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PACS IBDR
27/28 Feb 2002
Optical System Design
N. Geis
MPE
Optical System Design
1
PACS IBDR
27/28 Feb 2002
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 IBDR
27/28 Feb 2002
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 IBDR
27/28 Feb 2002
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)
•
Chopper throw (on sky) reduced to 1 full array size to allow
for larger FOV of bolometers with same entrance fieldstop/mirror sizes as previous design.
Optical System Design
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PACS IBDR
27/28 Feb 2002
Optical Design – Top Optics
Optical design for calibration sources
•
Acceptable image quality of pupil
• Köhler-type illumination (pupil on source aperture + a 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 IBDR
27/28 Feb 2002
Uniformity of Illumination by Calibrators
The two sources produce
mirrored illumination
distributions, as seen from
the detectors

Maximum (unwanted)
modulation of the calibration
signal by non-uniformity is ~ 5%
Compatible with the goal of
having relative signal changes of
10% when chopping.
E.g., one could set operating points
such that the range of signal is 7.5–
12.5% when chopping.
Optical System Design
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PACS IBDR
27/28 Feb 2002
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 IBDR
27/28 Feb 2002
Telescope
Cal. Source 1
C2 Active 3
C1 Active 3
Cal. Source 2
TO Fold 1
C1 Active 1
(Lens)
TO Fold 2
C2 Active 1
(Lens)
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 IBDR
27/28 Feb 2002
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
B Fold R1
B Fold B1
S Fold 3
B Active R1
B Active B1
S Fold 4
B Active R2
B Active B2
S Active 3
Filter
Filter Wheel
Red
Bolometer
Array
Blue
Bolometer
Array
Slicer
Optics
Optical
components
after Top
Optics
S Active 4
S Active 5
S Active 6
Dichroic
Beamsplitter
Filter
S Fold 5
Red
Spectrometer
Array
Optical System Design
Filter Wheel
Pupil
Field
Blue
Spectrometer
Array
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PACS IBDR
27/28 Feb 2002
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 (use for alignment and
baffling purposes)
• 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 compact unit at FPU external wall
Optical System Design
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PACS IBDR
27/28 Feb 2002
Optical Design – Spectrometers
No Changes in optical design for spectrometer since IIDR
•
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 of detector filters & alignment
gap of 0.75 mm between slit mirrors
gap of 3.6 mm between detector blocks for filter holder
•
Image quality diffraction limited
•
Excellent baffling situation
• end optics for both spectrometers separated on “ground floor”
• exit field stop of spectrometer inside a “periscope”
• extra pupil and field stops possible in end optics (alignment, baffles)
Optical System Design
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PACS IBDR
27/28 Feb 2002
The Image Slicer
Optical System Design
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PACS IBDR
27/28 Feb 2002
Image Slicer and Grating (in)
Slit Mirror
Slicer Mirror
Capture Mirror
Grating
Optical System Design
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PACS IBDR
27/28 Feb 2002
Image Slicer and Grating (in+out)
Slit Mirror
Periscope
Optics
Slicer Stack
Capture Mirror
Grating
Optical System Design
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PACS IBDR
27/28 Feb 2002
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
•
Excellent image quality in both, photometers, and
spectrometers
Optical System Design
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PACS IBDR
PACS Envelope -filled
Optical System Design
27/28 Feb 2002
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PACS IBDR
27/28 Feb 2002
PACS Optical Functional Groups
Optical System Design
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PACS IBDR
Photometer
Optics
Blue Bolometer
27/28 Feb 2002
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 IBDR
Entrance Optics
& Photometer
27/28 Feb 2002
Chopper
Lyot Stop
Telescope Focus
Dichroic
Calibrator
I+II
Filter
Wheel
Blue
Bolometer
Cryo
cooler
Red
Bolometer
Optical System Design
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PACS IBDR
27/28 Feb 2002
Chopping Left
Optical System Design
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PACS IBDR
27/28 Feb 2002
Chopping Right
Optical System Design
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PACS IBDR
27/28 Feb 2002
The Spectrometer Section
Optical System Design
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PACS IBDR
27/28 Feb 2002
PACS Filter Scheme
Optical System Design
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PACS IBDR
27/28 Feb 2002
Filter Rejection Requirements
(determined from template observation scenarios)
The requirements from 3
demanding astronomical
scenarios...
...lead to the required filter
suppression factors.
