K-D-PR Fabrication and testing of KGMT FSM prototype Oct Ho-Soon Yang, Hak-Yong Kihm, Il-Kwon Moon, Jae-Bong Song, Yun-Woo Lee Korea Research Institute of Standards and Science, Korea Young-Soo Kim Korea Astronomy & Space Science Institute, Korea

Contents  Objective of project  Recent experiences in KRISS  Manufacturing flow diagram  Optical testing  Working cell design  Future works 2

Objective 3  Technology development related to manufacturing of FSM for GMT project in advance  For FSMP Use a model very close to FSM  FSM prototype in KGMT project Fabrication of FSMP with its own working cell designed by KRISS Optical testing of FSMP after assembly with a cell (very close to real FSM cell)

4 Recent KRISS  800 mm lightweighted mirror for space application (1/2) Gravity Quilting (30 g/cm2) And other environmental effects …

5 Recent KRISS  800 mm lightweighted mirror for space application (2/2) - Measurement of wavefront error with two different methods : CGH (computer generated hologram) and conventional null lenses CGH Conventional null lens /30 rms

6  1 m off-axis paraboloid for collimator (1/2) - Focal length = 6 m - Distance between the center of OAP and optical axis = 700 mm Recent KRISS Flat mirror Interferometer OAP

7  1 m off-axis paraboloid for collimator (2/2) Reference flat Interferometer OAP mirror Alignment telescope Recent KRISS

8 Fabrication facilities in KRISS Rotating table with aspheric surface under test Measuring tower with null interferomete r 2 m CNC machine 1 m polishing machine 2 m coating chamber Assembly room 0.6 m polishing machine & testing tower 2 m polishing machine

9 Manufacturing flow diagram Curve generation Chemical Etching Assembly with cell FATFAT 10% larger size Mirror blank procurement LightweightingLightweighting PolishingPolishingGrindingGrinding InspectionInspection InspectionInspection Figuring (98%) InspectionInspection Rounding to target dimension Final figuring

10 Spherometer Profilometer ~ mm Grinding Profilometer Interferometer ~ μm Polishing Interferometer with CGH ~ nm Figuring Scheme of Optical testing  Efficient testing process is a key for successful fabrication of mirror

11 KRISS profilometer  Measurement accuracy ~ 1 um  Compensation of measurement errors of probe system using a high quality reference mirror Reference mirror

150 mm CGH layout : (Line spacing : ~ 20 um) Adjustment between interferometer and CGH Test configuration of FSMP off-axis segment (off-axis type CGH ) Interferometric testing using CGH

Adjustment I between CGH and FSMP segment Adjustment II between CGH and FSMP segment CGH alignment

14 Turn-over tool KRISS Profilometer Testing tower setup

15 Optical testing during fabrication Optical testing after assembly with FSMP cell Testing configuration

Concept for working cell design  Minimization of print through  Load cell on the back plate of the mirror  24 axial support location  Polishing pressure at the pocket of mirror with 0.1 psi

FE Model of M2  Thin shell elements  Material : Zerodur  Diameter : 1064 mm  Thickness : 140 mm  Hole size : 100 mm  Mass : 94 kg  Face sheet : 20 mm  Back plate : 14 mm  Rib : 5 mm  Web : 10 mm Top view bottom view

Axial support x y supportRadius (mm)Force (N) Ring Ring Ring Ring Support locations

Print through  Gravity z Displacement PV 165 nm Surface error Without correction PV 166 nm RMS 48 nm Surface error With correction PV 50 nm RMS 9 nm

Print through  Gravity z M2 structure function

Surface error  Quilting effect from polishing pressure of 0.1 psi Surface error Without correction PV 110 nm RMS 31 nm Surface error With correction PV 35 nm RMS 6 nm Displacement PV 110 nm

Surface error  Polishing pressure 0.1 psi M2 structure function

23 Future works  Manufacturing of two types of CGH  Manufacturing of testing tower  Optimization of mirror working cell  Fabrication of FSMP by the end of next year  We hope to take parts in the fabrication of FSM in future