As part of a study funded by NASA headquarters, we are developing a probe-class mission concept called the Cosmic Evolution through UV Spectroscopy (CETUS). CETUS includes a 1.5-m aperture diameter telescope with a large field of view (FOV). CETUS includes three scientific instruments: a far ultraviolet (FUV) and near ultraviolet (NUV) imaging camera (CAM); a NUV multiobject spectrograph (MOS); and a dual-channel point/slit spectrograph (PSS) in the Lyman ultraviolet (LUV), FUV, and NUV spectral regions. The large FOV three-mirror anastigmatic (TMA) optical telescope assembly (OTA) simultaneously feeds the three separate scientific instruments. That is, the instruments view separate portions of the TMA image plane, enabling parallel operation by the three instruments. The field viewed by the MOS, whose design is based on an Offner-type spectrographic configuration to provide wide FOV correction, is actively configured to select and isolate numerous field sources using a next-generation micro-shutter array. The two-channel CAM design is also based on an Offner-like configuration. The PSS performs high spectral resolution spectroscopy on unresolved objects over the NUV region with spectral resolving power, R ∼ 40,000, in an echelle mode. The PSS also performs long-slit imaging spectroscopy at R ∼ 20,000 in the LUV and FUV spectral regions with two aberration-corrected, blazed, holographic gratings used in a Rowland-like configuration. The optical system also includes two fine guidance sensors, and wavefront sensors that sample numerous locations over the full OTA FOV. In-flight wavelength calibration is performed by a wavelength calibration system, and flat-fielding is also performed, both using in-flight calibration sources. We describe the current optical design of CETUS and the major trade studies leading to the design.
We report on the early phases of a NASA-sponsored study of CETUS (Cosmic Evolution Through Ultraviolet Spectroscopy), a Probe-class mission concept. By definition, the full lifecycle cost of a Probe mission is greater than $400M (i.e. Explorer missions) and less than $1.00B (“Flagship” missions). The animating idea behind our study is that CETUS can help answer fundamental questions about galaxy evolution by carrying out a massive UV imaging and spectroscopic survey of galaxies and combining its findings with data obtained by other survey telescopes of the 2020’s. The CETUS mission concept comprises a 1.5-m wide-field telescope and three scientific instruments: a near-UV multi-object slit spectrograph with a micro-shutter array as the slit device; a near-UV and far-UV camera with angular resolution of 0.42” (near-UV) or 0.55” (far-UV); and a near-UV or far-UV single-object spectrograph aimed at providing access to the UV after Hubble is gone. We describe the scientific rationale for CETUS and the telescope and instruments in their early design phase.
Refinements in computer controlled optical surfacing allow efficient grinding and polishing of meterclass
optics to accuracy limited only by the surface metrology. We present a categorization of
metrology methods and their implementation for meter-class optical components. Interferometry with
computer generated holograms provides nanometer accuracy for full surface measurements of a wide
range of convex and concave aspheric surfaces. For measuring off-axis and freeform aspheric
surfaces, the holograms include features that provide references for alignment. Very high spatial
resolution is achieved with subaperture interferometric measurements which can be stitched together to
provide a full-aperture map. Scanning systems complement the capabilities of interferometry by
achieving larger dynamic range and providing independent corroboration. Optical coordinate
measurement machines (CMMs) provide non-contact measurements of surfaces in their ground state to
guide figuring, as well as highly accurate measurements of finished optics. Scanning systems for
measuring flat mirrors provide excellent resolution and absolute accuracy. The performance and
practical issues for this full array of measurement techniques are presented to show the relative
strengths of each method.
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 HET uses an 11-m fixed elevation segmented
spherical primary mirror, with pointing and tracking performed by moving the prime focus instrument package (PFIP)
such that it rotates about the virtual center of curvature of the spherical primary mirror. 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.
The 8-meter mirror production capacity at the University of Arizona is well known. As the Arizona Stadium facility is
occupied with giant mirrors, we have developed capability for grinding, polishing, and testing 4-m mirrors in the large
optics shop in the College of Optical Sciences. Several outstanding capabilities for optics up to 4.3 meters in diameter
are in place:
A 4.3-m computer controlled grinding and polishing machine allows efficient figuring of steeply aspheric and nonaxisymmetric
Interferometry (IR and visible wavelengths) and surface profilometry making novel use of a laser tracker allows quick,
accurate in-process measurements from a movable platform on a 30-m vertical tower.
A 2-meter class flat measured with a 1-m vibration insensitive Fizeau interferometer and scanning pentaprism system;
stitching of 1-m sub-apertures provides complete surface data with the technology ready for extension to the 4 m level.
These methods were proven successful by completion of several optics including the 4.3-m Discovery Channel
Telescope primary mirror. The 10 cm thick ULE substrate was ground and polished to 16 nm rms accuracy,
corresponding to 80% encircled energy in 0.073 arc-second, after removing low order bending modes. The successful
completion of the DCT mirror demonstrates the engineering and performance of the support system, ability to finish
large aspheric surfaces using computer controlled polishing, and accuracy verification of surface measurements. In
addition to the DCT mirror, a 2-meter class flat was produced to an unprecedented accuracy of <10 nm-rms,
demonstrating the combined 1-m Fizeau interferometer and scanning pentaprism measurement techniques.
New developments in fabrication and testing techniques at the College of Optical Sciences, University of Arizona have
allowed successful completion of 1.4-m diameter convex off-axis aspherics. The optics with up to 300 μm aspheric
departure were finished using a new method of computer controlled polishing and measured with two new optical tests:
the Swingarm Optical CMM (SOC) and a Fizeau interferometer using a spherical reference surface and CGH correction.
This paper shows the methods and equipment used for manufacturing these surfaces.
Surface plates and blocking tools are commonly made of granite because of its good stability. But how stable is the granite, and which type of material is optimal? We have explored several materials and manufacturing processes for a 4-m aspheric reference surface that would serve as a tool for laying up composite optics. In this paper, we discuss the materials selection, stability to thermal and moisture effects, and parameters for processing the surface to give sub-micron accuracy and stability.
The ability to grind and polish steep aspheric surfaces to high quality is limited by the tools used for working the surface. The optician prefers to use large, stiff tools to get good natural smoothing, avoiding small scale surface errors. This is difficult for steep aspheres because the tools must have sufficient compliance to fit the aspheric surface, yet we wish the tools to be stiff so they wear down high regions on the surface. This paper presents a toolkit for designing optimal tools that provide large scale compliance to fit the aspheric surface, yet maintain small scale stiffness for efficient polishing.
The Optical Science Center, University of Arizona, has designed and constructed an optical system for infrared target simulator for Carco Electronics Inc. The system is a f/3.36 three mirror telecentric system with 12 cm aperture. Due to the working environment, finite element analysis was performed to assure the system quality.