Andrew E. Lowman

Interferometer errors due to the presence of fringes

Phase-shifting interferometry permits analysis of complex interferograms. However, the measurement accuracy is reduced as the number of fringes is increased. The wave-front from a defocused spherical surface is used to demonstrate this degradation for several different transmission reference objectives.

Modulation transfer function measurement of sparse-array sensors using a self-calibrating fringe pattern

A simple method for the measurement of the pixel modulation transfer function (MTF) of sparse-array (extended MTF) sensors has been developed. We use a phase-shifting Twyman-Green interferometer to generate a series of single spatial-frequency fringe patterns incident on the sensor The resulting signal modulation is measured. We achieve self-calibration by restricting the measured spatial frequencies to multiples of the Nyquist frequency. The aliased patterns at these frequencies are unique and easily identifiable. Spatial frequencies of 480 cycles/mm are generated and measured. This frequency value is more than ten times that of the sensor sampling frequency. The expected MTF shape is obtained at multiples of the sampling frequency. At odd multiples of the Nyquist frequency, the MTFs are affected by the electronic bandwidth and cross talk in the charge-injection device sensor.

Sub-Nyquist interferometry: implementation and measurement capability

Sub-Nyquist interferometry (SNI) provides a method for measuring wavefronts with large departures from a reference sphere, such as those encountered when testing steep aspheric surfaces. SNI allows wavefronts with several hundred waves of departure to be recorded and analyzed. The theory of SNI is reviewed, its experimental implementation described, and limitations in the hardware and potential improvements are discussed. The importance of calibrating the interferometer for non-null testing is demonstrated.

Modeling an interferometer for non-null testing of aspheres

Aspheric surface testing would be greatly facilitated if the requirement for a null condition were removed. Testing an optic in a non-null configuration introduces aberrations into the wavefront. The wavefront measured at the sensor is different from the wavefront initially produced by the test surface, and the interferometer must be calibrated if useful measurements of aspheres are to be made. One potential calibration technique is reverse optimization, where a lens design program is used to retrieve the prescription of the interferometer. Various problems in modeling an interferometer, and potential solutions, are discussed. A defocused sphere was used to generate a non-null wavefront with 100(lambda) of departure at the surface. The reverse optimization results matched the experimental data to better than (lambda) /4 PV.

Interferometer-induced wavefront errors when testing in a nonnull configuration

Sub-Nyquist interferometry and other extended range techniques have the potential to allow measurement of aspheric surfaces with large departures from a reference sphere. The optic is tested in a non-null configuration, and aberrations are introduced into the wavefront by the interferometer optics. Consequently, the wavefront measured at the sensor is different from the wavefront initially produced by the test surface, and the interferometer must be calibrated if useful measurements of aspheres are to be made. The aberrations produced by a Twyman- Green interferometer for this application were examined. To study the severity of these interferometer induced errors, a defocused spherical surface was used to generate a non-null configuration. With wavefront departures up to 400 waves, errors up to 12 waves rms were found to be introduced by the non-null test setup.

Fabrication and testing of 4.2m off-axis aspheric primary mirror of Daniel K. Inouye Solar Telescope

Daniel K. Inouye Solar Telescope (formerly known as Advanced Technology Solar Telescope) will be the largest optical solar telescope ever built to provide greatly improved image, spatial and spectral resolution and to collect sufficient light flux of Sun. To meet the requirements of the telescope the design adopted a 4m aperture off-axis parabolic primary mirror with challenging specifications of the surface quality including the surface figure, irregularity and BRDF. The mirror has been completed at the College of Optical Sciences in the University of Arizona and it meets every aspect of requirement with margin. In fact this mirror may be the smoothest large mirror ever made. This paper presents the detail fabrication process and metrology applied to the mirror from the grinding to finish, that include extremely stable hydraulic support, IR and Visible deflectometry, Interferometry and Computer Controlled fabrication process developed at the University of Arizona.

