Lee R. Dettmann

Active supports and force optimization for the MMT primary mirror

We describe the active support system and optimization of support forces for the 6.5 m primary mirror for the Multiple Mirror Telescope Conversion. The mirror was figured to an accuracy of 26 nm rms surface error, excluding certain flexible bending modes that will be controlled by support forces in the telescope. On installation of the mirror into its telescope support cell, an initial optimization of support forces is needed because of minor differences between the support used during fabrication and that in the telescope cell. The optimization is based on figure measurements made interferometrically in the vibration- isolated test tower of the Steward Observatory Mirror Lab. Actuator influence functions were determined by finite- element analysis and verified by measurement. The optimization is performed by singular value decomposition of the influence functions into normal modes. Preliminary results give a wavefront accuracy better than that of the atmosphere in 0.11 arcsecond seeing.

Fabrication of mirrors for the Magellan telescopes and the Large Binocular Telescope

We describe the fabrication and testing of the 6.5 m f/1.25 primary mirrors for the Magellan telescopes and the 8.4 m f/1.14 primary mirrors for the Large Binocular Telescope (LBT). These mirrors, along with the 6.5 m MMT primary, are the fastest and most aspheric large mirrors made. Steward Observatory developed special methods to polish and measure these and other fast mirrors. We use a stressed-lap polishing tool to fit the aspheric surface while providing strong passive smoothing, and computer-generated holograms to verify the measurement of up to 1.4 mm peak-to-valley asphericity to an accuracy of 0.01%. The Magellan mirrors are diffraction-limited at visible wavelengths, with surface accuracies of about 20 nm rms on active supports. We are currently polishing the first LBT primary mirror and preparing to make the thin shells for the LBT adaptive secondary mirrors.

Optical fabrication of the MMT adaptive secondary mirror

We describe the optical fabrication of the adaptive secondary mirror for the MMT. The 640 mm f/15 secondary consists of a flexible glass shell, 1.8 mm thick, whose shape is controlled by 336 electromagnetic actuators. It is designed to give diffraction-limited images at a wavelength of 1 micron. For generating and polishing, the shell was supported by attaching it to a rigid glass blocking body with a thin layer of pitch. It could then be figured and measured using techniques developed for rigid secondaries. The highly aspheric surface was polished with a 30 cm stressed lap and small passive tools, and measured using a swing-arm profilometer and a holographic test plate. The goal for fabrication was to produce diffraction-limited images in the visible, after simulated adaptive correction using only a small fraction of the typical actuator forces. This translates into a surface accuracy of less than 19 nm rms with correction forces of less than 0.05 N rms. We achieved a surface accuracy of 8 nm rms after simulated correction with forces of 0.02 N rms.

Fabrication of ultrathin mirrors for adaptive and space optics

We describe the optical fabrication of thin glass shells which will be combined with rigid active supports for adaptive secondary mirrors and for space optics. These applications require glass shells about 2 mm thick, with diameters up to about 1 m for adaptive optics and possibly 6 m for space optics. The extreme flexibility presents unique fabrication challenges which are overcome by a simple adaptation of traditional glassworking techniques. Here we describe the fabrication of concave spherical shells 20 cm and 55 cm diameter. A method of handling and supporting the thin substrates for loose abrasive grinding and polishing is demonstrated and some variations on this approach are compared. Extension of the technique to aspheric adaptive secondary mirrors and to ultra-light mirrors up to 6 meters in diameter is discussed. The subsequent integration and optical testing of a 55 cm shell with a 36 point active support is reported.

Polishing of a 6.5 m f/1.25 mirror for the first Magellan telescope

We describe the optical fabrication and testing of the 6.5 m f/1.25 primary mirror for the first Magellan telescope. Figuring was performed with a 1.2m stressed lap, which bends under active control to match the local curvature of the optical surface, and a variety of small passive tools. The figure was measured with RI and visible interferometers, using refractive null correctors to compensate 810 microns of aspheric departure. After subtraction of Seidel astigmatism and spherical aberration, the finished mirror is accurate to 14 nm rms surface error, and has an encircled energy of 80 percent in 0.06 inch diameter at 500 nm.

