Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens

College of Optical Sciences

The University of Arizona

Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens

Chunyu ZhaoRene Zehnder

Jim BurgeBuddy Martin

College of Optical Sciences, University of Arizona



Outline

The mirror to be tested Testing system design and assembly Optical alignment Initial testing result Summary



The mirror

Off-axis parabolic mirror with 1.6m diameter of clear aperture

Parent: f/0.7 parabola with 7.7m ROC.

Offset from the parent vertex: 1.84m



Surface profile

P-V: 2.767mm RMS: 508um RMS asti: 497um RMS coma: 108um RMS trifoil: 9um RMS spherical:

5.8um Residue: 2.5um



Surface quality spec

Lower bending mode can be subtracted by the following amount: Astigmatism: 200nm Coma: 17nm Trefoil: 50nm Quatrefoil: 20nm Spherical: 25nm

Residual: 40nm rms



Requirement for testing system

Amount of lower order mode: Astigmatism: 170nm Coma: 15nm Trefoil: 42nm Quatrefoil: 17nm Spherical: 20nm

Residual: 20nm rms



System Configuration

A spherical mirror removes most of astigmatism and some coma – residual 0 astigmatism 22um and coma 46um

A CGH removes rest of the aberrations.



Lens + CGH + Spherical Mirror



Error Budget

in nm CGH Spherical mirror ROC

Thermal

RSS

Actuator

dz tilt x tilt y dz tilt x tilt y

Amount Correctabl

eBy 15 N

ForcesNeede

d(N)

um 7

7 @Edg

e7 @

Edge 77 @

Edge

7 @Edg

e 201deg

C

Z5 0.2 0.0 -5.6 0.0 0.0

-37.6 0.0 0.0 38.1 346.0 1.6

Z6 133.9 -7.0 1.6 93.0 -149.5 3.1 63.0 206.5 309 346.0 13.4

Z9 -16.1 1.7 -0.3 -4.3 19.1 -0.5 -3.0 -12.2 28.4 87.0 4.9

Z10 0.0 0.0 -1.8 0.0 0.0

-15.0 0.0 0.0 15.1 87.0 2.6

Z11 5.9 1.2 0.0 1.9 -4.2 0.0 1.2 4.7 9.0 43.0 3.1

Z14 1.1 3.1 0.0 0.0 -0.6 0.0 -0.1 0.2 3.3 35.0 1.4

Z15 0.0 0.0 2.8 0.0 0.0 -0.5 0.0 0.0 2.9 35.0 1.2

RSS 15.0



Residual wavefront errors Total allowed: 40nm Testing system budget: 20nm Errors in lens, CGH and spherical mirror will be backed out

in nm CGH Spherical mirror ROC Thermal

RSSdz tilt x tilt y dz tilt x tilt y

um/mdeg 7 7 7 7 1.4 1.4 20 1degree

RMSFit

error 2.0 2.6 1.7 1.2 2.6 2.5 0.7 3.5 6.4

Z12 -6.3 -2.0 -0.1 -0.9 4.4 -0.2 -0.6 -3.2 8.6

Z13 0.0 0.0 -0.5 0.0 0.0 3.2 0.0 0.0 3.2

RSS subtotal 11.2

Lens+CGH 8

Mirror 8

RSS Total 15.9



Optical Bench and Testing Tower



NST testing system

System assembled and aligned in lab

System mounted and aligned in test tower (looking up)



Alignment

Using CGH patterns to align the CGH itself

Using CGH patterns and metering rods to align the spherical mirror

Additional CGH patterns to create cross hairs for position the test mirror



The CGHs

10 segments create 8 wavefronts.

Main CGH creates the testing wavefront.

The ring type CGH aligns the substrate to the interferometer.

3 segments create a crosshair and 1 segment creates a clocking line to align the NST mirror to the test optics.

4 circular CGHs send beams to align the lateral positions of the 4 balls mounted on the surface of the fold sphere.

Main CGH

Substrate alignment CGH

CGH creating clocking line

3-segment CGH creating crosshair

4 CGHs creating beams for spherical mirror alignment



Alignment of CGH

With the reflection fringes from the alignment CGH controlled to 0.5 in power, the CGH substrate is aligned within 7μm.



Alignment of spherical mirror

Position a ball at focus of the 0th order diffraction beam after CGH, use it as a reference to position the spherical mirror.

Put a few balls at the mirror surface, patches of CGH direct spherical beams toward the ball and the reflection fringes are used to position the balls accurately in lateral direction, and metering rod with LVDTs are used to control the distance from these balls to the ball at focus. The mirror is adjusted so that all the balls are at proper position.

Ball at focus

Ball at mirror

CGH

Metering rod w/ LVDTs

Lens Spherical mirror



• The metering rods are made of low CTE carbon fiber tubes with invar tips glued on both ends

• Metering rods are calibrated between two balls separated by known distance.

• Since the tip of the rod has proper curvature slight misalignment does not alter calibration.

Metering rod calibration



•The calibration bench serves as master reference for the metering rods.

• The distance between the balls was measured by a laser tracker.

• In order to minimize measurement errors the tracker was aligned to direction of motion.

•The calibration bench is made of ULE and mechanically mounted to minimize environmental influence.

Metering rod calibration bench



Spherical mirror is aligned with metering rods.

4 balls mounted on the mirror surface. Their lateral positions are controlled with beams from the CGHs. Small stages position the balls laterally to give retroreflection.

Initial alignment scheme based on reflected wavefront did not work. New scheme based on reflected image is being implemented.

1 ball mounted on the 0th order beam focus. Its position is controlled by nulling the reflection fringes from its surface.

Metering rods lengths are calibrated to 3 μm.

Reflection fringesFrom the ball @focus->

Alignment of spherical mirror relative to CGH



A crosshair and a clocking line are generated by CGHs to align the NST mirror to the test optics.

Usage of crosshair and Line

Crosshair to get x-y-positionLine to get clocking

Intensities plotted in logscale

Aligning the test mirror: projected crosshair and clocking line



Initial testing results



Morphed surface map

P-V: 8.6 wave RMS: 1.5 wave



Summary

We have built a system for interferometrically testing an off-axis parabolic mirror

A CGH and a spherical mirror is used as null lens

Initial testing results are encouraging

Experience and know-how acquired will be applied to testing GMT mirrors which are 5x scale off-axis parabolas

GMT Telescope

Testing an off-axis parabolic mirror with a CGH and a spherical mirror as null lens

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