Another Look at Open Back Zerodur Another Look at Open Back Zerodur Primary Mirrors (OBZ PMs)Primary Mirrors (OBZ PMs)

Robert Besuner

12 May 2005

2

Evolution as of 2 weeks agoEvolution as of 2 weeks ago

• Mounting is near 2/3 radius rather than peripheral.

• Offloading schemes are better optimized.

• Cathedral ribs behind front face sheet to reduce gravity and polishing quilting.

• Tapered-back option is examined.

SAGEM design—basis for early OBZ studies Present tapered-back design

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This week’s evolutionThis week’s evolution

• Smaller cell size (294mm202mm leg length).

• No cathedral ribs, thinner front face (10mm8mm).

• Struts mount to sides of a (thicker) rib instead of to a partial closed cell.

• Tapered design is less aggressively lightened.

Tapered-back design from 2 weeks ago—294mm cells Present tapered-back design—202mm cells

4

Constant-Thickness MeniscusConstant-Thickness Meniscus

• 200mm thick mirror.

• 8mm front face.

• 8mm thick outer periphery.

• 6mm thick ribs.

• 20mm thick strut-mounting ribs.

• Glass mass = 245kg (compare to 230kg for prev. OBZ, 248kg PBZ, 204kg CBULE).

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Tapered-Back DesignTapered-Back Design

• 200mm thick at bipods.

• Tapers to 110mm thick at OD, flat back from bipods to ID.

• 8mm front face.

• 8mm thick outer periphery.

• 6mm thick ribs.

• 20mm thick strut-mounting ribs.

• Glass mass = 206kg (saves 39kg rel. to meniscus, comparable to CBULE).

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Performance ComparisonPerformance Comparison

closed back ULE

partial closed back

Zerodur

meniscus open back

Zerodur (294mm)

tapered open back

Zerodur (202mm)

meniscus open back

Zerodur (202mm)

tapered open back

Zerodur (202mm)

glass mass (kg) 204 248 230 178 245 206glass safety factor @ limit loads 16 11 5 5 5 7nat'l freq. on bipods (Hz) 111 110 103 93 102 100free-free nat'l freq. (Hz) 283 211 179 136 183 155rms surf. distortion in 1g lateral (nm) [indicates sensitivity to lateral loads] 81 187 217 313 82 * 82 *rms surf. distortion in 1g axial (nm) 757 726 1120 1710 1230 1740* different strut mounting pad detail accounts for much of the difference

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Offloader Count TrendsOffloader Count Trends

Trend for Offloading Primary Mirror in Vertical Beam Orientation

1

10

100

1000

1 10 100Number of support points (including bipods)

su

rfa

ce

dis

tort

ion

(n

m r

ms

)

partial closed back Zerodur

closed back ULE

open back Zerodur (294mm)

open back Zerodur (202mm)

tapered open back Zerodur (202mm)

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ConclusionConclusion

• A tapered open back Zerodur design can compete with a closed back ULE design in performance and mass.