thermal behavior of millimeter wavelength radio telescopes

IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 40, NO. 11, NOVEMBER 1992 1375

Thermal Behavior of Millimeter Wavelength Radio Telescopes

A. Greve, M. Dan, and J. Penalver

Abstract-The success of millimeter astronomy lies-for a substantial part-in the construction of large high-precision radio telescopes. An important aspect in the construction of such telescopes is a passive/active thermal control of significant structural components, as for instance the backstructure and the feed legs. We explain the thermal protection applied to the IRAM 15-m mm and 30-m mm wavelength telescopes, and illustrate their thermal behavior. The design of the passive/ active thermal protection of these telescopes was supported by dynamic time-dependent model calculations, which are explained and compared with in situ recorded temperatures of the telescope components.

I. INTRODUCTION E success of millimeter astronomy at wavelengths

F A ) of 0.6 I A I 3 mm is based-for a substantial part-on technological progress in the construction of large high-precision radio telescopes (cf. Ill), as, for instance:

the production of exact panel contours of = 5-30 p m precision,

the application of homologous backstructures ( [ 2 ] , [31) based on optimization finite element calculations,

high-precision alignment of panels by (phase retrieval) radio holography ([4], [5]),

accurate pointing employing precise encoders and sophisticated servo-loop control systems (cascade and state controllers, [61),

efficient passive and/or active thermal control of telescope structures ([7], [SI), etc.

After introduction of the subject of heat transfer between telescope components and/or the environment, we dis- cuss the thermal design and thermal behavior of the IRAM/SEST (Swedish ESO Submillimeter Telescope; ESO, Chile) full aperture 15-m telescopes ([9]-[11]) and the IRAM full aperture 30-m telescope ([121) as illustra- tive cases of efficient passive and/or active thermal control of large telescopes for observations at millimeter wavelengths. By comparing recorded temperature distributions of these telescopes and thermal model calcula-

Manuscript received November 8, 1991; revised February 3, 1992. A. Greve is with IRAM, Nucleo Central, Avda. Divina Pastora 7,

M. Dan is with IRAM, Domaine Universitaire, 200 rue de la Piscine,

J. Penalver is with IRAM, Nucleo Central, Avda. Divina Pastora 7,

IEEE Log Number 9204895.

18012 Granada, Spain.

38406 St. Martin d'Heres, France.

18012 Granada, Spain.

tions, we demonstrate that the usually applied investiga- tions of static thermal load cases (based on finite element calculations) may today be extended by dynamic, time- dependent thermal model calculations of the basic telescope components. Such calculations provide predictions of the thermal behavior under variable ambient load cases (for instance variable air temperature, variable solar illumination, etc.) and thus allow the theoretical investigation of appropriate passive and/or active thermal control systems of telescope structures. The thermal protections applied on the IRAM telescopes may give guidelines for future constructions.

Full aperture radio telescopes for observations at millimeter-wavelengths require, primarily, reflector surfaces of E = A/20 root-mean-square accuracy ([13]) and pointing accuracies of approximately a tenth of the beam width, i.e., 5-1 arcs. Von Hoerner [2] estimated the limita- tions in telescope size (diameter 0) and telescope performance ( D / E ) under the influence of gravity deformations, wind loads, and thermal loads; an updated graphical illustration for millimeter-wavelength telescopes is shown in Fig. 1 (cf. [l]). This figure indicates that the high mechanical precision required for telescopes operating at millimeter-wavelengths demands elaborate passive and/or active thermal control to reduce the influence of ambient thermal perturbations. For typically A = 1 mm, E = h/20 = 0.05 mm. For any member of the telescope structure of typical dimension L (for instance a panel, a member of the backstructure, etc.) the mechanical deformation AL(rms) of the structure induced by temperature inho- mogeneties AT(rms) is

AL(rms) = a L AT(rms) (1) For L = 3 m (as a typical value), AL(rms)/K = 0.05 (mm)/K when built in conventional way of aluminum/ steel with thermal expansion coefficient a = 0.015 - 0.02 (mm/m/K), or A L(rms)/K = 0.006 (mm)/K when built of carbon fiber with a = 0.002. Hence, for millimeter telescopes it is evident that, when built from conventional materials either passive (paint, insulation, (raldome, etc.) and/or active (ventilation, climatization, etc.) thermal control is required to guarantee thermal homogenity AT(rms) I 0.5 - 1" of the backstructure and feedlegs (IRAM 30-m telescope), or alternatively the backstructure and feedlegs must be built from low thermal expansion material like carbon fiber (IRAM 15-m telescope).

