1. Development of surface metrology for the Giant Magellan Telescope primary mirror
J. H. Burgea,b, W. Davisona, H. M. Martina, C. Zhaob
aSteward Observatory, University of Arizona
bCollege of Optical Sciences, University of Arizona

2. GMT primary mirror segments
25 meter telescope, requires 7 mirror segments
Each mirror segment is 8.4 meters in diameter
The off axis segments have 14.5 mm aspheric departure

3. Principal measurement using interferometer and reflective null corrector with CGH
Interferometer at M1 center of curvature

4. Test tower at Steward Observatory Mirror Lab
New tower
Original tower
New tower 28 meters tall, 80 tons of steel floated on 400 ton concrete pad accommodates other UA projects (LBT, LSST) lowest resonance of 4.8 Hz with 9 ton 3.75-m fold sphere + cell

5. New test tower at Mirror Lab
Test optics for GMT segment
Test of 3.75 m fold sphere
28 m vibration-isolated tower was installed Supports all GMT tests, plus LSST, future 6.5 m and 8.4 m mirrors.
GMT off-axis segment

6. Measurement of center segment
The center segment can be measured by tilting the fold sphere to point straight down, then a small computer generated hologram will compensate the residual errors.
Cone defined by light from outer edge of mirror
50 mm CGH compensates only 20µm aspheric departure
Cone defined by light from edge of central hole
Vibration insensitive interferometer

7. Optics of Sam ~1.5 m CGH M2 Insert a CGH to test Sam
Point source microscope aligned to M2
~1.5 m
Interferometer for GMT measurements
CGH M2

8. Invar cradle provides stable reference for M2 and CGH

9. M2 is aligned to CGH
Point Source Microscope
M2 CoC reference ball
M2 CoC reference ball
To M2
Computer generated hologram
CGH and M2 CoC reference ball are aligned using CMM to 10 µm
M2 aligned to CoC reference ball using PSM

10. M2 mount

11. Use of a Point Source Microscope to align M2
Use cradle to locate ball at location where M2 center of curvature should be (cradle geometry defined by CMM)
PSM is adjusted to the ball
The ball is then removed. The PSM is looking at mirror directly.
Adjust the mirror until reflection from it is focused on the same spot as the ball on the camera

12. Interferometer alignment to CGH
Use return into interferometer from reference patterns on CGH for
tilt (using fold flat)
shifting interferometer for focus

13. CGH test of Sam CGH inserted into light coming from Sam
Reflection back through system is used to verify wavefront
CGH mounted on invar plate with other references for M1 alignment

14. Alignment of M1, GMT M1 is aligned to Sam with ~100 µm tolerances
Reference hologram is aligned to Sam. Then it is used to represent Sam.
A laser tracker measures the 3-space position of the reference hologram and M1.
M1 is aligned to the reference hologram according to the measurements.
The laser tracker also provides the reference for the GMT location in the test

15. References co-aligned with CGH
CGH, coaligned with:
Corner cube tracker reference
Flat mirror, angular reference for tracker

16. Alignment error budget
Effect on primary mirror segment in telescope
radial shift (mm)
clocking (arcsec)
correction force
(N rms)
residual rms surface (nm)
Interferometer
0.0
2.8
2.4 M2
5.8
5.4
Reference Hologram
0.6 2
6.0
6.9 M1
1.0 5
7.2
9.1
GMT
0.2
2.7
3.2
Sam not measured by reference hologram 3
7.1
7.4
System total
1.2 6
13.4
15.4

17. 3.75 m fold sphere Cast in the Mirror Lab spinning oven
Figure of fold sphere will be measured in situ and subtracted.
Accuracy of correction depends on slope errors, and magnitude of small-scale structure that cannot be subtracted.
Finished fold sphere meets requirements:
< 2 nm/cm rms slope error
small-scale errors < 15% of GMT segment specification
Overall accuracy < 20 nm rms over clear aperture.
Cast in the Mirror Lab spinning oven
Polished at the Mirror Lab
Coated at Kitt Peak

18. Support of fold sphere
3750 mm mm
455 mm mm
Hangs from “Active” support, allowing quasi-static force adjustment based on in situ measurement

19. Scanning pentaprism test
Pentaprism rail lies in plane perpendicular to parent axis.
Hub rotates rail to scan different diameters.
Scanning pentaprism measures slope errors by producing collimated beams parallel to parent axis. Displacement of focused spot is measured with camera in focal plane.
Scanning pentaprism test as implemented for GMT off-axis segments. Pentaprism rail is suspended from tower.

20. Pentaprism test of 1.7 m off-axis NST mirror
1/5 scale GMT pentaprism test
This was done in late 2007 before the mirror was finished.
The pentaprism test only samples lowest order aberrations
The PP results agree with results from interferometry
Poster paper by P.Su et al
pentaprism measurement
interferometric test
aberration
interferometer
pentaprism
nm rms surface
astigmatism 0° 8
9 ± 23
astigmatism 45°
-2 ± 23
coma 0°
-87
-98 ± 12
coma 90° -4
16 ± 12
trefoil 0°
-50
-32 ± 35
trefoil 30° 9
23 ± 30
spherical
-32
-35 ± 8
nm surface

21. Laser Tracker Plus Poster paper by T. Zobrist et al
laser tracker & distance-measuring interferometers (DMI)
sphere-mounted retro-reflector for laser tracker
laser tracker
DMI laser and remote receivers
PSD
10% BS
DMIs
Retroreflector for interferometer and position sensing detector (PSD)
assemblies in 4 places at edge of mirror
DMI retroreflector
Poster paper by T. Zobrist et al

22. Laser Tracker Plus measurement of 3.75 m fold sphere M1
R = 25.5 m, tracker distance = 22 m
93 sample points, measured 4 DMIs with each sample
Subtracted best-fit sphere (R = m)
before DMI correction: 1.4 μm rms
after DMI correction: 0.75 μm rms

23. Shear test Each segment has axisymmetry about parent axis
Rotate segment about this axis under the optical test and separate effects that move with the mirror from those that remain with the test.

24. Summary
We are building the hardware to measure the GMT segments.
We expect to meet a tight error budget
Low order modes controlled by active optics, using < 5% Force
Uncorrectable features fit well within the allotted GMT PM structure function
We take this problem seriously and have implemented a comprehensive set of crosschecks
Scanning pentaprism system
Laser tracker Plus
Shear test
We have invested in metrology for making the first of the segments. Others will be made at low risk, low cost.