Optical System Design 1
PACS IIDR 01/02 Mar 2001
Optical System Design
N. GeisMPE
Optical System Design 2
PACS IIDR 01/02 Mar 2001
Pacs Optical System Overview
Anamorphic System
Grating Spectrometer
Telescope
Entrance Optics -- chopper -- calibration optics
Field splitter
Spectrometer
Image Slicer
To SlicerBolometer Optics
Dichroic
Bolometer
Red Bolometer Array
Blue Bolometer Array
Filter Filter Wheel Filter Filter Wheel
Red Photoconductor Array
Blue Photoconductor
Array
Bolometer Optics Bolometer Optics Dichroic
Optical System Design 3
PACS IIDR 01/02 Mar 2001
Definition of Image Scale
SubsystemPixel Pitch on Sky
(Physical)
Field-of-View
Spectrometer9.4 arcsec
(3.6 mm)47 x 47 arcsec2
Photometer (60–130 µm)3.2 arcsec
(0.75 mm)214 x 106 arcsec2
Photometer (130-210 µm)6.4 arcsec
(0.75 mm)211 x 102 arcsec2
Optical System Design 4
PACS IIDR 01/02 Mar 2001
Optical design for astronomical optical path
• Image inverter (3 flats) at the beginning to
compensate for telescope image tilt
• Chopper assembly on outer side of FPU (servicing)
• Labyrinth configuration for baffling (see straylight
analysis)
• Reduced chopper throw (sky) to allow for larger
FOV of bolometers with same entrance field stop /
mirror sizes
Optical Design – Top Optics
Optical System Design 5
PACS IIDR 01/02 Mar 2001
Optical design for calibration sources
• Acceptable image quality of pupil • Köhler-type illumination (pupil on source aperture + field
stop)
• Source aperture is projected onto M2/Cold Stop
• No physical match in source for “field” stop => excellent
uniformity expected
• Re-use of existing entrance optics mirrors in
reverse
• Excellent baffling situation • Sources are outside of Instrument Cold Stop• Initial calibration path & field stop outside of Instrument Cold
Stop
Optical Design – Top Optics
Optical System Design 6
PACS IIDR 01/02 Mar 2001
Top OpticsAstronomical
Common Focus, Top
Optics
TO Active 5
TO Active 4
Chopper
TO Fold 4
TO Active 3
TO Active 2
Lyot Stop
TO Active 1
TO Fold 3
TO Fold 2
TO Fold 1
Telescope
PupilField
Optical System Design 7
PACS IIDR 01/02 Mar 2001
Top OpticsCalibration
TO Fold 1
TO Active 1
TO Fold 3
TO Fold 2
Common Focus, Top
Optics
TO Active 5
C2 Active 3C1 Active 3
C1 Active 2
C1 Active 1
C2 Active 2
TO Active 4
Chopper
TO Fold 4
TO Active 3
Cal. Source 1
TO Active 2
Lyot Stop
Telescope
C2 Active 1
Cal. Source 2
PupilField
Calibrator 2Calibrator 1
Optical System Design 8
PACS IIDR 01/02 Mar 2001
Overall optical arrangement has favorable mechanical
layout
• clean separation between optical paths (no interpenetrating
beams)
• better accommodation for mechanical mounts
• most mechanisms and sub-units can be mounted close to
FPU outer walls for modularity
Overall Optical Design
Optical System Design 9
PACS IIDR 01/02 Mar 2001
Optical component
safter Top
Optics
Photometer
Filter
B Active R2
Filter Wheel
Dichroic Beamsplitter
S Active 6
Slicer Optics
BlueBolometer
Array
Dichroic Beamsplitter
B Active R1
B Active B2
S Fold 2
S Active 2
S Active 1
B Active 1S Fold 1
Common FocusTop Optics
RedBolometer
Array
B Active B1
RedSpectrometer
Array
S Active 5
S Active 4
S Active 3
Spectrometer
S Fold 4
S Fold 3
Blue Spectrometer
Array
S Fold 5
Filter Filter Wheel
Grating
S Collimator 2
S Collimator 1
S Collimator 2
S Collimator 1
B Fold B1B Fold R1
PupilField
Optical System Design 10
PACS IIDR 01/02 Mar 2001
Optical design for bolometer cameras finished
• very good image quality
• good geometry
• excellent baffling situation•fully separate end trains•extra pupil and field stops possible on the way to detectors•exit pupil with filter at entrance window to cold (1.8K) detector housing
• Bolometer arrays mounted close together on top of cryocooler
• Photometers are a self-contained unit at FPU external wall
Optical Design – Photometers
Optical System Design 11
PACS IIDR 01/02 Mar 2001
Changes in optical design for spectrometer since
ISVR • ILB column
Slicer output was reconfigured such that one pixel’s worth of
space is intentionally left blank between slices at the slit focus and on the detector array
•Reduces (diffraction-) cross-talk•helps with assembly & alignment
gap of 0.75 mm between slit mirrorsgap of 3.6 mm between detector blocks for filter holder
• Better image quality
• Excellent baffling situation•end optics for both spectrometers separated on “ground
floor”
•exit field stop of spectrometer inside “periscope”
•extra pupil and field stops possible in end optics
Optical Design – Spectrometers
Optical System Design 12
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The Image Slicer
