Dean A. Ketelsen

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.

Null test optics for the MMT and Magellan 6.5-m f/1.25 primary mirrors

The instruments used to interferometrically measure the optical surfaces of the 6.5-m f/1.25 primary mirrors for the MMT conversion and Magellan Telescopes must compensate over 800 micrometers surface departure from the best fitting sphere. The errors in the optical test must not contribute more than 0.04 arc seconds FWHM to the final image and the conic constant must be held to 0.01%. This paper presents the design, analysis, fabrication, and certification of the instruments used to measure these giant mirrors to such high accuracy.

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.

Manufacture of the second 8.4 m primary mirror for the Large Binocular Telescope

The second 8.4 m primary mirror and its active support system were delivered to the Large Binocular Telescope in September 2005. The mirror was figured to an accuracy of 15 nm rms surface after subtraction of low-order aberrations that will be controlled by the active support. The mirror was installed into its operational support cell in the lab, and support forces were optimized to produce a figure accurate to 20 nm rms surface with no synthetic correction. The mirror was polished on a new 8.4 m polishing machine that gives the Mirror Lab the capacity to process up to four 8.4 m mirrors simultaneously, with each mirror going through a sequence of stations: casting furnace, generating machine, polishing machine, and integration with its support cell. The new polishing machine has two carriages for polishing tools, allowing use of two 1.2 m stressed laps during loose-abrasive grinding and early polishing, followed by final figuring with a stressed lap and a small tool for local figuring.

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.

Optical Testing With Large Liquid Flats

We recently had two occasions to qualify optical flats of moderate size. Because of the mounting geometry, it was convenient to try measuring them with a Fizeau test against a liquid surface. In a preliminary literature search, we found few writings on the application of liquid flats. In this report, we briefly review the literature before describing our experiences and results.

Fabrication of the 6.5-m primary mirror for the Multiple Mirror Telescope Conversion

The Steward Observatory Mirror Lab is in the process of fabricating the 6.5 m mirror for the conversion of the multiple mirror telescope (MMT) to a single primary mirror. For this purpose the lab has developed a versatile polishing system built around the stressed lap polishing tool. The system must produce an f/1.25 parabolic surface with an accuracy corresponding to 0.09 arcsecond FWHM seeing and 1.5% scattering loss at 500 nm wavelength.

Machine for complete fabrication of 8-m class mirrors

The Large Optical Generator (LOG) was originally installed as a precision generator at the University of Arizona. It has since been relocated to the Steward Observatory Mirror Laboratory, where, in addition to its tasks as generator, it can be reconfigured as a polishing machine. As such, utilizing the Mirror Labs stressed-lap techniques, LOG has recently finished a series of three 3.5 meter mirrors to high accuracy. It is currently configured as a generator for work on the 6.5 meter MMT upgrade. LOGs operating parameters and level of performance both as generator and polisher will be discussed, along with some of the unique safety features that have been built into its operation.

Optical metrology for two large highly aspheric telescope mirrors

We describe a relatively simple, but highly effective, approach to the system design and alignment of an all-refractive Offner null corrector and phase-measuring Shack cube interferometer. In addition we outline procedures for fabricating and testing the optical components. Allowable errors for all parameters are determined by a tolerance analysis that separates axisymmetric and residual figure errors. An open construction optics frame provides a high degree of metering flexibility by incorporating simple kinematic mounts that provide adjustment of each lens while also allowing the lens to be removed and replaced with <2microm absolute repeatability. Nonaxisymmetric alignment errors are removed by rotating the optics on a high-precision bearing. Axial spacings are measured with contact transducers attached to both ends of an Invar metering rod. Two completed systems have guided the stressed-lap polishing of 1.8-m f/ 1.0 and 3.5-m f/ 1.5 aspheric mirrors.