1 / 17

Topics

Optical Figure Measurement on Convex or Concave, Meter-class Aspheres with Nanometer-class Accuracy Paul Glenn, Bauer Associates, Inc. 8 Tech Circle, Natick, MA 01760; paul@bauerinc.com OSA Optical Fabrication and Test 2010. 1. Topics.

kasimir-sullivan
Download Presentation

Topics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Optical Figure Measurement on Convex or Concave, Meter-class Aspheres with Nanometer-class Accuracy Paul Glenn, Bauer Associates, Inc. 8 Tech Circle, Natick, MA 01760; paul@bauerinc.com OSA Optical Fabrication and Test 2010 Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 1

  2. Topics • Review of Bauer’s “4PSP” (4-Point Stitching Profilometer) • Operating principles • Unique features • Specific capabilities • Summary of measurement campaigns to date • Work in process Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 2

  3. “4PSP” architecture: An integrated, in situ approach • A hexapod structure moves a “platter” in all degrees of freedom over the test piece • At any position over the test piece, the platter rotates to measure a “hoop” profile Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 3

  4. The three principles of “4PSP”… • If a rotating platter axis intersects the optic center of curvature, then a probe on the platter nominally sees no change in standoff (convex or concave substrate!) Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 4

  5. The three principles of “4PSP”… • If four or more probes are on a platter, then there is some linear combination of their readings that • (1) is insensitive to all rigid body motions • (2) tells something about the shape of the test piece • A continuous measurement of this linear combination as the platter rotates yields a circular “hoop” profile Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 5

  6. The three principles of “4PSP”… • If one measures multiple “hoops” around a test piece, with each overlapping at least two others, then it is possible to “stitch” the profiles to obtain the total surface height map. Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 6

  7. Summary of advantages Does not work from the center of curvature, thus not requiring a massive test tower Equally able to test concave, convex, and flat optics Easy to reconfigure probes for each new test piece size and radius of curvature Completely self referencing (i.e., no reference surfaces) Insensitive to rigid body motions of the test piece during measurements Close probe proximity to the test piece, drastically reducing turbulence problems Specially developed laser gauge probes provide absolutely reliable scale factor and mm-class range By replacing the laser gauge probes with coarse probes (touch probes or non-contact alternatives), the instrument can measure optics in their ground state. Thus, one metrology instrument can take an optic from generation through final polish. Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 7

  8. Surprising insensitivities It might seem that each of the four probes must maintain an absolutely constant, known height offset with respect to the measurement platter. However, there are some very important insensitivities that make the instrument extraordinarily robust: Each probe may have an arbitrary, unknown bias During the rotation of the platter when its rotation axis is declined from vertical, each probe may move out of the platter plane (i.e., up and down) due to varying self-weight sag, in any unknown amount and with any unknown phasing During the rotation of the platter, any probe’s bias with respect to the platter plane may change linearly (e.g., because of a change in environmental temperature) Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 8

  9. Current Capabilities Maximum diameter of mirror with mount: 2.0 meters Range of testable “speeds”: f-0.7 concave to f-0.7 convex Maximum aspheric departure: ~ 4 mm P-V Maximum aspheric slope: ~ 50 arc-minutes Maximum testable diameter: 1.3 meters over full f-0.7 range 1.5 meters for much slower “speeds” (Up to ~ 2 meters if measuring and combining multiple subapertures) Testing time: ~1 minute per “hoop” (200-600 hoops typical) Currently assessed full-aperture measurement uncertainty: ~2 nm rms for large spheres ~5-10 nm rms for typical large aspheres (depending on f-number, conic constant, and surface condition) Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 9

  10. Loading a large piece… • Safety shield • Thermal enclosure • Test piece (under protective cover) • Guard ring • Pneumatic “TriTable” with 3-point radially kinematic support Paul Glenn - Optical Fabrication and Testing (OF&C) 2010

  11. Fiducialization Naturally, accurate fiducialization is critical to the accurate measurement of an asphere Multiple fiducials (tooling balls or surface marks) are captured by our proprietary “ball-finder” sensor Ball-finder is integral to the measurement platter, with its location on the platter having been measured as part of the hexapod calibration We can move the ball finder to a ball (or surface mark), and find and record its location, in approximately two minutes Measured locations of a set of four tooling balls have agreed with independent measurements by a large CMM to 1-2 microns, thanks to high accuracies of the CMM and our hexapod coordinate system calibration (see text of paper), and high sensitivity in the ball-finder Paul Glenn - Optical Fabrication and Testing (OF&C) 2010

  12. Ball-finder Sensor Sensor is mounted at the end of one of the four sensor arms on the measurement platter Paul Glenn - Optical Fabrication and Testing (OF&C) 2010

  13. Measurement campaigns • Not counting two non-pedigreed “first light” pieces that we used to commission the instrument, we have measured six optics: • Sizes from 0.6 to 1.6 meters • Concave and convex • Speeds to f-1.1 • Spheres and off-axis aspheres, with mm-class P-V asphericity • Surface cosmetic qualities from pristine to poor • Coated and uncoated • Full-aperture measurements, as well as a week-long measurement campaign with multiple repositionings and subaperture measurements combined into a full surface map Paul Glenn - Optical Fabrication and Testing (OF&C) 2010

  14. Repeatability tests Repeatability tests have included Day-to-day repetitions of full-aperture measurements Full rotation tests, where after an initial full-aperture measurement, the optic (along with its mount) was removed from the instrument, rotated, repositioned, realigned, refiducialized, and remeasured This very important test essentially shows that any rotationally symmetric systematic measurement errors are comparable to or smaller than the measurement repeatability Tilt tests, where the rotation operation is replaced by a tilt operation Although not intuitively obvious, this test can be used to show the lack of rotationally asymmetric systematic measurement errors – We discovered such an error at the 10-nm level, and eliminated it through a structural improvement Paul Glenn - Optical Fabrication and Testing (OF&C) 2010

  15. Dynamic range We fully expected to be able to measure the large off-axis aspheres, with mm-class P-V asphericity and rms aspheric slopes of a few arc-minutes, and we did… However, we were surprised in one test to find that we had ventured into an area (outside the clear aperture) where the surface had never undergone the aspheric generation The edge rollups and rolldowns in the sharp transition were a few hundred microns, with slopes of the better part of a degree Although this degraded our measurement repeatability in the clear aperture, the several-thousand-times larger edge effects were measured with great fidelity and repeatability, with no data dropouts or other fundamental problems Paul Glenn - Optical Fabrication and Testing (OF&C) 2010

  16. Work in process We are setting up to measure a 0.6-meter piece that was MRF-polished by a customer for us, to deliberately introduce patterns with periods from a few mm to several cm, and with amplitudes from a few nm to a few hundred nm We expect to have high-density measurements and repeatability assessments, and to compare them with the customer’s best interferometric metrology, by the end of the summer The goal is to further push the correlation of our results with those from other measurements, over the widest possible range of spatial periods Investigations are underway for larger versions (2-4 meters) Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 16

  17. Summary The “4PSP” (Four-Point Stitching Profilometer) offers several important capabilities and features: Ability to test large convex, concave, or flat optics Self-referencing operation Insensitive to rigid body drifts and vibrations during a measurement Instrument size determined by test piece size, not by its radius of curvature Nanometer-class limiting uncertainty, with the ability to measure mm-class aspheres and extremely large slope errors We have measured several pieces in the meter-plus-class, and are pursuing other applications as well as expansions to even larger sizes Paul Glenn - Optical Fabrication and Testing (OF&C) 2010 17

More Related