A useful extension of the optical mask pattern placement metrology is the measurement of critical dimensions (CD),
exploiting the outstanding mechanical resolution and stability of a corresponding mask metrology machine. In particular
the CD measurement on phase-shift masks (PSMs) poses a challenge on the optical measurement method. The paper
presents measurements and the corresponding computational modeling of the setup with respect to illumination beam
path (reflection, transmission), PSM properties and measurement optics for a dedicated edge detection method. Variables
have been the focus variation of the edge position and the critical dimension of the pattern. Based on the modeling
outcome the alignment and the illumination have been improved and verification measurements have been performed on
various machines of the type Vistec LMS IPRO3. The paper presents the measurements, the modeling and the
comparison to the practical measurement results for original and improved setup, showing the achievement of the
envisaged 2-nm repeatability.
The SUNRISE telescope is part of a balloon-borne instrument for spectro-polarimetric high-resolution observations of the solar atmosphere, to be flown 2007/2008 in the Antarctic summer stratosphere. It is a 1-m UV-VIS Gregory type telescope, operating close to the VIS diffraction limit. The telescope has a steel central frame and a lightweight CFRP trusswork structure with Serrurier properties, providing proper alignment of the optical elements over the varying eleva-tion angle. Mechanisms allow a fine adjustment of the optics. Aberrations caused by residual deformations of the stiff silicon carbide (Cesic) primary mirror are lowered by a dedicated offset in the secondary mirror polish (imprint). The telescope is subjected to the changing heat loads caused by the sun and earth radiation, necessitating measures to provide thermal conditions suitable for high-performance observations. Adequate preliminary solutions for an effective baffling are outlined.
The Stratospheric Observatory For Infrared Astronomy (SOFIA) houses a 2.5-m infrared telescope in a Boeing 747SP aircraft. It will be operated at high altitudes above the atmospheric water vapor. The telescope is of Cassegrain type in a so-called Nasmyth configuration with a VIS and an IR focus. The 2.7-m primary mirror is a monolithic element of Zerodur with a milled honeycomb structure on the backside. Despite of its size, it has a high stiffness (approx. 160 Hz 1st natural frequency) and a mass of approx. 885 kg only. The mirror support structure is a lightweight structure made from CFRP panels and profiles, bonded and riveted together with metallic inserts and joints. The mirror mounting by dedicated flexures provides a very stiff but nevertheless isostatic mounting of the mirror in the support structure (first natural frequency around 70 Hz). The secondary mirror is made from SiC with a very high 1st natural frequency of approx. 2 kHz allowing noise-suppressing chopper operation without image distortions. The tertiary mirror assembly is implemented as a dichroic beam splitter providing the IR Nasmyth focus for the scientific instruments and as a fully reflective mirror providing the VIS Nasmyth focus for tracking purposes. The paper describes the optical system with its subassemblies, their tested as-built performance as well as the predicted extrapolated overall image performance. The integration of the primary mirror assembly into the aircraft will be shown. The further integration and alignment steps, planned for summer 2004, will be explained.
The airborne Stratospheric Observatory for Infrared Astronomy SOFIA has a 2.7-m Cassegrain telescope with Nasmyth focus. The optical elements of the telescope are a 2.7-m diameter Zerodur parabolic primary mirror, mounted in a CFRP mirror support structure, a 0.35-m diameter convex silicon carbide secondary mirror, mounted on a chopping mechanism, and a plane dichroic tertiary mirror element. The nominal telescope focal length is 49.14 m. There are several distortion contributions to the total image quality. Among these are alignment errors, fabrication imperfections stemming from the manufacturing and polishing processes, static and dynamic mirror surface deformations caused by the mounting forces due to gravity and temperature effects and forces induced by aircraft vibra-tions and aerodynamic effects, and diffraction effects. The paper presents simulations for the image quality using wave-front performance data actually measured with the finished mirrors, where the primary was even mounted in its support structure. The influence of the static distortion contributions is also quantified.
The primary mirror assembly is the key opto-mechanical subsystem of the airborne SOFIA telescope. It consists of a 2.7-m primary mirror and a mirror support structure, the so-called primary mirror cell. The mirror is a monolithic ele-ment of Zerodur with a milled honeycomb structure on the backside. Despite of its size it has a mass of approx. 885 kg only. The mirror cell is a lightweight structure made from CFRP panels and profiles, bonded and riveted together with metallic inserts and joints. It provides an isostatic but stiff mounting of the mirror. The first natural frequencies are pre-dicted to be above 70 Hz for the whole 2000-kg assembly. The paper presents the actual structural properties of the primary mirror assembly determined in a modal survey test as well as the optical performance of the mirror mounted in the cell measured in horizontal and vertical orientation of the optical axis.
