The <i>Infrared Multi-Object Spectrometer (IRMOS)</i> is a facility-class instrument for the Kitt Peak National Observatory 4 and 2.l meter telescopes. <i>IRMOS</i> is a near-IR (0.8-2.5 μm) spectrometer and operates at ~80 K. The 6061-T651 aluminum bench and mirrors constitute an athermal design. The instrument produces simultaneous spectra at low- to mid-resolving power (R = λ/Δλ = 300-3000) of ~100 objects in its 2.8×2.0 arcmin field.
We describe ambient and cryogenic optical testing of the <i>IRMOS</i> mirrors across a broad range in spatial frequency (figure error, mid-frequency error, and microroughness). The mirrors include three rotationally symmetric, off-axis conic sections, one off-axis biconic, and several flat fold mirrors. The symmetric mirrors include convex and concave prolate and oblate ellipsoids. They range in aperture from 94×86 mm to 286×269 mm and in f-number from 0.9 to 2.4. The biconic mirror is concave and has a 94×76 mm aperture, R<sub>x</sub>=377 mm, k<sub>x</sub>=0.0778, R<sub>y</sub>=407 mm, and k<sub>y</sub>=0.1265 and is decentered by -2 mm in X and 227 mm in Y. All of the mirrors have an aspect ratio of approximately 6:1. The surface error fabrication tolerances are < 10 nm RMS microroughness, best effort for mid-frequency error, and < 63.3 nm RMS figure error.
Ambient temperature (~293 K) testing is performed for each of the three surface error regimes, and figure testing is also performed at ~80 K. Operation of the ADE PhaseShift MicroXAM white light interferometer (micro-roughness) and the Bauer Model 200 profilometer (mid-frequency error) is described. Both the sag and conic values of the aspheric mirrors make these tests challenging. Figure testing is performed using a Zygo GPI interferometer, custom computer generated holograms (CGH), and optomechanical alignment fiducials.
Cryogenic CGH null testing is discussed in detail. We discuss complications such as the change in prescription with temperature and thermal gradients. Correction for the effect of the dewar window is also covered. We discuss the error budget for the optical test and alignment procedure. Data reduction is accomplished using commercial optical design and data analysis software packages. Results from CGH testing at cryogenic temperatures are encouraging thus far.
We have designed a nd built a phase-measuring LUPI interferometer to use pre-aligned custom CGH nulls for high accuracy figure metrology of deep aspherics. The CGH nulls operate in double pass, first producing an aspheric test wavefront and then recollimating the return wavefront. This eliminates any need to locate the CGH at an image of the test pupil THe CGH is common to both test and reference paths, allowing the use of photomask quality substrates. Tho enable the CGH-LUPI to test a wider variety of aspheres, we have designed and built a set of 100 mm aperture accessory optics for use in combination with CGH nulls. These accessory optics consist of five singles, each approximately F/3, which may be kinematically stacked in numerous combinations and permutations to produce test wavefronts ranging from nearly collimated to F/0.75 A CGH null compensates for asphericity of the test optic and design aberrations of the accessory optics. The interferometer and accessory optic designs permit independent verification of all aspects of system accuracy and calibration without the need for disassembly. Designing a custom CGH null involves raytracing the accessory optics but not the interferometer mainframe optics. Depending on the phase measuring algorithm selected, known system aberrations due to manufacturing tolerances may be software compensated in real time.
The wavelength scaling of an f# 2.5 off axis HOE from 488 nm to 1064 nm has been done. We canceled large induced astigmatism, and other higher order aberrations using a combination of 1 curved reflector, 1 cylindrical lens and one Null CGH bonded to the cylindrical lens. The task was made more difficult by a requirement to fill a 404 nm round aperture and make it focus to a 53 micron diameter spot at the 1/e clip level. The design procedure, the construction sequence and the measured results are presented as a work in progress.
Computer generated holograms (CGHs) are an alternative to refractive or reflective null optics when testing spheric optic components. A key attraction is that the difficulty of designing and fabricating a CGH null is largely independent of the detailed shape of the test asphere. CGH nulls have been used quite successfully in a number of high profile programs, but certification issues have limited their more widespread acceptance as a primary testing means. This is due largely to unfamiliarity with appropriate verification and certification methods. We here discuss specification and tolerancing of CGH nulls and present a comprehensive methodology for verification and certification.
We present a simple and general method of aspheric figure metrology using a CGH null mounted in the test beam of a conventional Fizeau or Twyman-Green interferometer. A 'standard' reflective CGH is used to establish optical alignment with respect to the interferometer's spherical test beam. This alignment is then mechanically trnaferred to a custom CGH null. The accuaracy of the alignment transfer is readily verified. The test method has been modeled by raytracing and verified experimentally by testing a perforated 8 inch F/1.5 on-axis paraboloid and a 50 mm off-axis paraboloid from their centers of curvature.
An interferometer for aspheric testing ([AT) is under development at APA Optics for testing of general aspherics using inexpensive electron-beam written computer generated holograms (CGHs) as null compensators. This 152-mm aperture Twyman-Green interferometer is compatible with standard transmission spheres fringe analysis software and phase measuring accessories. Aspheric departures of up to several hundred waves can be measured using only standard interferometer accessories. Deeper aspheres may be tested using simple auxiliary optics. The interferometer configuration methods of operation and performance specifications are presented. 1.
Aholographic combiner performing as a transmission combiner, consisting of two reflection holograms sandwiched
together, has been developed for display applications which may benefit in packaging and/or performance due to the
transmission geometry. The SRH combiner benefits from the reflection holographic components to reduce the spectral
bandwidth to less than 20 nm. Predicted performance of the SRH versus simple transmission holograms, experimental
data on the SRH, and practical display applications are presented.