Mirror Fabrication via Glass Slumping Techniques Anita Schael Max-Planck-Institute for...

Mirror Fabrication via Glass Slumping Techniques

Anita SchaelMax-Planck-Institute for Extraterrestrial Physics

in collaboration with:Monika Vongehr, Peter Friedrich

Mirror Fabrication via Glass Slumping

Outline

Metrology of mirror glasses

Thickness variation: Interference• Motivation

• Theory

• Implementation

X-Ray test: Gravitation-free mounting• Motivation

• Theory

• Implementation


Thickness Measurement

current method: LPKF measuring table accurate optical sensors clean room

uncertainty due to refraction index problems with curved segments time consuming

new method:

Interference patterns very time-efficient no problem with

refraction index very accurate (~ /10)


Interference: Theory I

Thickness measurement through double-reflection in glass

‘ d

C

In point C: interference of both beams A and B

path difference:

BA

22 sin2 ndl


Interference: Theory II

If path difference = n /2 (n integer) maximum / minimum

Interference pattern depending on d and

In perfectly plane-parallel layer: ring pattern

22 sin2 ndl


Interference: Haidinger Rings

in different angles : different l „Fringes of equal inclination“

‘ d

C


Interpretation

Real image = combination of patterns of: perfectly plane-parallel layer thickness variations in glass

a true profile of the glass can be derivedfrom residual of thereal pattern and the rings in perfect layer


Interference: Implementation I

Sodium light: = 589.5 nm via beam splitter onto glass segment observed through the beam splitter

diffuse sodium light

ObserverGlass segment

beam splitter


Implementation II


Thickness variation:before and after slumping

Do thickness variations in the glass change in the slumping process?


Quantitative Analysis I

In reality: more than two interfering rays with decreasing intensity

d

need to take reflectivity into account:more beams within the plate finer and sharper fringes

also: angle of slumped glass...


Quantitative Analysis II Thickness profile from interference pattern?

residual between two pictures taken before slumping – at two different positions

ring pattern of perfect plate stays on optical axis fringes of thickness variations move with plate derivation of thickness profile

Before and after slumping change in thickness profile

Results: thickness variations do not increase during slumping slight decrease...?

Further analysis in progress


Outline

Metrology of mirror glasses

Thickness variation: Interference• Motivation

• Theory

• Implementation

X-Ray test: Gravitation-free mounting• Motivation

• Theory

• Implementation


Mounting for X-Ray Test

Test of glass segments at X-Ray testing facility „PANTER“ Problem: glass sags under its own gravity distortion of

shape

Simulation of sag with glass fixed in y-axis only on both long edges

kink between parabola and hyperbola stabilises centre

maximum sag at short edges: ~26 µm


Compensation of Gravity

Idea: built-up of air pressure under the glass equivalent

to gravitational force using a water column

glass segmentPenclosed

PAir

Advantages: easy physical principle balance of air pressure variations through open

water column


Theory: Principle

Water column: Pascal‘s Principle

with = density, g = grav. acceleration, h = height of water column, P = pressure

Using with FG = 0,2698 N P = 13,49 Pa

water column of h = 1.3789 mm

Phg

A

FP

h


Problems

Main problem: dependence on temperature variations

Pressure

Temperature

Experiment over 24 hours in lab conditions

2

22

1

11

T

VP

T

VP

Solution: volume under glass needs to be kept very small less change in pressure


Improved design

Two containers of plexiglass, the inner one open at bottom

glass segment

manometer

plexiglass container

Oil layer on water to avoid evaporation


Results

Durchhängeverhalten bei 13,0 pa und ohne Druck (begradigt)

7450

7500

7550

7600

7650

20 25 30 35 40

realative Länge des Kastens [cm]

Ab

stan

d z

ur

Gla

spla

tte

[μm

]

Reihe2

Reihe4

Reihe6

Reihe8

measurements with optical sensor of glass plate on container

pressure applied sag reduced by ~15 µm in concordance with simulations

But: total sag too big container edges inaccurate? influence of glue?

next steps: improvement of container detailed measurement of container edges simulations including boundary conditions


Summary

Thickness measurement: Interference time-efficient and accurate ring pattern plus true thickness profile deviation through residual of both potential decrease in thickness variations through slumping

X-Ray test: Gravitation-free mounting Compensation of graviational sag via water column air volume needs to be kept small to minimise influence of

temperature changes too large total sag, but correct reduction of sag

Need for improved set-up and detailed simulations

Thank you!

with thanks to:Monika Vongehr, Peter Friedrich, Peter Predehl, Andrea Müller

Mirror Fabrication via Glass Slumping Techniques Anita Schael Max-Planck-Institute for...

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