The thermal protections applied for several modern millimeter-wavelength telescopes are shown in Table I.

0018-926X/92$03.00 0 1992 IEEE

1376 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 40, NO. 11, NOVEMBER 1992

I I - I : I - 1 - I _ I I -

_ - -I- : I - I _ I _ I I - . 1 : I :

I \.

- _ _ - _

h .

TABLE I

Reflector Shortest Telescope diameter (m) wavelength (mm) Thermal Protection

CSO (Hawaii) 10 Kitt Peak (USA) 12 JCMT (Hawaii) 15

IRAM/SEST 15 (France, Chile)

IRAM (Spain) 30

Noheyama (Japan) 45

- 0.65 Dome ( p ) - 1 Dome ( p ) - 0.65

1 - 0.8

Dome ( p ) , ventilation (a ) Carbon fiber panels ( p ) Panels and backstructure of carbon fiber ( p ) Insulation ( p ) , radiat. shield ( p ) Insulation ( p ) , paint ( p ) Climatization (a ) Insulation (p), ventilation (a )

1 - 0.8

3

Note: p = passive; a = active control.

I , ' " 1 ' ' ' 1 ' ' " " " 1 ' ' ' 1 ' ' ' I

b. /

'/'. . \ \

~ . . . 1 . 1 1

4 mrn \ \ \

GRAVITY \ \ \ \ I ' STRESS

Fig. 1. Reflector precision D / E ( E = surface rms value) against diameter D with natural limits for stress, gravity and thermal effects, for millimeter-wavelength telescopes. The lines labeled 1 mm, 4 mm result from the relation A,,, = 16 u (taken from [l]; see also [2]). 1,2 are the IRAM 15-m and 30-rn telescopes (1991).

There are relatively few publications on the thermal design and actual thermal behavior of modern millimeter-wavelength radio telescopes. The recent publication by Bregman and Casse [14] explains model calculations of the JCMT 10-m telescope, by Delannoy [9] the design and model calculations of the IRAM 15-m telescopes, by Baars et al. [8] the design, thermal behavior and model calculations of the IRAM 30-m telescope, and by Akabane 1151 the design and behavior of the Nobeyama 45-m telescope. The thermal properties of several Russian telescopes are discussed by Baimarov et al. [16], and Polyak and Bervalds [17]. A more detailed version of this paper is published by Greve [7].

Thermal model calculations of radio telescopes are made (a) to investigate deformations of the shape of important components (reflector, feedlegs, etc.) under static temperature loads; and (b) to investigate time- dependent temperature variations of bulk components of the telescope structure (backstructure, fork or yoke structure, feedlegs, etc.). Here we concentrate on the latter aspect of model calculations and the prediction of the thermal behavior.

11. DEFINITION OF THE SUBJECT A. Static Analysis

So far, the majority of thermal model calculations investigated mainly the variation of the reflector surface (backstructure) shape, denoted symbolically by S, under static thermal loads. For instance, in case the shape S(To) is known to be perfect for a homogeneous temperature To, the question investigated is whether the shape S(T, + AT) remains acceptable in case the temperature is homo- geneously varied by AT, or is varied by a gradient VT across the backstructure, i.e., S(T, + V T ) , or is varied by a random distribution T(rms) of temperature fluctuations, i.e., S(To + T(rms)), etc. The resulting static shape variations SS = ( d S / d T ) G T , usually expressed as associated changes of the reflector surface topography and of the surface rms value 8~ = ( & / d S > 8 S , are studied from finite element calculations taking into account the thermal dilatations of the individual elements of the backstructure. Typical examples are shown in the publications collected by Mar and Liebowitz [NI.

