Mirror Fabrication via Glass Slumping Techniques Anita Schael Max-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

Mirror Fabrication via Glass Slumping 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 (~ l / 10 )

Mirror Fabrication via Glass Slumping Interference: Theory I Thickness measurement through double-reflection in glass a a‘a‘ d C In point C: interference of both beams A and B path difference: B A

Mirror Fabrication via Glass Slumping Interference: Theory II If path difference = n l /2 (n integer)  maximum / minimum Interference pattern depending on d and a In perfectly plane-parallel layer: ring pattern

Mirror Fabrication via Glass Slumping Interference: Haidinger Rings in different angles a : different D l „Fringes of equal inclination“ a a‘a‘ d C

Mirror Fabrication via Glass Slumping Interpretation Real image = combination of patterns of: perfectly plane-parallel layer thickness variations in glass  a true profile of the glass can be derived from residual of the real pattern and the rings in perfect layer

Mirror Fabrication via Glass Slumping Interference: Implementation I Sodium light: l = nm via beam splitter onto glass segment observed through the beam splitter diffuse sodium light Observer Glass segment beam splitter

Mirror Fabrication via Glass Slumping Implementation II

Mirror Fabrication via Glass Slumping Thickness variation: before and after slumping Do thickness variations in the glass change in the slumping process?

Mirror Fabrication via Glass Slumping Quantitative Analysis I In reality: more than two interfering rays with decreasing intensity a d  need to take reflectivity into account: more beams within the plate  finer and sharper fringes also: angle of slumped glass...

Mirror Fabrication via Glass Slumping 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

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

Mirror Fabrication via Glass Slumping 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

Mirror Fabrication via Glass Slumping Compensation of Gravity Idea: built-up of air pressure under the glass equivalent to gravitational force using a water column glass segment P enclosed P Air Advantages: easy physical principle balance of air pressure variations through open water column

Mirror Fabrication via Glass Slumping Theory: Principle Water column: Pascal‘s Principle with r = density, g = grav. acceleration, h = height of water column, D P = pressure Using with F G = 0,2698 N  D P = 13,49 Pa  water column of h = mm h

Mirror Fabrication via Glass Slumping Problems Main problem: dependence on temperature variations Pressure Temperature Experiment over 24 hours in lab conditions Solution: volume under glass needs to be kept very small  less change in pressure

Mirror Fabrication via Glass Slumping Improved design Two containers of plexiglass, the inner one open at bottom glass segment manometer plexiglass container Oil layer on water to avoid evaporation

Mirror Fabrication via Glass Slumping Results 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

Mirror Fabrication via Glass Slumping 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