Optical Design & Analysis Paul Martini · Optical Design & Analysis Paul Martini July 6th, 2004. Harvard-Smithsonian Center for Astrophysics MMIRS PDR PM 2 Outline • Optical Design

Harvard-SmithsonianCenter for Astrophysics

MMIRS PDR PM 1

Optical Design & AnalysisPaul Martini

July 6th, 2004


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Outline

• Optical Design• Filters and Grisms• Pupils• Throughput Estimate• Ghost Analysis• Tolerance Analysis• Critical Areas• Task List


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Requirements

• Multislit spectroscopy over a wide field low to moderate resolution

• Wide field imaging• Slit mask exchange during daylight• Illuminate a 2048x2048 HAWAII-2 array

with adequate sampling


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Heritage

• Two-dewar design from R. Elston• Original MMIRS design by H. Epps• Modifications by D. Fabricant


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Overview

• Fully illuminated 7’x7’ field of view• Multislit spectroscopy over 4’x7’

– Some loss of spectral coverage outside 2’x7’• J+H or H+K spectroscopy at R=1500• J, H, or K spectroscopy at R=3000• Y, J, H, K, J+H, and H+K filters


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Optical Layout

Corrector

Slit Plane

Collimator

Camera

Detector


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Corrector DetailCaF2

D=14’139.4mm


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Full Corrected FieldGuide camera field Science Field

D = 14 arcmin 7x7 arcmin


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Corrector PerformanceSlit Plane

Guider

1 pixelbox


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Collimator/Camera Detail

CaF2

FQTZ

BaF2

ZnSe

S-FTM16


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Collimator/Camera RMS Spots

2 pixelbox


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Collimator/Camera Spots & EEJ H K


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Filters

• 2 filter wheels with 6 positions each• Y, J, H, K filters• J+H and H+K filters for spectroscopy• Large filters: 120mm diameter• Identified a consortium to reduce cost• Space for several future filters


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Grisms

• Five Grisms on CaF2

15000.08021H+K

15000.09418J+H

30000.1341K

30000.17541H

30000.2341J

RGrooves/µmAngleBand Flamingos-2

18 0.098

21 0.082


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Pupils

• Two pupils as compromise between throughput and thermal blocking

• Fixed, oversized pupil used by all observing modes

• Undersized pupils directly attached to K and H+K filters (one for each) to minimize thermal contamination


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Throughput Estimate

• Imaging estimate: 40%• Spectroscopy estimate: 30%• Assumptions:

– Surface losses: 2% per surface (28 surfaces in imaging mode) or 56%

– Filter losses: 10% (2 more surfaces)– Pupil losses: 0-5%– Grism losses: 25% (3 more surfaces)– Detector QE 75%


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Ghost Analysis

• Lenses assumed to be 1% reflective, detector 40%

• Ghosts minimized in optical design stage• Imaging mode

– Reflection off filters avoided by tilting filters• Spectroscopy mode

– Reduced transmission, although addition of surface at a conjugate point


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Ghost Results

• Reflectance of (back of) slit masks should be minimized

• No ghost images identified• Strongest ghost pupils are either very

vignetted or uniformly fill the field


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Tolerance

• Present design produces rms spot radius of 10 microns on the detector and 4 microns at the slit plane

• Goal: <14 micron spot radius at the detector and <10 micron spot radius at the slit plane


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Error Budget

• Lens constraints:– surface radii and quality– surface decenter and tilt– element thickness

• Element constraints:– element decenter and tilt– spacing between elements– group decenter and tilt


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Input Lens Constraints

• Radii: 0.1%• Flats: 2 fringes• Surface Irregularities: 0.5 fringes• Thickness 0.05mm• Surface tilt: 0.01mrad• Surface decenter: 0.01mm


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Input Element/Group Constraints

• Spacing: 0.1mm• Element tilt: 0.1mrad• Element decenter: 0.05mm• Group tilt: 0.2mrad• Group decenter: 0.1mm


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Analysis Method

• Inverse Sensitivity analysis in ZEMAX to compute first pass tolerance budget

• Identify critical constraints and rebalance the tolerance budget

• Set goal of <14 micron spot radius• Run Monte Carlo realizations • Determine mechanical constraints that

achieve goal with 90% probability


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Tolerance Simulations

4 5 6 78 9 10

11 12 13 - 20 21 22 - 27 28-34

L1 L2 L3 L4 L5 - L8 L9 - L11 L12 - L14

Lenses and Surfaces

Lens Groups

G1 G2 G3 G4

G5 G6


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Potential degradation

As designed “Worst case”


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Mechanical Requirements

• Lens spacings: several require 0.05mm• Lens tilts: many require 0.1mrad; one 0.05

mrad (although most closely packed)• Lens decenter: several require 0.025mm• Group tilt: several require 0.1mrad• Group decenter: several require 0.05mm;

one 0.025mm


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Critical Areas

• Lens spacing: L4-L5, L9-L10, L13-L14• Lens tilt: L3, L4, L10-L13• Lens decenter: L4, L5• Group tilt: G2, G3, G4, G6• Group decenter: G2


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Task List

• Identify vendors for optics and coatings• Finalize design with melt sheet data• Test plate fitting to reduce costs• Thermal gradient sensitivity study• Mechanical deflection sensitivity study• Finalize selection of Grisms• Obtain filters• Stray light analysis (in progress)


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Summary

• “Penultimate” optical design in hand• Lens sizes and materials are available

from multiple vendors• Filters and Grisms are attainable• Ghost images and pupils are negligible• Tolerance analysis has identified critical

areas for consideration in mechanical design

Documents

Optical Design & Analysis Paul Martini · Optical Design & Analysis Paul Martini July 6th, 2004. Harvard-Smithsonian Center for Astrophysics MMIRS PDR PM 2 Outline • Optical Design