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Optics•Design Overview

–Overview–Performance

•Coude Feed•Gregorian

•Prime Focus

•Primary Mirror Overview•Secondary Mirror Overview

Jim OschmannRon Price

Optical Design Overview

• Primary Mirror– 4 meter clear aperture, off axis (12 meter parent)– F/2

• Heat stop at prime focus– Allows 3-5 arc minutes through system– Potential occulting disk area

• Secondary Mirror– 600 mm off axis gregorian– Gregorian focus at f/13

• Feed Optics– Relay image to Coude area– F/69 (over elevation & azimuth axis)– Contains Deformable and tip/tilt mirrors for AO– Further interface optics for specific instruments needed

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M2

M4

DM

M3

M3

M4

M2

M1

Coude Focus

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Coude Focus

0.1”

Gregorian Focus

4

Gregorian Focus

0.2”

Prime Focus

5

Prime Focus

13 arc sec

Primary Mirror Overview

• Blank Configuration• Blank Material• Polishing• Support System• Procurement Schedule

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Preliminary Mirror Blank Configuration

Ø12100 PARENT PARABOLOID

4000

(4100)

100

ALL DIMENSIONS IN MM.

Ø4237

SURFACE A

SURFACE B

ATST PRIMARY MIRROR

PARENT PARABOLOID

GEOMETRICAL AXIS OF ATST PRIMARY MIRROR BLANK

AXIS A,GEOMETRICAL AXIS OF

PARENT PARABOLOID

A

SECTION A-A

Mirror Blank Material

• Selection of mirror blank material is driven by large temperature gradients from front to back

• Ultra-low-expansion material is needed to maintain optical figure under these conditions

• Preferred materials:– Schott Zerodur® low expansion glass ceramic– Corning ULE® titanium doped fused silica

• Alternate materials– Silicon Carbide example being worked for Gregor– LZOS AstroSitall

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Mirror Blank Thickness

• Mirror blank thickness is a trade-off based on several competing factors

Thinner:

More support print-thru

Decreased weight

Decreased thermal inertia

Decreased resistance to wind buffeting

Higher handling stress

Higher resonant frequency

Thicker:

Less support print-thru

Increased weight

Increased thermal inertia

Increased resistance to wind buffeting

Lower handling stress

Lower resonant frequency

Blank

Thickness

100 mm

Mirror Blank Status

• On-going discussions being held with Schott and Corning

• ROMs for cost and schedule provided• No significant difference between off-axis and

on-axis blanks• Blank will be generated to a best-fit spherical

contour (~10mm deviation from final shape)

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Primary Mirror Polishing

• Preliminary Specifications have been developed:– F/number: f/2 (parent mirror Ø12m f/0.7)– Surface figure: off-axis paraboloid– Conic Constant: K= -1.00– Paraxial ROC: 16 meters– Surface Roughness: 20 A rms or better– Figure Accuracy: 85% encircled energy within

0.13 arc second at 550 nm– Testing Support: supported on actual system

hardware or equivalent– Optical Testing: Full and sub-aperture

interferometry

Primary Mirror Polishing Feasibility Study

• Due to unique off-axis configuration, ATST issued an RFP in July, 2002 for a Primary Mirror Polishing Feasibility Study

• RFP was sent to:

U of A/SOML Brashear LP (Contraves ) Kodak Goodrich (Hughes Danbury) Rayleigh Optical SAGEM (REOSC)

• Contracts to produce Polishing Feasibility Studies awarded to:

U of A Brashear LP Rayleigh Optical SAGEM

• Each contractor’s study will propose and evaluate grinding & polishing processes, testing methods, specifications achievable, timescale, costs

• Concentration on f/2 configuration• Compare with on-axis case• Studies to be completed in 6 months / available for ATST CoDR

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Primary Mirror Support System

• Actively controlled mirror supports– 80 – 120 axial supports in

concentric rings– 48 – 72 lateral supports

around periphery of mirror

• Two general approaches:– Passive hydraulic 3-zone

system with superimposed active forces

– Electro-mechanical actuators

PRELIMINARY SUPPORT PATTERNFOR 120 AXIAL SUPPORTS

Primary Mirror Procurement Schedule

• General Discussions / Visits to Blank Fabricators 2 months• Prepare / Issue RFP for Primary Mirror Blank 1 month• Contractor Response Time 2 months• Source Selection Process 1 month• Contract Negotiations / Approval / Award 2 months• Blank Fabrication 20 months• Blank Generation 6 months• Acid Etching of rear and sides of blank 1 month• Transporation of blank to Polisher 1 month• Grinding / Polishing / Testing 30 months• Transportation of finished mirror to site 1 month• Integration of mirror into primary mirror cell 2 months• Coating of primary mirror 1 month

Total Time Required 70 months(~6 years)

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Basis for Procurement Schedule Estimates

• ROM cost and schedule inputs provided by blank fabricators

• Discussions / Feasibility Studies by mirror polishing contractors

• Direct and indirect experience from WIYN, SOAR, Gemini and VISTA telescope projects

Secondary Mirror Overview

• Blank Properties• Polishing• Support / Positioning System

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Secondary Mirror Blank

• 60 cm diameter• Lightweight, structured design• Materials under evaluation:

– Silicon Carbide– ULE– Zerodur– Beryllium

• Silicon Carbide has the best resistance to bending due to thermal gradients (solar heat load)

Secondary Mirror Polishing

• Secondary Mirror will be a concave ellipsoid• Preliminary specifications will be developed over

the next 2-3 months• Discussions with polishers will be conducted to

identify any risk or cost issues

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Secondary Mirror Support/Positioning System

• Support – Secondary mirror will attach to positioning system kinematically via three points

• Positioning – Five (possibly six) degrees of freedom required

• Hexapod or conventional five-axis system are being considered