B. Dynamic Analysis In the variable ambient thermal environment, the tem-

perature distributions T, of the telescope components [il are variable in time, i.e., T,(t) with, for instance, associated corresponding time-dependent changes of the reflector surface shape SS = Z ( d S / d T , ) ( d T , / d t ) 8 t . In case the associated surface deformations are too large, a thermal control is required to reduce S E either by using passive/active thermal protection such that the externally induced temperature variations of the components are negligible, i.e., d T , / d t = 0 + d S = 0, or/and by using low thermal expansion materials such that the shape is nearly independent of temperature variations, i.e., d S / d T , = 0 + d S = 0. Both types of thermal control are applied in telescope constructions, and in particular for the telescopes discussed here.

111. DYNAMIC THERMAL MODEL CALCULATIONS The aim of dynamic thermal model calculations thus is

a knowledge of the time-dependent temperature distributions T,(t) of selected telescope components [i], called thermal nodes. Each node is characterized by its mass M,, specific heat capacity c,, temperature T,(t), and contact surface area FE,, and thermal distance d l , J with the other

GREVE et al.: THERMAL BEHAVIOR OF MILLIMETER WAVELENTH RADIO TELESCOPES 1311

thermal nodes j and/or the ambient thermal environment. The skill of constructing a representative thermal model of a complex telescope lies in the selection of significant material components and the relations of their mutual thermal interactions and interactions with the environment.

The change in temperature AT,(t) of component [il is related to a change of its heat content AQi( t ) by

AT,( t ) = A Q i ( t ) / c i M i ( 2 ) The change of heat content depends on the thermal interactions, as there are [see Figs. 2, 9(b)]:

conductive heat transfer AQCD between bodies in contact;

convective heat transfer AQcv via a moving body (usually air);

radiative heat transfer AQR via electromagnetic waves;

(cf. Chapman [191) which are briefly discussed in order to explain the parametric dependence.

Conduction /Fig. 2(a)]: Assume two bodies 1,2 in contact with contact surface area F,,2. With dl ,2 a characteristic distance between the bulk material of the bodies (the thermal nodes), k the heat conductivity of the material, AT = Tl - T2 the temperature difference of the components, the conductive heat transfer AQc.(l, 2) between the bodies is

A Q c D ( ~ , ~ ) = kF1,2 AT/d1,2 (3) Convection /Fig. 2(b)]: The heat transfer between two

bodies 1,2 occurs via energy transport in a moving body, generally air. The efficiency of the heat transfer depends on the flow speed U of the moving body (the windspeed of the air), the type of flow (laminar or turbulent), the geometry of the surface, and its contact with the moving body. A realistic approximation of convective heat transfer is

A Q c v ( l , 2 ) = A T / [ h(alv" + a21ATI ')] (4)

with a,, a2 constants ( a , = u2 = l) , CY = 0.66, /3 = 0.33, and h a factor dependent on the geometry of the interaction (cf. [19]).

Radiation /Fig. 2(c)]: The diffuse radiative energy transfer between two bodies 1 + 2 with surfaces F,, F2 depends on the temperatures T I , T2 of the surfaces, their emissivity e, and absorptivity a2, and their distance s and orientations p l , P2

AQ(1,2) = ela2aFl(Tf - 7';)(~(1,2) ( 5 ) with

the shape factor of the geometrical surface configuration that, in most cases, must be derived from numerical calculations. v = 5.67 lo-' W/m2K4 is the Stefan- Boltzmann constant. Important parameters for the evalu- ation of radiative energy exchange are the emission ( e )

Fig. 2. Illustration of the modes of heat/energy transfer: CD =

conductive coupling, CV = convective coupling, RC = radiative coupling.

and absorption ( a ) coefficients of the surface finish (paint), where e and a may be functions of the wavelength of the radiation and, under certain conditions, may also have specular reflection characteristics (like for anodized aluminum).

A telescope (not enclosed in a (ra)dome) interacts with the ambient thermal environment [as illustrated in Fig. 9(b)], i.e., with

the ambient air [an infinit reservoir of temperature TA(t)] by convection and, to a lesser degree, by conduction,

the sky [an infinit reservoir of calorimetric temperature TJt)] by radiation,

the ground [an infinit reservoir of temperature T,(t)l by radiation,

the solar radiation S ( t ) as direct external heat source.

