The Lyman-UV Imaging Spectrograph (LUVIS) is a NASA SMEX mission concept. Here, we describe the basic scientific requirements of LUVIS and instrumental requirements derived from the scientific requirements. Other papers in this conference by Woodruff et al. and Kendrick et al. describe the LUVIS instrument in detail.

We are developing the design for the Lyman-Ultraviolet (LUV) Imaging Spectrograph, LUVIS to propose for a small explorer (SMEX) mission. LUVIS will provide true long slit (six arc minute) imaging spectroscopic capability with large spectral resolving power, R. Minimizing the number of optical components to the required minimum of three enables large spectral throughput. The design uses a two-mirror Cassegrain Ritchey-Chretien Optical Telescope assembly (OTA), a single optic Rowland-like spectrometer, and a windowless 50 x 127 mm curved microchannel plate (MCP). The design is optimized over the 102 to 140 nm spectral range providing spectral imaging at R ~ 20K in a single exposure. Lyman-β enhanced Al + LiF mirror and grating coatings with the LiF protected with an atomic layer deposition (ALD) fluoride encapsulating overcoat provide high throughput over that spectral range. Line-of-sight (LOS) jitter control utilizes time-tag photon arrival to compensate field position jitter by re-registering pixel location in post-processing as well as tip/tilt active control of the secondary mirror of the OTA. This paper will describe the design as well as some of the key design trades that defined the design.

The Lyman UV imaging spectrograph (LUVIS) accomplishes priority UV science contained in the budget of a SMEXclass mission. LUVIS consists of a 0.5-m f/24 Cassegrain optical telescope assembly feeding a UV/ far-UV spectrometer. LUVIS has a long 6 arcmin slit enabling spectral imaging and is optimized for 102-140 nm with a resolving power of 20,000 on a micro-channel plate detector with a CsI photocathode. The light gathering power is designed to reach galaxies with near-UV fluxes as low as 10-14 erg/s/cm2 /Å (and lower with long time exposures). The design approach encompasses a simple but elegant optical design, minimum number of reflective surfaces, limited mechanisms, and an orbit minimizing fuel requirements while offering operational advantages. All components are already at a high technology readiness level further reducing technical and cost risk to meet a SMEX budget with healthy cost reserves.

The Cosmic Evolution Through UV Surveys (CETUS) concept has three UV instruments to achieve its science goals that work in the near ultraviolet (NUV) and far ultraviolet (FUV). The NUV multi-object spectrograph (MOS) and the NUV/FUV Camera operate simultaneously with their separate field of views. The key enabling technologies will be discussed including the micro-shutter array, detectors, and optical coatings. The NUV MOS can target up to 100 objects at a time which will allow over 100,000 galaxies to be observed during the mission lifetime. The UV Camera has the capability to image from the FUV to the NUV at the same time the MOS is operating at 180-350 nm. The UV Camera has a selection of bandpass filters, longpass filters, and two separate detectors to optimize observing in either the FUV or the NUV utilizing a sealed CsI solar blind micro-channel plate and a 4Kx4K CCD respectively. Both instruments have a tip/tilt/focus mechanism on one of their optics allowing independent focus correction and dithering of the image at the focal plane.

The Cosmic Evolution Through UV Spectroscopy (CETUS) concept1-3 enables parallel observations by the UV multiobject spectrometer (MOS) and near-UV/far-UV camera which operate simultaneously but independently with their separate field of views. The near-UV MOS can target up to 100 objects at a time without confusion with nearby sources or background zodiacal light. This multiplexing will allow over 100,000 galaxies to be observed over a typical mission lifetime. The MOS includes a next-generation micro-shutter array (NGMSA), an efficient aspheric Offner-like spectrometer design with a convex grating, and nanotube light traps for suppressing unwanted wavelengths. The NUV/FUV Camera has the capability to image in a range of sub-bands from 115-400 nm at the same time the MOS is operating at 180-350 nm. The UV camera has a similar Offner-like relay, selectable filters, and two separate detectors to optimize observing in either the far-UV (115-175 nm) or the near-UV (180-400 nm) utilizing a CsI Micro-Channel Plate detector (MCP) and a CCD respectively.

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 Multi-Object Spectrograph (MOS); and a dual-channel Point Source 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 of 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 (MSA). The two-channel camera design is also based on an Offner-like configuration. The Point Source Spectrograph (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 (FGS), and Wavefront Sensors (WFS) that sample numerous locations over the full OTA FOV. In-flight wavelength calibration is performed by a Wavelength Calibration System (WCS), and flat-fielding is also performed, both using in-flight calibration sources. This paper will describe the current optical design and the major trade studies leading to the design.

The ultraviolet multi-object spectrograph (MOS) for the Cosmic Evolution Through UV Spectroscopy (CETUS) concept(1,2) is a slit-based instrument allowing multiple simultaneous observations over a wide field of view. It utilizes a next-generation micro-shutter array, an efficient aspheric Offner spectrometer design with a convex grating, and carbon nanotube light traps for suppressing unwanted wavelengths. The optical coatings are also designed to optimize the UV throughput while minimizing out-of-band signal at the detector. The UV MOS will be able to target up to 100 objects at a time without the issues of confusion with nearby sources or unwanted background like zodiacal stray light. With this multiplexing, the scientific yield of both Probe and Great Observatories will be greatly enhanced.

