Abstract

Segmented mirrors present unique challenges to fabrication and testing that are absent for monolithic optics. Since traditional asphere tests do not address segmented optics adequately, we validate a previously developed method to test large quantities of segments accurately, quickly, and economically. In this test, the aspheric shape of each segment is controlled to high accuracy by use of computer-generated holograms, and the radius of curvature is tightly controlled by use of the reference plate. In an adjoining paper [Appl Opt 43, 5303 (2004)] we developed the theory for this test, and now we present a complete system design and optimization for measuring the 1.4-m segments from a 30-m F/1 primary. A complete tolerance analysis predicts a test accuracy of 4.8-nm rms surface and excellent accuracy for controlling the geometry of the segment. In addition, a laboratory demonstration using 30-cm optics is presented that demonstrated 3.9-nm rms surface accuracy.

© 2004 Optical Society of America

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References

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  1. J. Nelson, T. Mast, S. Faber, “The design of the Keck Observatory and telescope,” Keck Observatory Report 90 (W. M. Keck Library, Kamuela, Hawaii, 1985).
  2. F. Pan, J. Burge, Y. Wang, Z. Shan, “Fabrication and testing issues of segmented optics,” Appl. Opt. 43, 2632–2642 (2004).
    [CrossRef] [PubMed]
  3. F. Pan, “Measurement of aspherical surfaces using a test plate and computer generated holograms,” Ph.D. dissertation (University of Arizona, Tucson, Ariz., 2002).
  4. F. Pan, J. Burge, “Efficient testing of segmented aspherical mirrors by use of a reference plate and computer generated holograms: 1. Theory and system optimization,” Appl. Opt. 43, 5303–5312 (2004).
    [CrossRef] [PubMed]
  5. “Enabling a giant segmented mini telescope for the astronomical community (National Optical Astronomy Observatory, Tucson, Ariz., 24May2004), www.aura-nio.noao.edu/book/ch4 .
  6. F. Pan, J. Burge, “Design study for testing primary mirror segments from a a 30-m GSMT (Giant Segmented Mirror Telescope) using a test plate with computer generated holograms”, a report submitted to the National Optical Astronomy Observatories, available at http://www.aura-nio.noao.edu/book/ch4/4.5.D.pdf .

2004 (2)

Burge, J.

F. Pan, J. Burge, Y. Wang, Z. Shan, “Fabrication and testing issues of segmented optics,” Appl. Opt. 43, 2632–2642 (2004).
[CrossRef] [PubMed]

F. Pan, J. Burge, “Efficient testing of segmented aspherical mirrors by use of a reference plate and computer generated holograms: 1. Theory and system optimization,” Appl. Opt. 43, 5303–5312 (2004).
[CrossRef] [PubMed]

F. Pan, J. Burge, “Design study for testing primary mirror segments from a a 30-m GSMT (Giant Segmented Mirror Telescope) using a test plate with computer generated holograms”, a report submitted to the National Optical Astronomy Observatories, available at http://www.aura-nio.noao.edu/book/ch4/4.5.D.pdf .

Faber, S.

J. Nelson, T. Mast, S. Faber, “The design of the Keck Observatory and telescope,” Keck Observatory Report 90 (W. M. Keck Library, Kamuela, Hawaii, 1985).

Mast, T.

J. Nelson, T. Mast, S. Faber, “The design of the Keck Observatory and telescope,” Keck Observatory Report 90 (W. M. Keck Library, Kamuela, Hawaii, 1985).

Nelson, J.

J. Nelson, T. Mast, S. Faber, “The design of the Keck Observatory and telescope,” Keck Observatory Report 90 (W. M. Keck Library, Kamuela, Hawaii, 1985).

Pan, F.

F. Pan, J. Burge, Y. Wang, Z. Shan, “Fabrication and testing issues of segmented optics,” Appl. Opt. 43, 2632–2642 (2004).
[CrossRef] [PubMed]

F. Pan, J. Burge, “Efficient testing of segmented aspherical mirrors by use of a reference plate and computer generated holograms: 1. Theory and system optimization,” Appl. Opt. 43, 5303–5312 (2004).
[CrossRef] [PubMed]

F. Pan, “Measurement of aspherical surfaces using a test plate and computer generated holograms,” Ph.D. dissertation (University of Arizona, Tucson, Ariz., 2002).

F. Pan, J. Burge, “Design study for testing primary mirror segments from a a 30-m GSMT (Giant Segmented Mirror Telescope) using a test plate with computer generated holograms”, a report submitted to the National Optical Astronomy Observatories, available at http://www.aura-nio.noao.edu/book/ch4/4.5.D.pdf .

