WFST is a proposed 2.5m wide field survey telescope intended for dedicated wide field sciences. The telescope is to operate at six wavelength bands (u, g, r, i, z, and w), spanning from 320 to 1028 nm. Designed with a field of view diameter of 3 degree and an effective aperture diameter of 2.29 m, the WFST acquires a total optical throughput over 29.3 m2deg2. With such a large throughput, WFST will survey up to 6000deg2 of the northern sky in multiple colors each night, reaching 23th magnitude for high-precision photometry and astrometry. The optical design is based on an advanced primary-focus system made up of a 2.5 m f/2.48 concave primary mirror and a primary-focus assembly (PFA) consisting of five corrector lenses, atmospheric dispersion corrector (ADC), filters, and the focal-plane instrument. For zenith angles from 0 to 60 degrees, 80% of the polychromatic diffracted energy falls within a 0.35 arcsec diameter. The optical design also highlights an enhanced transmission in the UV bands. The total optical transmission reaches 23.5% at 320 nm, allowing unique science goals in the U band. Other features include low distortion and ease of baffling against stray lights, etc. The focal-plane instrument is a 0.9 gigapixel mosaic CCD camera comprising 9 pieces of 10K×10K CCD chips. An active optics system (AOS) is used to maintain runtime image quality. Various design aspects of the WFST including the optical design, active optics, mirror supports, and the focal-plane instrument are discussed in detail.
Dome A 5m Terahertz Explorer (DATE5) is a proposed telescope to be deployed at Dome A, Antarctica to explore the excellent terahertz observation condition unique to the site. One of the key challenges of the telescope is to realize and maintain the required 10 μm rms overall reflector surface accuracy under the extreme site conditions and unmanned operating mode. Aluminum panels on carbon fiber backup structures is one of the candidate options for the 5 meter main reflector. For aluminum panels, three major technical risks were identified: 1) the large CTE of aluminum causes significant panel deformation due to the large seasonal soak temperature change; 2) internal stress may cause additional surface deformation when operating under a cold environment; 3) reflector panels working at Dome A run high risks of icing (which degrades antenna efficiency and increases noise) and automatic active de-icing mechanisms has to be implemented on the panels. In order to verify the feasibility of the aluminum panels for DATE5 and identify possible technical risks, a prototype panel was fabricated and went through rigorous tests. The manufacture error at the room temperature is 3.2 μm rms, which meets the budget. The panel surface is then measured at various ambient temperatures down to -60°C in a climate chamber using photogrammetric techniques. The additional surface error at the low temperatures is found to be mainly contributed by defocusing error, and the dependence of the panel focal length on temperature is well predictable. No additional surface error caused by internal stress has been observed. Next, the icing condition of the panel is analyzed and a prototype de-icing system based on polyimide film heaters was installed on the panel. The performance of the de-icing system was tested in a climate chamber as well as in the field experiments to simulate a variety of operating environments. The experiments indicate that the power required for de-icing the entire main reflector is less than 1kW and the temperature field produced by the de-icing system has trivial effect on the surface accuracy of the panel. This study indicates that aluminum panels have the potential to meet the reflector surface error budget under the harsh environment of Dome A.
DATE5 antenna, which is a 5m telescope for terahertz exploration, will be sited at Dome A, Antarctica. It is necessary to keep high surface accuracy of the primary reflector panels so that high observing efficiency can be achieved. In antenna field, carbon fiber reinforced composite (CFRP) sandwich panels are widely used as these panels are light in weight, high in strength, low in thermal expansion, and cheap in mass fabrication. In DATE5 project, CFRP panels are important panel candidates. In the design study phase, a CFRP prototype panel of 1-meter size is initially developed for the verification purpose. This paper introduces the material arrangement in the sandwich panel, measured performance of this testing sandwich structure samples, and together with the panel forming process. For anti-icing in the South Pole region, a special CFRP heating film is embedded in the front skin of sandwich panel. The properties of some types of basic building materials are tested. Base on the results, the deformation of prototype panel with different sandwich structures and skin layers are simulated and a best structural concept is selected. The panel mold used is a high accuracy one with a surface rms error of 1.4 μm. Prototype panels are replicated from the mold. Room temperature curing resin is used to reduce the thermal deformation in the resin transfer process. In the curing, vacuum negative pressure technology is also used to increase the volume content of carbon fiber. After the measurement of the three coordinate measure machine (CMM), a prototype CFRP panel of 5.1 μm rms surface error is developed initially.
