrographs:
Advances on their Optical Design
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Optical details
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Advances on their Optical Design
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Slits and spectra
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Advances on their Optical Design
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Mechanical layout
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Advances on their Optical Design
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Summary
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DMD spectrographs with enhanced FoV can be designed
with faster beams
TIR prisms can be very effective to keep size very small
Simpler designs if Offner-type configurations are
selected
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Advances on their Optical Design
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