pacs ibdr 27/28 feb 2002 optical system design1 n. geis mpe
Post on 01-Jan-2016
216 Views
Preview:
TRANSCRIPT
Optical System Design 1
PACS IBDR 27/28 Feb 2002
Optical System Design
N. GeisMPE
Optical System Design 2
PACS IBDR 27/28 Feb 2002Pacs 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 IBDR 27/28 Feb 2002
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 IBDR 27/28 Feb 2002
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 field-stop/mirror sizes as previous design.
Optical Design – Top Optics
Optical System Design 5
PACS IBDR 27/28 Feb 2002
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 Design – Top Optics
Optical System Design 6
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 7
PACS IBDR 27/28 Feb 2002
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 8
PACS IBDR 27/28 Feb 2002
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 (Lens)
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 (Lens)
Cal. Source 2
PupilField
Calibrator 2Calibrator 1
Optical System Design 9
PACS IBDR 27/28 Feb 2002
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 IBDR 27/28 Feb 2002
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 Design – Photometers
Optical System Design 11
PACS IBDR 27/28 Feb 2002
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 mirrorsgap 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 Design – Spectrometers
Optical System Design 12
PACS IBDR 27/28 Feb 2002
The Image Slicer
Optical System Design 13
PACS IBDR 27/28 Feb 2002
Image Slicer and Grating (in)
Slicer Mirror
Capture Mirror
Slit Mirror
Grating
Optical System Design 14
PACS IBDR 27/28 Feb 2002
Image Slicer and Grating (in+out)
Slicer Stack
Capture Mirror
Slit Mirror
Grating
Periscope Optics
Optical System Design 15
PACS IBDR 27/28 Feb 2002
• 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 Design Summary
Optical System Design 16
PACS IBDR 27/28 Feb 2002
PACS Envelope -filled
Optical System Design 17
PACS IBDR 27/28 Feb 2002
PACS Optical Functional Groups
Optical System Design 18
PACS IBDR 27/28 Feb 2002
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 19
PACS IBDR 27/28 Feb 2002
Dichroic
FilterWheel
BlueBolometer
Cryocooler
RedBolometer
Chopper
Telescope Focus
Lyot Stop
Calibrator I+II
Entrance Optics &
Photometer
Optical System Design 20
PACS IBDR 27/28 Feb 2002
Chopping Left
Optical System Design 21
PACS IBDR 27/28 Feb 2002
Chopping Right
Optical System Design 22
PACS IBDR 27/28 Feb 2002
The Spectrometer Section
Optical System Design 23
PACS IBDR 27/28 Feb 2002
PACS Filter Scheme
Optical System Design 24
PACS IBDR 27/28 Feb 2002Filter Rejection Requirements
(determined from template observation scenarios)
The requirements from 3 demanding astronomical scenarios...
• planet with high albedo• deep imaging (Galactic/extragalactic)• FIR excess around bright star
...lead to the required filter suppression factors.
Solid red line: total required suppression
Dashed blue line: model detector responsivity
Suppre
ssio
n f
act
or
Dotted green line: resulting required filter suppression factor
Wavelength [µm]
detector responsefilter transmissionoverall response
(bolometersonly)
Optical System Design 25
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 26
PACS IBDR 27/28 Feb 2002
PACS Filtering Scheme
Optical System Design 27
PACS IBDR 27/28 Feb 2002
Example: Prototype of Long Pass Edge filter
Examples of QMW filters
Examples of QMW filters
Optical System Design 28
PACS IBDR 27/28 Feb 2002
Example Filter Chain: Long-Wavelength Photometer
Dichroic beam splitter 130.µm
Long-pass edge filters
52.µm
110.µm125.µm
Short-pass edge filter 210.µm
Optical System Design 29
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 30
PACS IBDR 27/28 Feb 2002
Geometrical Optics Performance
Optical System Design 31
PACS IBDR 27/28 Feb 2002
Optical Performance - Blue Bolometer
Optical System Design 32
PACS IBDR 27/28 Feb 2002
Optical Performance - Geometry Blue Bolometer
1 2
3
Optical System Design 33
PACS IBDR 27/28 Feb 2002
Optical Performance - Red Bolometer
Optical System Design 34
PACS IBDR 27/28 Feb 2002
Optical Performance - Geometry Red Bolometer
Optical System Design 35
PACS IBDR 27/28 Feb 2002
Optical Performance - Spectrometer
Center of Array, center Corner of Array, extreme
Optical System Design 36
PACS IBDR 27/28 Feb 2002
Optical Performance - Geometry Spectrometer
“ILB”
175.0µm
175.4µm
174.6 µm
90% Strehl @ 80 µm75% Strehl @ 80 µm
Optical System Design 37
PACS IBDR 27/28 Feb 2002
PACS Optical Performance in a System Context
New Goal?
New Req?
Optical System Design 38
PACS IBDR 27/28 Feb 2002
Diffraction
Optical System Design 39
PACS IBDR 27/28 Feb 2002
Illumination of Lyot Stop
2 Strategiesdepending on outcome of system straylight analysis
1 M2 as system stop (baseline):oversize cold stop by ~ 10% area (if only cold sky visible beyond M2, and straylight analysis allows)
2 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)
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
• Diffraction ring ~10% of aperture area
• Cannot block “Narcissus effects” from M2 center at Lyot stop without throughput loss
Optical System Design 40
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 41
PACS IBDR 27/28 Feb 2002
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 42
PACS IBDR 27/28 Feb 2002
•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 43
PACS IBDR 27/28 Feb 2002
•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 44
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 45
PACS IBDR 27/28 Feb 2002
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
Diffraction Summary
top related