Optical System Design 13
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Image Slicer and Grating (in)
Slicer Mirror
Capture Mirror
Slit Mirror
Grating
Optical System Design 14
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Image Slicer and Grating (in+out)
Slicer Stack
Capture Mirror
Slit Mirror
Grating
Periscope Optics
Optical System Design 15
PACS IIDR 01/02 Mar 2001
• Clean separation between optical paths – a
result of the incorporation of the bolometers
• Realistic accommodation for mechanical mounts
• Significant savings in number of mirrors from
the photoconductor-only design
• Improved image quality in both, photometers,
and spectrometers
Optical Design Summary
Optical System Design 16
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A Walk Through PACS
Optical System Design 17
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PACS Envelope -filled
Optical System Design 18
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PACS Functional Groups
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PACS Envelope
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PACS Envelope + Top Optics
Optical System Design 21
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Top Optics Chopper
Telescope Focus
Lyot Stop
Optical System Design 22
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Calibrators Calibrator I+II
Optical System Design 23
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Chopping Left
Optical System Design 24
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Chopping Right
Optical System Design 25
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Entrance Optics + Blue Photometer
Dichroic
FilterWheel
BlueBolometer
Cryocooler
Optical System Design 26
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Entrance Optics + Blue Photometer
Optical System Design 27
PACS IIDR 01/02 Mar 2001
Entrance Optics + Blue Photometer + Red Photometer
Dichroic
RedBolometer
Filter
Optical System Design 28
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Entrance Optics + Blue Photometer + Red Photometer
Optical System Design 29
PACS IIDR 01/02 Mar 2001
Photometer UnitCommon Focus
Dichroic
Red
Dichroic
Blue
Bolometer
Common Focus
Fold Fold
Fold
Fold
Red Blue
Red
Blue Bolometer
Dichroic
Optical System Design 30
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The Spectrometer Section
Optical System Design 31
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Chopper
sGeGaDetectorRed Spectrometer
Blue Bolometer
Red Bolometer
Calibrator I and II
0.3 K Cooler
Filter Wheel I
Filter Wheel II
Grating
sGeGa DetectorBlue Spectrometer
Encoder
Grating Drive
Entrance Optics
Photometer Optics
Calibrator Optics
SlicerOptics
SpectrometerOptics
Optical System Design 32
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Geometrical Optics Performance
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Optical Performance - Blue Bolometer
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Optical Performance - Geometry Blue Bolometer
1 2
3
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Optical Performance - Red Bolometer
Optical System Design 36
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Optical Performance - Geometry Red Bolometer
Optical System Design 37
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Optical Performance - Spectrometer
Center of Array, center Corner of Array, extreme
Optical System Design 38
PACS IIDR 01/02 Mar 2001
Optical Performance - Geometry Spectrometer
“ILB”
175.0µm
175.4µm
174.6 µm
Optical System Design 39
PACS IIDR 01/02 Mar 2001
Diffraction
Optical System Design 40
PACS IIDR 01/02 Mar 2001
Illumination of Lyot Stop
2 Strategies1 Use of M2 as system stop
(baseline): oversize instrument Lyot stop by ~ 10% area (if only cold sky visible beyond M2 )
2 Use of Lyot stop as system stop (optional); suppresses diffracted emission/reflection from M2 spider, but we lose 5–10% throughput
GLAD 4.5diffraction analysis = 175 µm
Radius [cm]
Inte
nsi
ty (
arb
. unit
s)
• M2 is system aperture• Image quality of M2 on Lyot stop determined
by diffraction from PACS entrance field stop• Maximum size of entrance field stop is limited
by payload accommodation (M3) and thermal/ stray radiation
• Diffraction ring ~10% of aperture area
Optical System Design 41
PACS IIDR 01/02 Mar 2001
Diffraction Analysis - Slicer/Spectrometer
Diffraction Analysis of the Spectrometer was repeated with current (pre-freezing) mirror dimensions and focal lengths, and for a larger range of wavelengths.
