Download PACS FPU Optical System Design

PACS IIDR
01/02 Mar 2001
Optical System Design
N. Geis
MPE
Optical System Design
1
PACS IIDR
01/02 Mar 2001
Pacs Optical
Telescope
System Overview
Entrance Optics
-- chopper
-- calibration optics
Bolometer
Spectrometer
To Slicer
Field splitter
Bolometer Optics
Image Slicer
Grating
Spectrometer
Dichroic
Anamorphic System
Dichroic
Bolometer Optics
Bolometer Optics
Filter
Filter Wheel
Filter
Filter Wheel
Red Bolometer
Array
Blue Bolometer
Array
Red Photoconductor
Array
Blue
Photoconductor
Array
Optical System Design
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PACS IIDR
01/02 Mar 2001
Definition of Image Scale
Subsystem
Spectrometer
Pixel Pitch on Sky
(Physical)
9.4 arcsec
(3.6 mm)
3.2 arcsec
Photometer (60–130 µm)
Photometer (130-210 µm)
Optical System Design
Field-of-View
(0.75 mm)
6.4 arcsec
(0.75 mm)
47 x 47 arcsec2
214 x 106 arcsec2
211 x 102 arcsec2
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PACS IIDR
01/02 Mar 2001
Optical Design – Top Optics
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 System Design
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PACS IIDR
01/02 Mar 2001
Optical Design – Top Optics
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 System Design
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PACS IIDR
01/02 Mar 2001
Telescope
TO Fold 1
TO Fold 2
TO Fold 3
TO Active 1
Lyot Stop
TO Active 2
TO Active 3
Top Optics
Astronomical
TO Fold 4
Pupil
Field
Chopper
TO Active 4
TO Active 5
Optical System Design
Common
Focus, Top
Optics
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PACS IIDR
01/02 Mar 2001
Telescope
Cal. Source 1
C2 Active 3
C1 Active 3
Cal. Source 2
TO Fold 1
C1 Active 1
TO Fold 2
C2 Active 1
C1 Active 2
TO Fold 3
C2 Active 2
TO Active 1
Calibrator 1
Calibrator 2
Lyot Stop
TO Active 2
TO Active 3
Top Optics
Calibration
TO Fold 4
Pupil
Field
Chopper
TO Active 4
TO Active 5
Optical System Design
Common
Focus, Top
Optics
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PACS IIDR
01/02 Mar 2001
Overall Optical Design
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
Optical System Design
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PACS IIDR
01/02 Mar 2001
Common Focus
Top Optics
Spectrometer
S Collimator 1
S Collimator 1
S Collimator 2
S Collimator 2
Photometer
B Active 1
S Fold 1
Dichroic
Beamsplitter
S Active 1
S Active 2
Grating
S Fold 2
S Fold 3
Slicer
Optics
S Fold 4
S Active 3
Optical
components
after Top
Optics
S Active 4
S Active 5
B Fold B1
B Active R1
B Active B1
B Active R2
B Active B2
Filter
Filter Wheel
Red
Bolometer
Array
Blue
Bolometer
Array
S Active 6
Dichroic
Beamsplitter
Filter
Filter Wheel
S Fold 5
Red
Spectrometer
Array
Optical System Design
B Fold R1
Pupil
Field
Blue
Spectrometer
Array
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PACS IIDR
01/02 Mar 2001
Optical Design – Photometers
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 System Design
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PACS IIDR
01/02 Mar 2001
Optical Design – Spectrometers
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 mirrors
gap 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 System Design
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PACS IIDR
01/02 Mar 2001
The Image Slicer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Image Slicer and Grating (in)
Slit Mirror
Slicer Mirror
Capture Mirror
Grating
Optical System Design
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PACS IIDR
01/02 Mar 2001
Image Slicer and Grating (in+out)
Slit Mirror
Periscope
Optics
Slicer Stack
Capture Mirror
Grating
Optical System Design
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PACS IIDR
01/02 Mar 2001
Optical Design Summary
•
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 System Design
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PACS IIDR
01/02 Mar 2001
A Walk Through PACS
Optical System Design
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PACS IIDR
01/02 Mar 2001
PACS Envelope -filled
Optical System Design
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PACS IIDR
01/02 Mar 2001
PACS Functional Groups
Optical System Design
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PACS IIDR
01/02 Mar 2001
PACS Envelope
Optical System Design
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PACS IIDR
01/02 Mar 2001
PACS Envelope + Top Optics
Optical System Design
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PACS IIDR
Top Optics
Optical System Design
01/02 Mar 2001
Chopper
Lyot Stop
Telescope Focus
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PACS IIDR
Calibrators
Optical System Design
01/02 Mar 2001
Calibrator I+II
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PACS IIDR
01/02 Mar 2001
Chopping Left