Shan, Z.

Wang, Y.

Appl. Opt. (2)

Other (4)

J. Nelson, T. Mast, S. Faber, “The design of the Keck Observatory and telescope,” Keck Observatory Report 90 (W. M. Keck Library, Kamuela, Hawaii, 1985).

F. Pan, “Measurement of aspherical surfaces using a test plate and computer generated holograms,” Ph.D. dissertation (University of Arizona, Tucson, Ariz., 2002).

“Enabling a giant segmented mini telescope for the astronomical community (National Optical Astronomy Observatory, Tucson, Ariz., 24May2004), www.aura-nio.noao.edu/book/ch4 .

F. Pan, J. Burge, “Design study for testing primary mirror segments from a a 30-m GSMT (Giant Segmented Mirror Telescope) using a test plate with computer generated holograms”, a report submitted to the National Optical Astronomy Observatories, available at http://www.aura-nio.noao.edu/book/ch4/4.5.D.pdf .

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Figures (14)

Fig. 1
Fig. 1

New test comparing a concave segment with a convex spherical reference surface of the test plate whose size matches that of the mirror segment. CGHs are used to compensate any aspherical departure of the segment from the spherical reference surface. The test plate reference surface is spherical.

Fig. 2
Fig. 2

Optimization of the reference ROC. A ROC of 60.920 m is optimal, because it minimizes the tangential slope. (y slope) for the extreme segments. Both inner and outer segments have a slope variation of 0.351 mrad; therefore the nominal tilt in the test is three times this, or 1.05 mrad.

Fig. 3
Fig. 3

Spot diagrams for the closest and furthest segments when the reference ROC is 60.96 and 62.8 m, respectively. Two segments of the y slope match for a ROC of 60.96 m, and the x slope matches at a ROC of 62.8 m, but the x slope has a smaller value and is thus used to determine the system parameter reference ROC.

Fig. 4
Fig. 4

Ring of six alignment marks etched around the hologram to aid the alignment. This is imaged onto the test plate.

Fig. 5
Fig. 5

Five error sources contributing to a total of 4.85-nm uncertainty in surface figure measurement.

Fig. 6
Fig. 6

Error accumulated through calibration of reference sphere and test plate reference surface.

Fig. 7
Fig. 7

Sample CGH (plotted here with each fringe representing a 10-λ optical path difference) used to validate the new test method. Here, a 30-cm convex sphere with a known surface quality was tested by use of a CGH and a 30-cm test plate. The CGH was designed and used in the same manner as the testing of an asphere. The dominating feature on the CGH is a large tilt carrier fringes (126 λ across the 20-mm hologram), so test sensitivity is the same as that of testing an asphere.

Fig. 8
Fig. 8

Sample interferogram showing excellent contrast.

Fig. 9
Fig. 9

Sample surface measurement of the test sphere. To reduce the random system noise, we averaged a collection of 146 such measurements to obtain results shown in Fig. 10.

Fig. 10
Fig. 10

Comparison of corrected measurement data. Left, 0.05542 cms wave-front, traditional method; right, 0.0489.2 rms wave-front, new CGH method.

Fig. 11
Fig. 11

Measurement difference when alignment marks are not used for the new method; rms wave-front error is 0.0116 λ.

Fig. 12
Fig. 12

Measurement difference (same as shown in Fig. 11) when lower-order coma and astigmatism are removed; rms wave-front error is 0.0069 λ.

Fig. 13
Fig. 13

Error budget for the as built system (λ/4 optics shows 0.0124-λ rms wave-front errors).

Fig. 14
Fig. 14

Subtraction of two consecutive measurements showing typical rms test repeatability of 0.009 λ (wave front).

Tables (9)

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Table 1 Summary of the System Parameters

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Table 2 Figure Error Budget for Test of the T13 Segment (the Most Difficult)a

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Table 3 Tolerance Analysis for the Projection Optics Used in the NOAO Computer Modela

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Table 4 Error Budget for Measuring Figure from the Test Plate and the Reference Sphere

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Table 5 Error Budget for Measuring Surface Figure from the Test Plate and the Reference Sphere

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Table 6 Error Budget for Position and Angle for the T13 Segment Test (the Farthest-out Segment)

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Table 7 Error Budget for ROC Matching for the T13 Segment Test (the Most Extreme)

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Table 8 Test Accuracy for the Most Severe Segment

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Table 9 Tolerance Analysis on the Projection Optics for the Experimental Setupa

Equations (1)

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