The beam pattern of the IRAM 30m telescope has been derived fr om differential total power scans across the limb of the Moon. The 30m beam is described by the main beam and three error beams which are given by residual large-scale deformations of the primary mirror , and its frames and panels. Total power scans across the Moon taken shortly after Full Mo on at 86, 145, 210, 280 and 340 GHz in February 2010 under best late-night conditions and at opt imum elevation were used to derive the beam pattern of the 30m telescope to a level of less than −30dB and to a full width of upto2000. The analysis shows the improvement of the IRAM 30m telescope refl ctor from panel frame adjustments and application of ventilation and temperature control in t he elescope yoke and counter weight done between 1998 and 2002. At the present low level of the error beam , transient thermal panel buckling becomes an observational effect. The paper gives an updated table of the error beam parameters and telescope efficiencies which are valid since September 2002. Recipes are g iven to derive the beam parameters at other frequencies. We also briefly discuss the possible degradation o f the beam when observing under non-optimum conditions.

Aims. The prime motivation of this project was to design and build a state-of-art mm-wave heterodyne receiver system to enhance the observing throughput of the IRAM 30-m radiotelescope. More specifically, the requirements were i) state-of-art noise performance for spectroscopic observations; ii) simultaneous dual polarization and dual-frequency observing; iii) coverage of the atmospheric transmission windows from 83 to 360 GHz; iv) compact footprint and minimal maintenance.Methods. Key elements for low noise performance of heterodyne mixers are the superconducting Niobium junctions, operating at similar or equal to 4 K. These junctions are embedded in carefully designed coupling structures; furthermore, since atmospheric radiation is a significant contributor to the system noise budget, all mixers are either sideband separating or sideband rejecting. To achieve low noise, it is also essential to maximize the coupling of the receiver to the astronomical source, and to minimize the coupling to thermal radiation from the ground-based environment; this is achieved through mirror optics that realize a wavelength-independent coupling to the telescope. A flexible configuration of mirrors and frequency selective surfaces permits various combinations of frequency bands, as well as dual-load radiometric calibration. Low noise intermediate frequency amplifiers and bias electronics also play an important role in the system performance.Results. The EMIR receiver in operation at the 30m telescope offers four frequency bands: B1: 83-117 GHz, B2: 129-174 GHz, B3: 200-267 GHz, and B4: 260-360 GHz. In each band, the two orthogonal polarizations are observed simultaneously. Dual-band combinations B1/2 B1/3, and B2/4 are available. Bands 1 and 4 (also 3 as of Nov.-2011) feature sideband separation. In dual-band configuration, including sideband separation and polarization diplexing, up to eight IF channels are delivered to the spectrometers, totaling up to 64 GHz of signal bandwidth (of which 32 GHz can be transported and processed by spectrometers, status Nov.-2011). The EMIR receiver has been in continuous operation for more than two years and has allowed, through a qualitative jump in performance, observations not possible before, as shown by a few selected examples of astronomical results.

We summarize earlier and unpublished long-slit spectroscopic measurements of radial velocities of ionized gas and stars along and parallel to M 82's major axis to a radial distance of similar to +/- 2.5 kpc (similar to +/- 140 '') from the center. In the position-velocity diagram, these measurements indicate a velocity reversal of similar to 100 km s(-1) of gas and/or stars at similar to +/- 1.0 kpc (similar to +/- 50-70 '') on either side of the center, outside the bar. Although seen in earlier observations, and perhaps neglected because they were assumed to be only an effect of heavy local extinction, the positional symmetry of the velocity reversal with respect to the center of M 82, as well as the absence of the reversal in the motion of stars seen in the near-IR Ca II absorption lines, points to another origin. M 82's two-armed spiral, as outlined by Mayya et al. (2005, ApJ, 628, L33), may explain a part of the velocity reversal, although this interpretation leaves inconsistencies so that other explanations must also be investigated. A simple, conclusive explanation of the velocity reversal has not yet been found. While restricted observationally in radial distance to similar to 120 '' (2 kpc), the near-IR stellar Ca II absorption lines, which do not show the velocity reversal, indicate a flat radial velocity curve of the stellar disk that remained after the encounter with M 81.