A 5-meter terahertz telescope is proposed by the Chinese Center for Antarctic Astronomy (CCAA) for the East
Antarctica site of the Dome A plateau. The Dome A 5-m terahertz telescope (DATE 5) will be operated at sub-millimeter
waveband taking the unique advantage of the transparent atmospheric windows between 200 and 350 μm wavelengths at Dome A. A preliminary design has been conducted according to the given technical requirements and the special environmental conditions at Dome A. A symmetric R-C Cassegrain optical system is designed for the telescope, with a primary f-ratio of 0.4 and a wide field of view of 10 arcmin. The magnification of the sub-reflector is 9.4, leading to the final focal ratio of 3.76 and the focus 0.2 m below the vertex of the primary reflector. To ensure surface accuracy of the reflectors precise as small as 10 um RMS, we consider using Carbon Fiber Reinforced Plastics (CFRP) to build the backup structure (BUS) of the primary reflector and the sub-reflector itself. An alt-azimuthal mounting is adopted and a tall base structure beneath the telescope is set up to lift the telescope above the low atmosphere turbulent layer. All the mechanics, as well as control electronics, are strictly designed to fit the lower temperature operation in the Dome A environment. This paper is to generally present the mentioned systematic optical, structural and electronic design of the DATE 5 telescope.
The planned Square Kilometer Array (SKA) includes three thousand 15m antennas. The radio flux density from the sun is stronger, so that a solar array, such as Frequency-Agile Solar Radiotelescope (FASR) with hundreds of dishes can have smaller dish size. Therefore, light weight, low cost dish design is of vital importance. The reflecting surface supported by an antenna back-up structure, generally, should have an RMS surface error less than λ/20 (λ. is the operating wavelength). For resisting gravitational, wind, and ice-snow loadings, an antenna dish also requires reasonable mode frequencies. In this paper, different low cost small or medium back-up structure designs are discussed, including double-layer truss design and prestressed dish design. Based on discussion, an innovative light weight, prestressed back-up
structure is proposed for small or medium aperture antennas. Example of a small 4.5m aperture dish design working
below 3GHz is presented. This design is a one-layer prestressed truss structure with low weight, ease installation, and
low manufacture cost. Structural analysis and modal extraction results show the structure is much stiffer than the same
structure without prestressed loading.
Off-axis systems in radio and infrared wavelengths have obvious advantages in suppressing aperture blockage and
background noise. Therefore, the signal-to-noise ratio as well as system gain is improved. However, an off-axis optical
system involves complex aberrations which limit its field of view. This paper provides an overview of aberrations in
axial symmetric and off-axis optical systems. As the system deviates from an axial symmetric one, system aberrations
become more and more complicated. In a general two mirror off-axis focusing system, the field of view is nearly zero.
Even for off-axis system with an equivalent parabola, the field of view is still very small as the linear astigmatism
dominates the system. An optimized off-axis system is one in which the linear astigmatism and cross polarization-free
conditions are met. In this optimized system, coma aberration is dominant, so that the field of view is still limited. The
field of view of an optimized off-axis system can be improved using a simple coma corrector system.
Proc. SPIE. 7733, Ground-based and Airborne Telescopes III
KEYWORDS: Telescopes, Manufacturing, Control systems, Computer programming, Synthetic apertures, Radio telescopes, Directed self assembly, Antennas, Radio astronomy, Commercial off the shelf technology
The key factor influencing the size of future radio aperture synthesis telescope projects is the cost of the antenna dish and
its drive and axis system. A good drive and axis system is of vital importance to the antenna performance as it is directly
related to the antenna pointing and tracking accuracy. In this paper, various different drive systems have been
investigated and, finally, a compact, modular, scalable, combined elevation and azimuth drive and axis unit design is
presented. In this proposed design, the elevation drive and axis component shares virtually all parts with the azimuth
drive and axis component and many commercial off-the-shelf parts are used for both axis and drive linkage. This design
can also assure the required pointing and tracking accuracy is met. Since it is a self contained unit, the assembly,
installation, and maintenance are further simplified, resulting in overall lower cost.