In model calculations TA(t) is either taken from representative in situ recordings, or is, appropriate for many realistic conditions, approximated by 24-hr periodic variations

with w = 2 ~ / 2 4 hr, TAo = (T'(t)) the daily average temperature, 6 TA the amplitude of the temperature variation, and t , (of the order of 1-2 hr) the time delay of the maximum of the air temperature with respect to 12-hr noon.

The telescope surfaces are radiatively coupled to the sky, primarily by radiation at infrared wavelengths 8 p m s A 5 13 p m ([20]-[22]). The heat exchange between a

TA( t ) = TAo - 6TA COS [ W ( t - t , )] (7)

1378 IEEE TRANSACTIONS O N ANTENNAS AND PROPAGATION, VOL. 40, NO. 11, NOVEMBER 1992

surface and the sky can be expressed by (5) where in this case T2 = T,(t) with T, the calorimetric sky temperature.

For clear sky conditions the temperature T, is often represented by Swinbank's [23] relation T,(t) = 0.0553 T,(t)'.j, however, in the model calculations we always approximated the sky temperature by

T d t ) = U t ) - ST, (8) with S T , = 1O0-2o"C. The radiative constants of the sky are usually approximated by those close to a black body with a = e = 0.90; however, these constants depend on the height, the meteorological state and the water vapor content of the atmosphere (cf. [24]). Equation (8) seems to give a good representation of realistic conditions.

The telescope is radiatively coupled to the ground of equivalent gray body temperature T,(t); for the ground we used a = e = 0.6-0.7. However, the radiative constants of the ground depend very much on the nature of the soil, i.e., earth or snow layer, etc. In model calculations we approximated the ground temperature either by

TG( t ) = T A ( t ) + ST, (9) or a 24-hr periodic variation similar to (7).

The major source providing energy to a telescope is solar radiation (in case there are no significant internal heat sources). The amount of solar irradiation is ex- tremely variable in time (day-night, season, clouds, different illumination aspect according to telescope position, etc.) and thus difficult to model. Since there are only few experimental recordings of solar energy absorbed on telescope structures, in the model calculations we based the solar irradiation on the azimuth position and the elevation P ( t ) of the sun

sin p ( t ) = cos H ( t ) c o s cpcos S ( E ) + sin cpsin S ( E ) ( lo?

[ H ( t ) = hour angle, cp = geographic latitude, 6 ( E ) =

declination of the sun at epoch E ] and the solar radiation S ( t ) incident on 1 m2 surface

s(t) = S,(1 + d)e[kB/Fi"B(f)I (11) (So = 1300 W/m2 for normal incidence, d = diffusion coefficient - 0.1, B = transparency coefficient - 0.1).

The mathematical description of a thermal model results in a large number ( - a few three times the number of thermal nodes because there are three modes of heat transfer) of coupled differential equations connecting the temperatures of the individual nodes. These equations, with time-dependent transient effects included (like short-term shadowing by clouds), are solved for large models by iterative numerical methods. Since the thermal model of a complex mechanical structure is analog to a multicomponent electrical circuit (cf. [ 19]), the ESACAP program by Stangerup ([251, [26]), or similar programs for electrical circuit calculations, may be used for the evalua- tion of thermal models. We have used the ESACAP program.

The temperatures in the figures are in degree Celsius.

Iv. THERMAL DESIGN, BEHAVIOR AND MODEL CALCULATIONS OF THE 1- 15-M TELESCOPES

(PLATEAU DE BURE, FRANCE; SEST, CHILE) A. Thermal Design

A picture of the SEST 15-m telescope (which is a copy of the IRAM telescopes) is shown in Fig. 3(a); a description of the IRAM/SEST telescopes is given in [9]-[11]. The telescope may be divided into the following thermal substructures [Fig. 3(b)l:

pedestal (PE)-fork structure (FS)-central hub (CH) -secondary focus cabin (SFC)-reflector backstructure (RBS)-panels (PA)-feed legs (FL)-subreflector (SRI, which, in model calculations, are again subdivided into smaller components of thermal nodes.