There is a range of lessons learned when taking an optical system from design through deployment in space. I will summarize some key lessons and observations related to both airborne and spaceborne optical systems with an emphasis on light weighted optics and space telescopes. Areas discussed encompass 1) the initial architecture and system trades as constrained by system error budgets, 2) manufacturing considerations, 3) testing implications, and 4) real and perceived cost impacts.

Traditional mirror manufacturing, particularly for astronomical purposes, requires substantial lead time, due to the nature of the materials and the grinding/polishing process. We propose a new technique for rapid, low-cost production of large, lightweight precision optics by fusing several technologies which in combination we call frozen membrane mirror technology (FMMT). FMMT combines well-understood subsystem technologies, including electrostatic control of membrane mirrors, adaptive optics, wavefront sensing and control, and inflatable structures technology to shorten production time. The basic technique is to control the surface of a reflective coated membrane mirror with electrostatic actuation and wavefront sensor feedback and freeze the membrane shape. We discuss the details of the concept and present results of early lab testing. We focus on the optical regime, but this technology has applicability from the microwave to x-ray spectral bands. Starting with a flexible membrane mirror, one can envision techniques for deployment of large apertures in space. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

X-ray telescope architectures currently being examined for future missions such as concepts like the International X-ray Observatory (IXO) are composed of thousands of extremely thin mirror elements (0.2 to 0.4 mm thick) arranged in closely spaced arrays. The precise positioning, integration, and testing of those optical elements are some of the fundamental challenges for fabrication of future X-ray telescopes. We will describe a novel pneumatic actuator and initial testbed results for positioning a single mirror and subsequently an array of mirrors.

Herein we report on the development, sensing and control and our first results with the Vacuum Nuller Testbed to realize a Visible Nulling Coronagraph (VNC) for exoplanet coronagraphy. The VNC is one of the few approaches that works with filled, segmented and sparse or diluted-aperture telescope systems. It thus spans a range of potential future NASA telescopes and could be flown as a separate instrument on such a future mission. NASA/Goddard Space Flight Center (GSFC) has a well-established effort to develop VNC technologies, and has developed an incremental sequence of VNC testbeds to advance this approach and the enabling technologies associated with it. We discuss the continued development of the vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated testbed that operates under closed-loop control within a vacuum chamber. It will be used to achieve an incremental sequence of three visible-light nulling milestones with sequentially higher contrasts of 108, 109, and ideally 1010 at an inner working angle of 2*λ/D. The VNT is based on a modified Mach-Zehnder nulling interferometer, with a "W" configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent fiber bundle and achromatic phase shifters. We discuss the initial laboratory results, the optical configuration, critical technologies and the null sensing and control approach.

Three of the recently completed NASA Astrophysics Strategic Mission Concept (ASMC) studies addressed the feasibility of using a Visible Nulling Coronagraph (VNC) as the prime instrument for exoplanet science. The VNC approach is one of the few approaches that works with filled, segmented and sparse or diluted aperture telescope systems and thus spans the space of potential ASMC exoplanet missions. NASA/Goddard Space Flight Center (GSFC) has a well-established effort to develop VNC technologies and has developed an incremental sequence of VNC testbeds to advance the this approach and the technologies associated with it. Herein we report on the continued development of the vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated testbed that operates under high bandwidth closed-loop control within a vacuum chamber. It will be used to achieve an incremental sequence of three visible light nulling milestones of sequentially higher contrasts of 10(8), 10(9) and 10(10) at an inner working angle of 2*lambda/D and ultimately culminate in spectrally broadband (>20%) high contrast imaging. Each of the milestones, one per year, is traceable to one or more of the ASMC studies. The VNT uses a modified Mach-Zehnder nulling interferometer, modified with a modified "W" configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent fiber bundle and achromatic phase shifters. Discussed will be the optical configuration laboratory results, critical technologies and the null sensing and control approach.

A number of ongoing astrophysical mission concept studies are based on large aperture spaceborne telescopes. As optics get larger, both manufacturing and engineering trades come into consideration and must be balanced with the science goals and requirements. One of the top-level telescope trades examines the impact of a large monolithic primary mirror versus an array of smaller mirror segments to either fully or sparsely populate the same aperture. The first consideration is the scientific impact. Should the scattered edge effects and diffraction of a segmented design be acceptable, it then becomes a fabrication, test, and cost trade along with any associated risks. This paper will examine some of the key factors that go into such a trade and looks at manufacturing breakpoints. Examples such as the 4-m aperture New World Observer (NWO) and the 8-m aperture Advanced Technology Large Aperture Space Telescope (ATLAST) will be presented.

A number of upcoming astrophysical investigation concepts are based on large aperture spaceborne telescopes. The basic science goals drive the required aperture to gather sufficient resolution and signal for reasonable integrations to complete their planned design reference missions. In addition, certain fundamental requirements may dictate whether or not a monolithic aperture is required or a segmented mirror array is acceptable. The operating temperature and required performance (absolute and stability over time) are other important drivers. Based on such performance requirements a number of mirror manufacturing trades can be performed to balance the technical performance, cost, and schedule.We will discuss some of the overarching architectural and material trades along with particular manufacturing processes (and their related step functions) that are integral to selecting primary mirror approaches. We will include examples ranging from a few meters up to 16 meters which can be packaged into existing launch shrouds or in significantly expanded future resources such as the Ares V.