The SOFIA airborne telescope has a Tracking Subsystem for stellar acquisition, tracking, and pointing. The system has three high-performance imagers: the boresighted wide field (6 degrees FOV) and fine field imagers (70 arcminutes FOV), and the main-telescope-optics sharing focal plane imager (8 arcminutes FOV). The imagers are controlled by 3 CCD head controllers, an overall imager controller, and a tracker controller providing the tracking error signals from the objects observed by the imagers. There have been several test steps in the assembly, integration, and verification of the Tracking Subsystem. The paper presents the fully integrated system as actually built, the results of the thermal-vacuum and vibration tests of the fine field imager, the tested operational/functional S/W performance, as well as the results of the geometric and radiometric calibrations of the imagers.
SUNRISE is a balloon-borne instrument for spectro-polarimetric high-resolution observation of the solar atmosphere. It has a lightweight UV-VIS telescope of Gregory type with an aperture of 1 m, designed to be close to the VIS diffraction limit. The paper will first present the basic prescriptions of the optical design and the achievable performance. The re-quirements for the mechanisms in order to maintain the alignment over the range of environmental conditions will be derived. Secondly, the structural and thermal requirements will be discussed. Here, structural deflections due to gravity and residual thermal imbalances have to be taken into account. Preliminary structural and thermal designs will be out-lined.
For operation and monitoring, the SOFIA telescope assembly comprises a dedicated controls system. Basically this system appears to the user to consist of four major controller subsystems: Master Control Processor (MCP), Tracker Controller (TRC), Telescope Assembly Servo Control Unit (TASCU) and Secondary Mirror Controller (SMC). They are accessible via a single command driven interface link through the MCP. With the example of the subsystems MCP and TRC, commonality aspects in communication interface architecture and software protocol selection are discussed. The special requirements associated with the installation in an aircraft are considered as well as maintenance aspects for 20 years of operation. The paper covers user relevant hard-/software design aspects of the TA flight as well as ground support systems. The usage of the TA data structure definitions, provided in XML, throughout the whole observatory for interface description and implementation are explained.
The SOFIA telescope is ajoint NASA-DLR project for a 2.5 m airborne Stratospheric Observatory for IR Astronomy to be flown in a specially adapted Boeing 747 SP plane, Kayser-Threde being resopinsible for the development of the Telescope Optics. The φ 352 mm Secondary Mirror is mounted ona chopping mechanism to allow avoidance of background noise during IR observations. Stiffness associated to lightness is a major demand for such a mirror to achieve high frequency chopping. This leads to select SIlicon Carbide for the mirror blank. Its development has been run by the ASTRIUM/BOOSTEC joint venture SiCSPACE, taking full benefit of the instrinsic properties of the BOOSTEC SiC-100 sintered material, associated to qualified processes specifically developed for space borne mirrors by ASTRIUM. Achieved performances include a low mass of 1.97 kg, a very high stiffness with a first resonant frequency of 1865 Hz and a measured optical surface accuracy of 39 nm rms, using Ion Beam Figuring. It is proposed here to present the major design features of the SOFIA Secondary Mirror, highlighting the main advantages of using Silicon Carbide, the main steps of its development and the achieved optomechanical performances of the developed mirror.
The Tracking Subsystem of the SOFIA telescope consists of three high performance imagers and a dedicated tracking control unit. There are two boresighted imagers for target acquisition and tracking, one with a wide (6 degrees) and one with a fine (70 arcmin) field-of-view, and one main- telescope-optics sharing imager with a narrow field-of-view (8 arcmin) for high performance tracking. From the recorded stellar images, tracking error signals are generated by the tracker controller. The tracker controller has several features to support various tracking schemes such as tracking the telescope as an inertial platform, on- axis/offset tracking, and limb tracking. The tracker has three modes, i.e. positioning, tracking and `override'. Special features are the handling of so-called areas-of- interest in the inertial reference frame and the external imager synchronization. The paper presents the design and functional/operational performance of the imagers and the tracking control unit.
The stratospheric observatory SOFIA will provide astronomers routine access to celestial objects at wavelength bands which are not observable from the ground due to atmospheric absorption. SOFIA comprises a 2.5 m entrance pupil diameter telescope, including all required control systems, installed in a Boeing 747SP aircraft to form an observatory which is operated at altitudes above 12.5 km. Since January 1997 the telescope system is being developed by an industrial team for the German and US space agencies DLR and NASA. Operating a high precision instrument within the extreme environment of the stratosphere as well as stringent restrictions on the telescope mass demand technical solutions that represent the cutting edge of the state of the art optical instrument design.