Solid red line: total required
suppression
Suppression factor
• planet with high albedo
• deep imaging (Galactic/extragalactic)
• FIR excess around bright star
(bolometers
only)
detector response
filter transmission
overall response
Dashed blue line: model detector
responsivity
Dotted green line:
resulting required filter
suppression factor
Optical System Design
Wavelength [µm]
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PACS IBDR
27/28 Feb 2002
PACS Filters
• Filter Functions
– definition of spectral bands
• photometric bands
• order sorting for spectrometer grating
– in-band transmission (high)
– out-of-band suppression (thermal background, straylight, astronomical)
• Filter implementation
– Filter types (low-pass, high-pass, band-pass, dichroic)
– Technology: Metal mesh filters developed at QMW
• Proven technology
• Robust
• Excellent Performance
– Filter location in optical path chosen for
• rejection of thermal radiation from satellite
• instrument stray light management
Optical System Design
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PACS IBDR
27/28 Feb 2002
PACS Filtering Scheme
Optical System Design
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PACS IBDR
27/28 Feb 2002
Example: Prototype of Long Pass Edge filter
Examples of QMW filters
Examples of QMW filters
Optical System Design
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PACS IBDR
27/28 Feb 2002
Example Filter Chain: Long-Wavelength Photometer
Dichroic beam splitter
130.µm
Long-pass edge filters
52 µm
110 µm
125 µm
.
.
.
Short-pass edge filter
210.µm
Optical System Design
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PACS IBDR
27/28 Feb 2002
Filter Summary
• Filter scheme with 4 or 5 filters in series in each
instrument channel provides sufficient out-of-band
suppression
• Measured/expected in-band transmission
– > 80 % for long-pass and dichroic filters
– ~ 80 % for band-pass filters
 > 40 % for filter combination
– ~ 50 % expected
• Requirements will be met
Optical System Design
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PACS IBDR
27/28 Feb 2002
Geometrical Optics Performance
Optical System Design
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PACS IBDR
27/28 Feb 2002
Optical Performance - Blue Bolometer
Optical System Design
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PACS IBDR
27/28 Feb 2002
Optical Performance - Geometry Blue Bolometer
3
1
Optical System Design
2
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PACS IBDR
27/28 Feb 2002
Optical Performance - Red Bolometer
Optical System Design
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PACS IBDR
27/28 Feb 2002
Optical Performance - Geometry Red Bolometer
Optical System Design
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PACS IBDR
27/28 Feb 2002
Optical Performance - Spectrometer
Center of Array, center l
Optical System Design
Corner of Array, extreme l
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PACS IBDR
27/28 Feb 2002
Optical Performance - Geometry Spectrometer
174.6 µm
175.0µm
175.4µm
75% Strehl @ 80 µm
Optical System Design
90% Strehl @ 80 µm
“ILB”
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PACS IBDR
27/28 Feb 2002
PACS Optical Performance in a System Context
New
Req?
New
Goal?
Optical System Design
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PACS IBDR
27/28 Feb 2002
Diffraction
Optical System Design
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PACS IBDR
27/28 Feb 2002
Illumination of Lyot Stop
•
M2 is system aperture
•
Image quality of M2 on Lyot stop determined by
diffraction from PACS entrance field stop
•
Diffraction ring ~10% of aperture area
•
Cannot block “Narcissus effects” from M2 center at
Lyot stop without throughput loss
2 Strategies
1
Intensity (arb. units)
GLAD 4.5
diffraction
analysis
l = 175 µm
depending on outcome of system
straylight analysis
2
Radius [cm]
Optical System Design
M2 as system stop (baseline):
oversize cold stop by ~ 10% area
(if only cold sky visible beyond M2, and
straylight analysis allows)
Lyot stop as system stop (optional):
undersize cold stop by ~ 10% area —
throughput loss
(if diffracted emission/reflection from M2
spider, M2 edge, or straylight is problematic)
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PACS IBDR
27/28 Feb 2002
Diffraction Analysis - Slicer/Spectrometer
Diffraction Analysis of the Spectrometer repeated with final 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 IBDR
27/28 Feb 2002
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 IBDR
27/28 Feb 2002
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
42
PACS IBDR
27/28 Feb 2002
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
43
PACS IBDR
27/28 Feb 2002
Diffractive Walk-Off
Off-axis pixel diffraction throughput
For edge pixels, and long wavelength, asymmetric diffraction losses move the
PSF peak ~ 0.3 pixel (3’’) from its expected spatial position.
Image scale on the sky for the spectrometer depends on wavelength  Effect
needs to be fully characterized for astrometry/mapping.
Optical System Design
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PACS IBDR
27/28 Feb 2002
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 (i.e., image quality is
intact)
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 induced “chromatic aberration” needs further study
Optical System Design
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