In earlier work we have described the thermal modelling for design and control of a fully insulated, and sometimes ventilated, high precision radio telescope. For such an insulated telescope the modelling of the time-variable dynamic influence of the thermal environment (air, sky and ground radiation, insolation) is relatively simple. The modelling becomes however quite complex for an open-air radio telescope where each individual member of the reflector backup structure (BUS) and the support structure (fork or yoke) is exposed under a different and time-dependent aspect angle to the thermal environment, which applies in particular to solar radiation. We present a time-dependent 800-element thermal model of an open-air telescope. Using the IRAM 30-m radio telescope as the basic mechanical structure, we explain how the temperature induced, real-time pointing and reflector surface deformations can be derived when using as input the day of the year, the thermal environment, and the geographic position of the telescope and its changing pointing direction. Thermal modelling and results similar to those reported here can be used for radio telescope design and real-time control of pointing and surface adjustment of a telescope with active panels.

The local thermal environment interacts with a telescope, or its enclosure, and determines to a large extent the thermal behavior of a telescope. The thermal environment is therefore investigated with the intention to derive global parameters for the design and operation of a telescope and its enclosure. The global parameters are site dependent and dependent on the season, the month, and the day of the year. Each telescope site needs its own investigation for a considerable length of time to obtain reliable statistical data. A telescope may reach a (quasi-)equilibrium state in the thermal environment, or may follow its variations with a certain time delay and a reduced amplitude. The interaction of structural components with the environment can be reduced by paint, insulation, and ventilation, or even be excluded to a large extent by placing the telescope in an enclosure. Important for the design and operation of a telescope are the time constants of the thermal environment and the thermal time constants of the telescope components.

Radio telescopes are built for observations frommeter to sub-millimeter wavelengths (Table 1.1). The steerable telescopes have a parabolic reflector of 10–100–m diameter and the optics can be designed for observations in primary focus, Cassegrainian focus, or Gregory focus (Sect. 2.1). The telescopes have a support, that is, a pedestal, an alidade, or a fork; a reflector backup structure and panels; and a quadripod with subreflector (Fig. 1.1, Chap. 2). The construction material is steel, aluminium, low thermal expansion reinforced carbon fiber plastic (CFRP), and invar, and concrete for the pedestal and foundation (Sect. 2.4). The design of a telescope is based on a finite element analysis, a flexible body analysis (which considers the telescope, the drives, and the control system as one dynamical unit (Kärcher 2006; Gawronski 2007)), and static and/or dynamic thermal model calculations (Sects. 11.2 and 11.3). The backup structure can be a homologous design (Sect. 2.4.2). The technical efforts applied in the control of temperature (and wind) induced deformations must be seen in the context of the envisaged sensitivity and directivity, and also in the context of the time spent in focus and pointing measurements.

The temperature (T) and the temperature change with time (∂T/∂t) are the basic parameters in the discussion of the thermal behavior of a structural component. The temperature of a component defines, on the one hand, the heat transfer to other components of the telescope, the enclosure, and the environment, and on the other hand, the thermal expansion of a component and by this a possible force introduced in the structure.

A radio telescope operates with good performance if all relevant factors remain stable for a considerable period of time. Adverse influences may arise from gravity, temperature, and wind. They affect the focus, the pointing, the reflector surface, and the path length. Corrections can be made from pointing and focus measurements that may, however, consume a substantial part of the observing time. Telescopes with active main reflector or subreflector surface can, in addition, upgrade the performance from temperature monitoring and/or metrology measurements and subsequent real time actuator control.1

The elaborate technical efforts in the construction of radio telescopes and communication antennas are undertaken to obtain a clean beam and the best power sensitivity. This merits an explanation of the beam (image) formation of a telescope/antenna, at first hand expected to be perfect, in the end, however, always suffering from some degradation because of technical shortcomings and influences from the environment. The knowledge of the origin, the form, and the magnitude of beam degradations is the basis on which the tolerance theory of thermal deformations of a telescope is constructed in Chap. 13. An explanation of the diffraction theory of beam formation is found in the textbooks by Born and Wolf (1980), Love (1968), Rush and Potter (1972), Lo and Lee (1988), Christiansen and Högbom (1995), Kraus (1985), Baars (2007), Rohlfs and Wilson (1996), Thompson et al. (2001), and others.