The optical system of the airborne SOFIA telescope consists of a Cassegrain telescope with an effective aperture of 2.5 m and a so-called Nasmyth focus providing a lateral focus exit with access from the cabin side. The central optical part of the SOFIA Telescope is the 2.7m primary mirror made from a solid block of Zerodur as a monolithic element. The mirror will be lightweighted by making it to a dedicated 'double-arch' shape and by milling hexagonal holes from the backside. The lightweighting factor will be approximately 80 percent yielding a mirror mass of 850 kg only. The secondary mirror has a high-performance chopper actuator enabling an efficient background suppression especially for far IR observations. The tertiary mirror is implemented as a dichroic beamsplitter transmitting the visible part of the incoming radiation. This part is fed by an additional mirror to one of the tracking images, the focal plane imager, which allows a high-precision pointing of the star field under observation in the sub-arcsecond range. The two other imagers, the wide field imager and the fine field imager, are boresighted to the main telescope and will be used for the acquisition of the star field as well as for pointing and tracking of the telescope. The paper presents the current status of the development of the optical system including the imagers.
The 1-m STARS telescope was one of the five candidates for the coming ESA medium size (M3) mission. Based on the very compact Triply Reflecting Telescope concept, the STARS telescope provides diffraction limited image performance over a large field of view (+/- 0.75 degree(s)). Two different focal plane instruments, the Astero Seismology Detector and the Activity Line Monitor observe simultaneously in different wavelength bands in the range from 110 to 750 nm. Within the frame of an ESTEC conducted phase A study, the optical, mechanical, and thermal design of the complete telescope assembly including the accommodation on the service module has been investigated. A stiff and lightweight hexapod trusswork structure with struts from carbon fiber reinforced composites has been worked out as the most advantageous concept with respect to mass, opto- mechanical, and thermal properties. The major issues were the maintenance of an axial distance stability of +/- 10 micrometers between primary and secondary mirror as well as the fulfillment of certain thermal requirements for the two scientific instruments (Activity Line Monitor and Asteroseismology Detector). The finally chosen concept has a fully reflective optical design with higher order aspheric optical surfaces, a passive thermal design, is extremely lightweight (< 190 kg), and has a high stiffness (all eigenfrequencies are above 60 Hz). For the optics, a simpler approach with only conical reflective surfaces and an additional refractive correction element has been investigated.
The FT-IR system K300, primarily used for environmental monitoring by means of remote sensing techniques, has been modified in such a manner that it could be used for remote monitoring of a SiC chemical vapor infiltration process (CVI). This comprised hardware adaptations to a CVI reaction plant at DASA (MBB), as well as development of analytical methods. First measurements showed the good performance of the K300 system at the reactor. A lot of gaseous species could be detected and qualitatively analyzed (concentration changes).
A mobile environmental laboratory has been developed. This laboratory consists of a van which is equipped with different environmental sensors. The FT-IR system K300 by Kayser- Threde is the key instrument. With this K300 the van can be used for remote measurements of the gaseous emissions from smoke stacks. In addition the laboratory is equipped with standard ambient air analyzers as well as meteorological sensors. A large battery system ensures current source free operation the whole day. Reloading of the batteries takes only one night. remote measurements with this van were carried out at different power plants. Several pollutants could be analyzed. First results are presented.
The described double pendulum interferometer is a newly designed FTIR spectrometer based on the Michelson interferometer type. The unique optical layout allows a compact design in spite of high resolution and offers a very high stability. The presented device K300 is constructed as a stable instrument for field measurements. Apart from absorption measurements of trace gases, an innovation in the field of environmental measurements is the monitoring of smoke stack pollution by remote sensing. First results of emission as well as absorption measurements are presented.
The design and the main features of the new double pendulum type
michelson interferometer (DPI) by Kayser-Threde are presented.
The advantages of this spectrometer in comparison to conventional Fourier spectrometers are discussed. The DPI is compact in
design, mobile, insensitive to vibrations and temperature changes and, thus, well adapted to field measurements. The spectrometer was applied to emission as well as immission measurements
of air pollutants. Several molecules could be identified and
their concentrations could be estimated. The detection limit of
the DPI yields 15 - 60 ppm depending on the analyzed gas for
emission measurements, 6 - 84 ppb for immission measurements.
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