The key factor, which influences the size of future radio aperture synthesis telescope projects, is the cost of the antenna
dish. Lower antenna dish cost will allow more antennas to be built, providing more baselines and improving the
telescope sensitivity and image quality. In this paper, various methods of lowering the dish structure cost, including
using pre-stressed beam members, are discussed and, finally, an innovative low cost small antenna dish is proposed
which relies mostly on commercial off-the-shelf components.
More and more astronomical telescopes use carbon fiber reinforced composites (CFRP). CFRP has high stiffness, high
strength, and low thermal expansion. However, they are not isotropic in performance. Their properties are direction
dependent. This paper discusses, in detail, the structural and thermal properties of carbon fiber structure members, such
as tubes, plates, and honeycomb sandwich structures. Comparisons are provided both from the structural point of view
and from the thermal point of view.
Generally, panels of radio telescopes are mainly shaped in trapezoid and each is supported/positioned by four adjustors
beneath its vertexes. Such configuration of panel supporting system is essentially hyper-static, and the panel is overconstrained
from a kinematic point of view. When the panel is to be adjusted and/or actuated, it will suffer stress from its
adjusters and hence its shape is to be distorted. This situation is not desirable for high precision panels, such as glass
based panels especially used for sub-millimeter and shorter wavelength telescopes with active optics/active panel
technology. This paper began with a general overview of panel patterns and panel supports of existing radio telescopes.
Thereby, we proposed a preferable master-slave active surface concept for triangular and/or hexagonal panel pattern. In
addition, we carry out panel error sensitivity analysis for all the 6 degrees of freedom (DOF) of a panel to identify what
DOFs are most sensitive for an active surface. And afterwards, based on the error sensitivity analysis, we suggested an
innovative parallel-series concept hexapod well fitted for an active panel to correct for all of its 6 rigid errors. A
demonstration active surface using the master-slave concept and the hexapod manifested a great save in cost, where only
486 precision actuators are needed for 438 panels, which is 37% of those actuators needed by classic segmented mirror
active optics. Further, we put forward a swaying-arm based design concept for the related connecting joints between
panels, which ensures that all the panels attached on to it free from over-constraints when they are positioned and/or
actuated. Principle and performance of the swaying-arm connecting mechanism are elaborated before a practical cablemesh
based prototype active surface is presented with comprehensive finite element analysis and simulation.
In millimeter wavelength telescope design and construction, there have been a number of mysterious failures of simple
CFRF-metal joints. Telescope designers have not had satisfactory interpretations of these failures. In this paper, factors
which may influence the failure of joints are discussed. These include stress concentration, material creep, joint fatigue,
reasons related to chemical process and manufacture process. Extrapolation formulas for material creep, joint fatigue,
and differential thermal stresses are derived in this paper. Detailed chemical and manufacturing factors are also discussed.
All these issues are the causes of a number of early failures under a loading which is significantly lower than the strength
of adhesives used. For ensuring reliability of a precision instrument structure joint, the designer should have a thorough
understanding of all these factors.
The photogrammetry technique is widely used for measuring 3-D shape in diverse industries thanks to its easy implementation and straightforward algorithm. However, most measurements done by this technique are using a single camera with multi-exposure and the results are derived after a period of time. Along with the development of the CCD and computer technology, it is now possible to use multi-camera, real-time photogrammetric measurements and the results could be derived without much time delay. A multi-camera system avoids the error caused by time-delayed exposure. So the new technique could be widely used in telescope surface or position measurement and control. In this paper, the pertinent formulations, implementation and application details are discussed before a general conclusion is drawn.
