optical system design

Optical System Design 1

PACS IIDR 01/02 Mar 2001

Optical System Design

N. GeisMPE



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



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)

• Reduced chopper throw (sky) to allow for larger

FOV of bolometers with same entrance field stop /

mirror sizes

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 Design – Top Optics



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

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



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 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•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



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



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

• Improved image quality in both, photometers,

and spectrometers

Optical Design Summary



A Walk Through PACS



PACS Envelope -filled



PACS Functional Groups



PACS Envelope



PACS Envelope + Top Optics



Top Optics Chopper

Telescope Focus

Lyot Stop



Calibrators Calibrator I+II



Chopping Left



Chopping Right



Entrance Optics + Blue Photometer

Dichroic

FilterWheel

BlueBolometer

Cryocooler



Entrance Optics + Blue Photometer



Entrance Optics + Blue Photometer + Red Photometer

Dichroic

RedBolometer

Filter



Entrance Optics + Blue Photometer + Red Photometer



Photometer UnitCommon Focus

Dichroic

Red

Dichroic

Blue

Bolometer

Common Focus

Fold Fold

Fold

Fold

Red Blue

Red

Blue Bolometer

Dichroic



The Spectrometer Section



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



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



Diffraction



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



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



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



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




Width of grating sufficient: minimal loss at 205 µm

205µm

Grating

Grating



57µmGrating

Y-Axis has to be scaled by 1/cos(46.6°)






Grating

205µm


57µmGrating


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



• 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

optical system design

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