by MP Divakar, PhD, Stack Design Automation
Technical Editor, Electronics Cooling
Complementing Electronics Cooling’ article on Cooling the DARPA Space Surveillance Telescope, we are pleased to bring you a book review on the Thermal Design and Thermal Behaviour of Radio Telescopes and their Enclosures.
As you may have read from the article on DARPA’s SST, electronics cooling of such large hardware is a system-level exercise involving heat transfer and its coupled effects on the mechanical and electromagnetic behavior. Temperature induced deformations of the telescope can lead to performance degradation resulting in pointing errors and a decrease in sensitivity.
The opening salvo in the foreword of the book is guaranteed to catch thermal management engineers’ attention:
Every antenna, even those restricted to nighttime operation, is subject to thermal distortion. The greater the degree of surface precision demanded, the greater the relative importance of these distortions. Here again, the usual approximations are likely to be inadequate, and a computer analysis may be necessary. The choice of configuration of the antenna structure should minimize distortions due to temperature differentials in the structure and to changes in the ambient temperature. Consideration should be given to the use of reflective paints, and (for enclosed antennas) to environmental control. Lightweight insulation may be applied in some cases.
H. Simpson (1964)
A radio telescope is an instrument that collects and detects electromagnetic radiation from a preset area and direction in the sky. The calibrated measurements consist of either the total power or amplitude and phase, or a calibrated image of the object. The electromagnetic radiation observed in ground–based radio astronomy covers the radio window with wavelengths from several meters, for example 10m (= 30 MHz), to a fraction of a millimeter, for example, 0.3mm (= 1000GHz). The radio waves are collected on a parabolic dish antenna which are reflected to a radio receiver which are typically circuits with temperature controlled Heterostructure Field-Effect Transistors (HFET). Keeping the HFETs cooler reduces the thermal noise. Liquid Nitrogen is one of the common choices for cryogenic cooling of the receiver modules.
Unlike smaller electronic communication systems (for example, cell phone base stations) whose dimensions are an order or two smaller, the thermal environment inside a radio telescope has temperature effects that not only influence the embedded electronics but also result in thermal deformations of the structural components. Often, these effects manifest in a coupled manner that affects the electromagnetic behavior of the antenna which forces alignment changes in the radio telescope system. The required changes are implemented in a feedback control system where the ambient temperature (the temperature inside the antenna’s radome, if present), the heat dissipated by electronic components, and the effects of temperature on the structural deformations of the antenna backup structure (BUS) are all considered. This is one of the significant differences between radio telescopes and the smaller hardware systems for mobile and data center communication systems.
Organized in 15 chapters, the book begins by providing the requisite basic information on Radio Astronomy and Radio Telescopes. Chapter 1 also details the differences in the construction between meter and millimeter wave radios.
Chapter 2 on Radio Telescope Constructions in View of Thermal Aspects delves right into the effects of temperature fluctuations in the components of various types of radio telescopes. This chapter provides practical information on thermal protection solutions adopted in many functioning radio telescopes like ALMA (Chile), NRO (Japan), ALMA–J (USA), ASTE (Chile), etc. More importantly, the chapter also lists BUS ventilation and climatisation loads for many of the foregoing telescopes.
Chapter 3 discusses Telescope Enclosures such as radomes and astrodomes. Astrodomes, which are quite common, typically employ slits which are covered by membranes that are RF- transparent. Many others have features to open, as well as rotate about an azimuth for observations. Radomes on the other hand are hemispherical structures, typically stationary, closed, and are RF-transparent. The thermal load in a radome is the same as that of the telescope with adequate ventilation to reduce diffused background radiation.
Chapter 4 presents more treatment of the Variable Thermal Environment the telescope is subjected to. It considers the modes with which the external environment interacts with a telescope, or its enclosure, and hence the thermal behavior of a telescope. The figure below represents a model used to capture the various environmental variables influencing the thermal behavior of radio telescopes.
Chapter 5 discusses the calculation of Solar Illumination, a topic that is quite well known to thermal management professionals.
Chapter 6 presents various methods for temperature measurements and the finite element method (FEM) to compute thermal deformation calculations. This chapter is particularly noteworthy as FEM results are correlated to locations of temperature sensors. The figure below shows two views of the FEM grid and the sensor node locations modeled for the AEC ALMA telescope.
Chapter 8 presents rigorous treatment of radiative coupling between the outer surfaces of a telescope, its enclosure, the sky, and the ground. Significant temperature differences can exist between the top and bottom surfaces of the antenna reflector, some times as much as 5degC. Ignoring the radiative coupling can lead to significant errors in thermal calculations.
Chapters 9 and 10 cover useful and practical data on measured thermal behavior of radio telescopes and their enclosures, respectively. These chapters are filled with very real world measurement data from many of the radio telescopes previously cited.
Chapter 11, Thermal Model Calculations, is one of the most interesting chapters in the book, covering thermal modeling of the radio telescope components. Temperatures predicted from modeling are useful for both design and operational purposes.
Chapter 12 on Beam Formation and Beam Degradation discusses very important aspects of thermal modeling of telescope components – beam formation and the magnitude of beam degradations due to thermal deformations. The effect of thermal tolerances is discussed in Chapter 13. For example, good telescope performance is obtained if the deformations of the beam forming wavefront do not exceed ∼1/16 of the wavelength of observation λ, if the focus is stable within ∼λ /10 and if the pointing is stable within ∼1/10 of the beam width.
Chapter 14 on Optical Telescopes and Enclosures provides a brief discussion on the enclosures of optical/IR telescopes.
Chapter 15 A Summary and Further Studies, covers topics that are worthy of investigation for a future revision of the book. Aging effects on thermal protection systems such as paints, insulation, real time temperature monitoring and pointing corrections, including view factors of backup structures in radiation heat transfer, etc.
Books on thermal management technology and design of radio telescopes are not as common as books on heat transfer and thermal management! Monographs like this are therefore very valuable resources that a practitioner and novice alike would find very useful. The book is very practical, chockfull of data on operating radio telescopes worldwide. It closes the gap between design and simulation and correlates with real world operational data that validates assumptions and methodologies.
Book Information:
Thermal Design and Thermal Behaviour of Radio Telescopes and their Enclosures (2010)
Authors: Albert Greve and Michael Bremer
ISSN 0067-0057
ISBN 978-3-642-03866-2 e-ISBN 978-3-642-03867-9
DOI 10.1007/978-3-642-03867-9
Springer Heidelberg Dordrecht London New York