Several radio telescopes for millimeter and sub-millimeter wavelengths and several communication antennas are protected by a radome or an astrodome. The radome is an over-hemispherical enclosure of which the inside climate can be controlled by ventilation or air-conditioned ventilation. The radome is stationary and the Sun illuminates it from a gradually changing direction as explained in Sect. 5.8. Solar radiation is diffusely transmitted through the radome skin and is next to internal heat sources (receivers, telescope drives), the origin of a vertical temperature gradient of the inside air. The ventilation reduces the vertical temperature gradient and also the temperature gradient in the telescope structure. The radome of the Onsala 20-m telescope is shown in Fig. 3.4.

A radio telescope consists of a pedestal, or an alidade or fork, of a BUS connected to the elevation structure or connected to the secondary focus cabin, of panels on the BUS that form the reflector surface, and of a quadripod with the primary focus cabin and subreflector (Fig. 1.1). Not all of these components are in direct thermal contact, but they are all exposed to the variable thermal environment or an artificial environment in a radome or astrodome. As the components are mechanically connected, a thermal inhomogeneity in one or several of the telescope components may introduce a mechanical deformation that may affect the performance of the whole telescope.

Model calculations are made to obtain for certain load cases information on the thermal behavior of a telescope, or a telescope and its enclosure or of a specific structural component. The temperatures predicted from model calculations are used for design and/or operational purposes. Design questions addressed in thermal model calculations may concern the type of insulation required to obtain temperature stability, the type and amount of ventilation required to obtain temperature uniformity of a BUS under asymmetric solar illumination, the heat required to counterbalance radiative cooling of a BUS during the night, etc. The calculations provide numbers, like 4 cm thick insulation, 10,000m3 h-1 ventilation, 10kW heating etc., which are then realized in the construction. Thermal model calculations and associated finite element calculations may investigate for operational purposes the pointing stability of an alidade support, the focus stability of a BUS under the influence of the thermal environment, the development of transient reflector surface deformations due to temperature asymmetry in a yoke structure, etc. The thermal load case in the finite element calculation may be of a static nature in which the telescope and enclosure experiences one particular temperature change, or of a dynamic nature in which the telescope and enclosure experience a time variable temperature change. This temperature change may be of an artificial nature, for instance a sudden temperature drop to determine the thermal time constant of a structure, or it may represent the response of the telescope and enclosure to the change of the thermal environment in which they operate or are expected to operate. The quality of an answer of a thermal problem depends, evidently, on the quality of the thermal model, which is constructed from the basic relations of heat conservation and heat transfer. The confidence of an answer can be judged from thermal calculations of a similar structure of which temperature measurements are available for comparison.

Radio telescopes as well as communication antennas operate under the influence of gravity, temperature and wind. Among those, temperature influences may degrade the performance of a radio telescope th

An astronomical object somewhere in the Planetary System, the Galaxy, a Cluster of Galaxies, or far out in the Universe may generate radio waves by one or the other physical process (a topic of Astrophysics). If the radio emission is generated inside the object, some of the radio waves propagate through the object until they may leave the radio source at its surface (Radiative Transfer). The emitted radio waves then propagate through the intergalactic, interstellar, and interplanetary space (Radiation Propagation) and finally through the Earth’s atmosphere (Atmospheric Physics). On the way through space and the Earth’s atmosphere, the radio waves are more or less severely absorbed and deflected from a straight path (Atmospheric Physics, Refraction). In addition, the orientation of polarization of the radio waves may be rotated (Faraday rotation) in a medium with free electric particles located anywhere along the path of propagation. Finally, the radio waves are collected by the radio telescope (Radio Optics), detected by the receiver (Radio Electronics), and analyzed and recorded in the spectrometer and computer (Data Acquisition).

Although the specific thermal aspects under discussion may be different for a radio telescope and an optical telescope, for instance concerning, on the one hand, the thermal stability of the reflector backup structure and, on the other hand, temperature induced man-made dome seeing, the physics and the treatment of thermal problems is nevertheless in many ways similar. The similarity of thermal issues is high for (sub)millimeter-wavelength radio telescopes and optical/IR telescopes, although an optical telescope is always covered by a dome. The structural similarities between millimeter-wavelength radio telescopes and optical telescopes are summarized in Table 14.1.