The results were used• as inputs to a detailed grating size specification• for optimizing mirror sizes in the spectrometer path
=> Diffraction on the image slicer leads to considerable deviations from the geometrical footprint on the grating at all wavelengths
Optical System Design 42
PACS IIDR 01/02 Mar 2001
Diffraction Gallery at 175 µmtelescope focus, re-imaged “slice” through point spread
function
capture mirror
entrance slit field mirror
grating
pixel
Detectorarray
Optical System Design 43
PACS IIDR 01/02 Mar 2001
•Considerable difference from geometrical optics footprint.
•No noticeable spillover problem at short wavelength
•Non-uniform illumination profile will lead to change in effective grating resolution => calculate/measure
Grating: The worst offenderat long wavelength
Optical System Design 44
PACS IIDR 01/02 Mar 2001
•Major difference from geometrical optics footprint.
•Spillover of ~ 20% energy past grating & collimators at longest wavelength
•Non-uniform illumination profile will lead to change in effective grating resolution => calculate/measure
Grating: The worst offenderat long wavelength
Optical System Design 45
PACS IIDR 01/02 Mar 2001
Grating: The worst offenderat long
wavelength
Grating80mm x 320mm
X
Y
Before Grating
57µm 205µm
After Grating
57µm
Grating
Grating
Angle of incidence:46.6°3.Order
Angle of incidence:46.6°3.Order
Angle of incidence:60.4°1.Order
Angle of incidence:60.4°1.Order
Width of grating sufficient: minimal loss at 205 µm
205µm
Grating
Grating
Optical System Design 46
PACS IIDR 01/02 Mar 2001
57µmGrating
Y-Axis has to be scaled by 1/cos(46.6°)
Y-Axis has to be scaled by 1/cos(46.6°)
Y-Axis has to be scaled by 1/cos(60.5°)
Y-Axis has to be scaled by 1/cos(60.5°)
Angle of incidence:46.6°3.Order
Angle of incidence:60.4°1.Order
Grating
205µm
Angle of incidence:60.4°1.Order
57µmGrating
Angle of incidence:46.6°3.Order
Losses due to length of grating at 205 µm, 57 µm OK
CollimatorVignetting
GratingVignetting
Before Grating
205µm
After Grating
Grating80mm x 320mm
X
Y
Grating
Grating: The worst offenderat long
wavelength
Optical System Design 47
PACS IIDR 01/02 Mar 2001
• System stop should be M2 - oversize PACS cold stop
accordingly
• Diffraction lobes introduced by slicer mirrors can still be transferred through most of the spectrometer optics
• Considerable clipping occurs on collimator mirrors and
grating at long wavelength
• Losses due to “spill-over”:
– up to 20% (205 µm), 15% (175 µm) other wavelengths tbd.
80% “diffraction transmission” to detector for central pixel
Diffraction Summary