Alignment of four-mirror wide field corrector for the Hobby-Eberly Telescope

The Hobby-Eberly Telescope (HET) Wide Field Corrector (WFC) is a four-mirror optical system which corrects for aberrations from the 10-m segmented spherical primary mirror. The WFC mirror alignments must meet particularly tight tolerances for the system to meet performance requirements. The system uses 1-m class highly aspheric mirrors, which precludes conventional alignment methods. For the WFC system alignment a “center reference fixture” has been used as the reference for each mirror’s vertex and optical axis. The center reference fixtures have both a CGH and sphere mounted retroreflector (SMR) nests. The CGH is aligned to the mirror’s optical axis to provide a reference for mirror decenter and tilt. The vertex of each mirror is registered to the SMR nests on the center reference fixtures using a laser tracker. The spacing between the mirror vertices is measured during the system alignment using these SMR nest locations to determine the vertex locations. In this paper we present the procedures and results from creating and characterizing these center reference fixtures. As a verification of our alignment methods we also present results from their application in the WFC system alignment are also presented.

Manufacturing of super-polished large aspheric/freeform optics

Several next generation astronomical telescopes or large optical systems utilize aspheric/freeform optics for creating a segmented optical system. Multiple mirrors can be combined to form a larger optical surface or used as a single surface to avoid obscurations. In this paper, we demonstrate a specific case of the Daniel K. Inouye Solar Telescope (DKIST). This optic is a 4.2 m in diameter off-axis primary mirror using ZERODUR thin substrate, and has been successfully completed in the Optical Engineering and Fabrication Facility (OEFF) at the University of Arizona, in 2016. As the telescope looks at the brightest object in the sky, our own Sun, the primary mirror surface quality meets extreme specifications covering a wide range of spatial frequency errors. In manufacturing the DKIST mirror, metrology systems have been studied, developed and applied to measure low-to-mid-to-high spatial frequency surface shape information in the 4.2 m super-polished optical surface. In this paper, measurements from these systems are converted to Power Spectral Density (PSD) plots and combined in the spatial frequency domain. Results cover 5 orders of magnitude in spatial frequencies and meet or exceed specifications for this large aspheric mirror. Precision manufacturing of the super-polished DKIST mirror enables a new level of solar science.

Modern technologies of fabrication and testing of large convex secondary mirrors

Modern large telescopes such as TAO, LSST, TMT and EELT require 0.9m-4m monolithic convex secondary mirrors. The fabrication and testing of these large convex secondary mirrors of astronomical telescopes is getting challenging as the aperture of the mirror is getting bigger. The biggest challenge to fabricate these large convex aspheric mirrors is to measure the surface figure to a few nanometers, while maintaining the testing and fabrication cycle to be efficient to minimize the downtime. For the last a couple of decades there was huge advancement in the metrology and fabrication of large aspheric secondary mirrors. College of Optical Sciences in the University Arizona developed a full fabrication and metrology process with extremely high accuracy and efficiency for manufacturing the large convex secondary mirrors. In this paper modern metrology systems including Swing-Arm Optical Coordinate Measuring System (SOCMM) which is comparable to Interferometry and a Sub-aperture stitching interferometry scalable to a several meters have been presented. Also a Computer Controlled Fabrication Process which produces extremely fine surface figure and finish has been demonstrated. These most recent development has been applied to the fabrication and testing of 0.9m aspheric convex secondary mirror for the Tokyo Atacama Observatory’s 6.5m telescope and the result has been presented.

Development of a wide field spherical aberration corrector for the Hobby Eberly Telescope: Design, fabrication and alignment

A 4-mirror prime focus corrector is under development to provide seeing-limited images for the 10-m aperture Hobby- Eberly Telescope (HET) over a 22 arcminute wide field of view. The images created by the spherical primary mirror are aberrated with 13 arcmin diameter point spread function. The University of Arizona is developing the 4-mirror wide field corrector to compensate the aberrations from the primary mirror and present seeing limited imaged to the pickoffs for the fiber-fed spectrographs. The requirements for this system pose several challenges, including optical fabrication of the aspheric mirrors, system alignment, and operational mechanical stability. This paper presents current status of the program which covers fabrication of mirrors and structures and pretest result from the alignment of the system.