The fundamental information that the designer, construction engineer and telescope operator wants from the preceding chapters are thermal tolerance criteria on which to build a telescope and that can be used to evaluate the thermal performance of an operating telescope. The thermal tolerance criteria derived in this chapter are based on beam pattern calculations explained in Chap. 12, which indicate that a 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. Interferometer telescopes should have, in addition, a path length stability of ~λ/10 or better. These performance criteria should be fulfilled during a considerable length of time before a calibration of the telescope needs to be made. The performance criteria translate into mechanical tolerances and these into thermal tolerances of the telescope structure.

The reddening in and around emission nebulae is characterized by the extinction AV and the ratio R of the absolute-to-selective absorption. Both are usually derived from the photometry of a single star or cluster stars that are associated with an emission nebula. Using the parameterized reddening relation published by Cardelli et al. (1989, ApJ, 345, 245), we show that A(V) and R can be derived with good precision from the observation of a set of common upper level Paschen-Balmer hydrogen line ratios. The use of common upper level line ratios has the advantage of being nearly independent of the excitation condition of the nebula (n(e), T-e). The line ratio method can be applied in regions where no stars are available for photometry.

A significant radiative coupling can occur between the outer surfaces of a telescope, and enclosure, and the sky and the ground. This radiative coupling is illustrated in Fig. 8.1 for the IRAM 30-m telescope. In the left picture, the telescope is at horizon position so that the upper part of the reflector surface sees the warmer ground while the lower part sees the cooler sky. There exists an up.down temperature difference of the reflector panels (panel surfaces) of 4.5° C. In the right picture, the telescope points towards 45° elevation and the reflector surface sees more or less uniformly the cool sky so that the panels have a lower and more uniform temperature compared to the situation shown for horizon position. The measured temperatures are summarized in Fig. 8.2. In this interpretation, it is assumed that the contribution of reflected radiation from the sky and the ground is small (TiO2 painted panels with little specular reflection). A consequence of the asymmetric radiative cooling is shown in Fig. 8.3 for the IRAM 15-m telescope, which pointed during night towards horizon. The lower part of the reflector, facing primarily the cool sky, is iced up at the panel gaps, and the upper part, facing primarily the warmer ground (even when covered with snow), is free of ice.

Solar radiation is the most disturbing external heat source. With solar radiation incident on the ground between approximately 300 and 1000Wm-2, a white painted telescope or enclosure surface will absorb approximately 50–300Wm-2. This power will heat up the surface, but part of the absorbed power is radiated back into the environment at infrared wavelengths. The actual amount of incident and absorbed radiation depends on the shape of the telescope and enclosure surface, its orientation towards the Sun, and its surface finish. This chapter explains the calculation of the solar illumination for several telescope components and enclosures.

The large cm-wavelength telescopes at Jodrell Bank, Effelsberg, Parkes, Green Bank and others are open-air telescopes. The smaller telescopes, and in particular those for short wavelengths, are either closed or placed in an enclosure. A closed telescope is one in which the BUS is covered by the front panels and the rear cladding to prevent direct interaction with the thermal environment, and in some cases to allow ventilation. Several telescopes are placed in an astrodome with a slit that can be opened for observation, or which is more or less permanently covered by a radio transparent membrane. The sides and the rear of an astrodome are usually metal plate walls. The astrodome follows the motion of the telescope. This limits the size to manageable structures, at reasonable costs. The over-hemispherical radome, on the other hand, has a radiowave transparent skin, is fully closed, and is stationary. The telescope inside the radome can move and observe in all directions through the radome, with blockage by the supporting space frame. The astrodome and radome are part of the telescope’s concept of thermal and wind protection. The earlier mmwavelength telescopes were built from aluminium to reduce mass and placed in a ventilated radome to cope with the larger thermal expansion of aluminium. The HHT, CSO, and JCMT telescopes with astrodome protection are listed in Table 3.1 and are shown in Figs. 3.1–3.3, and the radome of the Onsala 20–m telescope is shown in Fig. 3.4.