Fabrication and measured quality of the MMT primary mirror

The primary mirror for the Multiple Mirror Telescope Conversion is the first 6.5 m honeycomb sandwich mirror cast and polished by the Steward Observatory Mirror Lab. We describe the optical fabrication and testing of the f/1.25 paraboloid, and present the final measurements of figure accuracy and inferred image quality. Figuring was performed with a 1.2 m stressed lap--which bends under active control to match the local curvature of the optical surface--and a variety of small passive tools. The mirror was pressurized to compensate for polishing loads and thereby eliminate print-through of the honeycomb structure. The net result is a smoother surface on scales of 5 - 20 cm than has been achieved on previous honeycomb sandwich mirrors. The figure was measured with IR and visible interferometers, using refractive null correctors to compensate 810 microns of aspheric departure. The final measurements were used to calculate synthetic stellar images in a variety of seeing conditions.

Survey of interferometric techniques used to test JWST optical components

JWST optical component in-process optical testing and cryogenic requirement compliance certification, verification & validation is probably the most difficult metrology job of our generation in astronomical optics. But, the challenge has been met: by the hard work of dozens of optical metrologists; the development and qualification of multiple custom test setups; and several new inventions, including 4D PhaseCam and Leica Absolute Distance Meter. This paper summarizes the metrology tools, test setups and processes used to characterize the JWST optical components.

Vibration stabilization of a phase-shifting interferometer for large optics

Phase shifting interferometry requires an intentional shifting of the relative phase between the reference arm and the test arm of fan interferometry. Vibration can lead to uncertainty in the relative phase difference with respect to time and result in erroneous surface measurements. We have developed a method for actively compensating for vibration using a closed-loop phase servo system. An essential feature of this is a high frequency phase measurement. The phase is modulated and the intensity variations are measured with a high sped photodiode and digitized. This information is processed by a DSP and a five step algorithm is used to determine the instantaneous phase. These high speed phase measurements are used in a closed loop phase servo to compensate for vibration and also allow for phase shifting interferometry. Test results with and without the vibration compensation will be presented.

Successful production of the engineering development unit (EDU) primary mirror segment and flight unit tertiary mirror for JWST

During 2009, Tinsley finished most of the Configuration 1 pre-cryo test Computer Controlled Optical Surfacing (CCOS) operations on the James Webb Space Telescope primary mirror segments and in mid-2009 we began the Configuration 2 post-cryo test CCOS operations. After completing the grinding and polishing operations, including final figuring to a cryo-null target, we delivered the finished Engineering Development Unit (EDU) to Ball Aerospace Technology Corporation on 4 December 2009. Achieving fabrication and metrology conditions to meet the specifications for this off-axis ~1.5 m hexagonal point-to-point segmented mirror required special methods. Achieving repeatable and accurate interferometric alignment of the off-axis aspherical mirror surface and stable thermal gradient control of the beryllium substructure during tests required rigorous component and system-level validation. Final optical wavefront measurements over the various spatial frequency ranges have demonstrated that all of the requirements are met. This success has validated our processes of fabrication and metrology and allows us to proceed with the production of the 18 flight mirror segments. The first finished flight mirror, the Tertiary Mirror, was shipped to BATC on 24 February, 2010. Performance of that mirror is reported here also.

Interferogram acquisition using a high-frame-rate CCD camera

Phase-shifting interferometry measures surface height error by acquiring multiple interferograms. Analysis of these data is affected by the accuracy of individual measurements and the ability to compare subsequent measurements. The accuracy of individual measurements relies on producing constant phase shifts between the interferograms. Interferometric testing o large aperture telescope optics can be limited by vibration as the optical path difference increase. To minimize the effect of vibration, a series of interferograms would be acquired as quickly as possible. To accurately compare subsequent measurements, fiducials are used to provide registration. Accurate fiducial location is essential. This paper will describe the implementation of a high frame rate CCD camera and frame grabber system operating on an IBM PC compatible computer platform. This system quickly acquires a series of six consecutive digitized interferograms which are evaluated for proper phase shift before being reduced in a commercial phase-shift analysis program. The speed and storage advantages of the frame grabber also allow the system to produce a time- averaged fringe contrast map which enables fiducials to be easily an accurate located.