Optical System Design

Optical System Design 1

PACS IBDR 27/28 Feb 2002

Optical System Design

N. GeisMPE


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



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 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 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



Uniformity of Illumination by Calibrators

The two sources produce mirrored illumination distributions, as seen from the detectorsMaximum (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.



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



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 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 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



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



The Image Slicer



Image Slicer and Grating (in)

Slicer Mirror

Capture Mirror

Slit Mirror

Grating



Image Slicer and Grating (in+out)

Slicer Stack

Capture Mirror

Slit Mirror

Grating

Periscope Optics



• 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


PACS IBDR 27/28 Feb 2002PACS Envelope -filled


PACS IBDR 27/28 Feb 2002PACS Optical Functional Groups



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



Dichroic

FilterWheel

BlueBolometer

Cryocooler

RedBolometer

Chopper

Telescope Focus

Lyot Stop

Calibrator I+II

Entrance Optics &

Photometer



Chopping Left



Chopping Right



The Spectrometer Section



PACS Filter Scheme


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

Supp

ress

ion

fact

or

Dotted green line: resulting required filter suppression factor

Wavelength [µm]

detector responsefilter transmissionoverall response

(bolometersonly)



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



PACS Filtering Scheme



Example: Prototype of Long Pass Edge filter

Examples of QMW filters

Examples of QMW filters


PACS IBDR 27/28 Feb 2002Example Filter Chain: Long-Wavelength

Photometer

Dichroic beam splitter 130.µmLong-pass edge filters

52.µm110.µm125.µm

Short-pass edge filter 210.µm



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



Geometrical Optics Performance



Optical Performance - Blue Bolometer



Optical Performance - Geometry Blue Bolometer

1 2

3



Optical Performance - Red Bolometer



Optical Performance - Geometry Red Bolometer



Optical Performance - Spectrometer

Center of Array, center Corner of Array, extreme



Optical Performance - Geometry Spectrometer

“ILB”

175.0µm

175.4µm

174.6 µm

90% Strehl @ 80 µm75% Strehl @ 80 µm



PACS Optical Performance in a System Context

New Goal?

New Req?



Diffraction



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

nsity

(arb

. uni

ts)

• 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



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



Diffraction Gallery at 175 µmtelescope focus, re-imaged “slice” through point spread

function

capture mirror

entrance slit field mirror

grating

pixel

Detectorarray



•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



•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



Diffractive Walk-Off

Off-axis pixel diffraction throughputFor 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.



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

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Optical System Design

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