A radio telescope is to be built at National Astronomical Observatory of China, which is designed to receive signal from pulsars for timing and relevant usage. The telescope will have an aperture of 50 m in diameter working at multi-wave bands of which the shortest wavelength is down to 13 cm. A fully steerable exposed scheme of the telescope within issued specification is studied. The design is essentially wheel and track style with 6 rollers grouped in three couples running on a track of 35 m in diameter. The main paraboloidal reflector is a mesh spanned with cramped stainless steel wire installed on a special "bowl-like" backup truss structure supported by 6 points on the bottom. The elevation motion is served by a couple of big spoke and brace welded gearwheels with "buoyant" unloading system for eliminating deflections due to deadweight and thermal effect. Besides the design of the main reflector back-up structure, this paper presents the special alidade layout, driving system, structural and wind hazard analyses, and includes servo control system before drawing a general conclusion.
Carbon fiber reinforced plastic (CFRP) is a suitable material for space and ground-based telescope structures. CFRP has a high stiffness-over-weight ratio and a low thermal expansion coefficient. Together with aluminum honeycomb, CFRP can form very strong light-weight sandwiched structures. These sandwiched structures, which can support high bending moments and shear forces without much deformation, are used widely in the existing and future large-space or ground-based telescopes. However, some special CFRP-aluminum honeycomb sandwiched structures have shape change problems when the absolute temperature changes. In this paper, some of these thermal shape changes are discussed. The designers of the future large telescopes should be fully aware of the shape change problem of these structures.
For millimeter wavelength antennas, solar radiation is a very important source of serious surface, phase, and pointing distortions. To reduce these distortions, carbon fiber composites are widely used in some existing and newly designed millimeter wavelength antennas. However, carbon fiber reinforced composites (CFRP) are different from other isotropic structural materials. Their properties are direction dependent; in the fiber direction, very favorable; perpendicular to the fibers, less favorable. Carbon fiber composite joints are also complex and costly. This paper discusses the properties of carbon fiber structure members, such as: tubes, beams, cones, thin plates, and honeycomb sandwiched thick plates. Comparisons are provided both from the structural and thermal points of view. The paper also gives the stress distribution of a simple composite glued joint.
The prospect of a joint Millimeter Array development effort between the U.S. and Europe has led to various antenna designs. This paper describes a new 12-m antenna design that has many new features which are not widely used among existing millimeter wavelength antennas. These include: light-weight machined aluminum panels; feedlegs with triangular roofing for reflecting scattered rays to the sky; double-layered carbon- fiber reinforced plastic (CFRP) trusses on large radius supports; rotating counterweight for reducing the moment of inertia; a yoke incorporating CFRP trusses and a steel beam structure; and a displacement-measuring metrology system. A design incorporating these features could achieve a combination of high performance and reasonable overall cost. The paper also discusses in detail a number of key issues of interest for future millimeter wavelength antenna development. The design is influenced by the large number of antennas required for the Millimeter Array.
A number of possible antenna designs have been considered for the Millimeter Array. This paper presents two designs which were studied in detail but are no longer being considered, as well as three of the designs currently being studied. The major performance goals for the antennas are listed. Two of the most challenging performance requirements, getting sufficient pointing accuracy and achieving fast position switching, and some of the possible approaches to meeting these challenges are discussed.
Recently, design examples of wide field three mirror Mersenne-Schmidt system had been presented by R.V. Willstrop. The work presented in this paper is an extension of Willstrop's early work. By applying overall system optimization, the maximum image size within a 4-degree field of a well-designed Mersenne-Schmidt system can be as small as 0.16 arc sec. If small edge vignetting is allowed, within a five-degree wider field of view, the maximum image size of the Mersenne-Schmidt system would be as small as 0.34 arc sec. In the second part of this paper, the Mersenne-Schmidt telescope is discussed in comparison with a Schmidt camera. Discussion shows that both systems can achieve a wider field of view. However, from astronomical requirements, the Mersenne-Schmidt telescope has better image quality, wider waveband coverage, greater light power, shorter tube length and larger usage area of the primary mirror. For a light collecting area larger than that of existing ones, the Mersenne-Schmidt telescope would be a better choice for astronomical frontier work.