Optical System Design
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PACS IIDR
01/02 Mar 2001
Chopping Right
Optical System Design
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PACS IIDR
01/02 Mar 2001
Entrance Optics + Blue Photometer
Dichroic
Filter
Wheel
Blue
Bolometer
Cryo
cooler
Optical System Design
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PACS IIDR
01/02 Mar 2001
Entrance Optics + Blue Photometer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Entrance Optics + Blue Photometer + Red Photometer
Dichroic
Filter
Red
Bolometer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Entrance Optics + Blue Photometer + Red Photometer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Common Focus
Photometer Unit
Dichroic
Dichroic
Fold
Fold
Red
Blue Bolometer
Fold
Red
Bolometer
Blue
Common Focus
Dichroic
Fold
Optical System Design
Red
Blue
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PACS IIDR
01/02 Mar 2001
The Spectrometer Section
Optical System Design
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PACS IIDR
Photometer
Optics
Blue Bolometer
01/02 Mar 2001
Filter Wheel I
Slicer
Optics
0.3 K Cooler
Red Bolometer
Grating
Grating Drive
Encoder
sGeGaDetector
Red Spectrometer
Spectrometer
Optics
Chopper
Calibrator I and II
Calibrator
Optics
Entrance Optics
Optical System Design
sGeGa Detector
Blue Spectrometer
Filter Wheel II
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PACS IIDR
01/02 Mar 2001
Geometrical Optics Performance
Optical System Design
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PACS IIDR
01/02 Mar 2001
Optical Performance - Blue Bolometer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Optical Performance - Geometry Blue Bolometer
3
1
Optical System Design
2
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PACS IIDR
01/02 Mar 2001
Optical Performance - Red Bolometer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Optical Performance - Geometry Red Bolometer
Optical System Design
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PACS IIDR
01/02 Mar 2001
Optical Performance - Spectrometer
Center of Array, center l
Optical System Design
Corner of Array, extreme l
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PACS IIDR
01/02 Mar 2001
Optical Performance - Geometry Spectrometer
174.6 µm
175.0µm
175.4µm
“ILB”
Optical System Design
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PACS IIDR
01/02 Mar 2001
Diffraction
Optical System Design
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PACS IIDR
01/02 Mar 2001
Illumination of Lyot Stop
•
•
•
•
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
GLAD 4.5
diffraction
analysis
l = 175 µm
Intensity (arb. units)
2 Strategies
Radius [cm]
Optical System Design
1 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
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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
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PACS IIDR
01/02 Mar 2001
Diffraction Gallery at 175 µm
telescope focus,
re-imaged
“slice” through point spread function
entrance slit field mirror
capture mirror
Detector
array
pixel
grating
Optical System Design
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PACS IIDR
01/02 Mar 2001
Grating:
The worst offender
at long wavelength
• 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
Optical System Design
43
PACS IIDR
01/02 Mar 2001
Grating:
The worst offender
at long wavelength
• 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
Optical System Design
44
PACS IIDR
01/02 Mar 2001
Grating:
The worst
offender
at long
wavelength
Before Grating
Angle of incidence:
60.4°
1.Order
Angle of incidence:
46.6°
3.Order
Grating
Grating
205µm
57µm
Grating
80mm x 320mm
Width of grating sufficient: minimal loss at 205 µm
Y
After Grating
Angle of incidence:
60.4°
1.Order
Angle of incidence:
46.6°
3.Order
X
Grating
Grating
Optical System Design
57µm
205µm
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PACS IIDR
01/02 Mar 2001
Grating:
The worst
offender
at long
wavelength
Before Grating
Angle of incidence:
60.4°
1.Order
Angle of incidence:
46.6°
3.Order
Collimator
Vignetting
Grating
57µm
Grating
80mm x 320mm
Grating
205µm
Y-Axis has to be
scaled by 1/cos(60.5°)
Y-Axis has to be
scaled by 1/cos(46.6°)
Losses due to length of grating at 205 µm, 57 µm OK
Y
After Grating
Angle of incidence:
46.6°
3.Order
X
Angle of incidence:
60.4°
1.Order
Grating
Vignetting
Grating
Grating
205µm
57µm
Y-Axis has to be
scaled by 1/cos(46.6°)
Optical System Design
Y-Axis has to be
scaled by 1/cos(60.5°)
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PACS IIDR
01/02 Mar 2001
Diffraction Summary
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
Optical System Design
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