The steel PE (transporter) of the movable Plateau de Bure telescopes is painted white (ordinary paint) to reduce the influence of solar radiation. The concrete PE of the stationary SEST telescope is covered with insulation, which itself is covered by shiny aluminum plates, leaving an air gap between the insulation and the plates. The P E of these telescopes contain a substantial amount of elec- tronics; the heat generated by these components may diffuse by natural ventilation into the air volume of the fork arms [see Fig. 3(b)].

The FS consists of two arms [see Fig. 3(a) and (b>l and the middle box section containing the A Z bearing. The steel plates (16 mm thickness) of the FS arms and the box section are covered by passive thermal protection consisting of insulation (4-cm polyurethane attached to the steel plates), an air gap (2 cm), and a radiation shield. The radiation shield consists of two aluminum plates ( - l-mm- thickness each) of 5-mm separation and connected by corrugated aluminum. The outer surface of the insulation is covered with aluminum foil; the surface finish of the radiation shield is anodized aluminum (outer surfaces). The air in the gap may be ventilated (applied at SEST). A cross section of the thermal protection (facade) is shown in the insert of Fig. 3(b). At one side of the FS, a cabin is attached that contains the He-compressor. This cabin is not insulated against the outside (environment) but is insulated towards the steel plates of the FS. The walls of the cabin are painted white (ordinary paint). The compressor produces heat (of the order of 5 kW) part of which may diffuse into the FS; however, the cabin can be ventilated by louvres.

The protection of the FS is applied to avoid asymmetric thermal dilatations of the fork arms-mainly due to asymmetric solar irradiation-and thus to prevent time-dependent (unpredictable) pointing errors. Estimates indicate that the pointing errors will not exceed - 2 arcs in case the temperature difference between the fork arms does not exceed - 2" and in case the temperature homogenity (rms) of the individual fork arms is better than - 0.5".

The RBS is a network of carbon fiber bars [denoted 2 in Fig. 3(b)] and steel bars [denoted 1 in Fig. 3(b)] supporting the reflector surface (PA). The rear side of the RBS is covered by radiation shield plates (identical to those of

G R E W er al.: THERMAL BEHAVIOR OF MILLIMETER WAVELENTH RADIO TELESCOPES 1379

(b) Fig. 3. (a) Picture of the SEST 15-m telescope (ESO, Chile), which is a copy of the IRAM telescopes [lo] (Photo ESO); (b) Thermal substructures of the IRAM 15-m telescope with indication of the location of the temperature sensors in the fork structure (0). The heavy solid line shows the thermal protection of the FS and SFC. RBS = reflector backstructure with 1-steel bars, 2-carbon fiber bars, 2 + 3-carbon fiber bars and panels; RS = radiation shield; CH = central hub; SFC = secondary focus cabin; CC = compressor cabin; FS = fork structure; PE = pedestal (transporter). The arrows show a possible heat flow in the FS. The insert shows a cross section through the thermal protection of the FS and SFC with S = steel plate, I = insulation, AF = aluminum foil, A = air gap, RS = radiation shield.

1380 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 40. NO. 11, NOVEMBER 1992

the FS), the front side of the RBS is closed by the panels. The inside air may be exchanged by natural ventilation through small louvres (mainly to prevent condensation). The RBS is attached to the CH, made of steel. As shown in Fig. 3(b), the carbon fiber bars form the top and bottom layer of the RBS while the vertical connections are made of steel bars. This network was chosen so that the RBS does follow the thermal dilatations of the heavy CH. The static thermal model calculations [27] of the initial design of a complete carbon fiber RBS indicated a severe buck- ling of the reflector surface introduced by differential thermal dilatations between the steel hub (CH) and the then-proposed vertical carbon fiber members (RBS).

The PA consist of aluminum honeycomb sandwiches (6 cm thickness) with a carbon fiber skin (top and bottom, 1 mm thickness) and a 40-nm-thin radiowave reflective aluminum layer (shiny) protected by a Hostaflon plastic film ( - 0.04 mm thickness). The PA can be heated ( N 100 W/m2) to prevent the formation of ice.

The RBS is attached to the CH, which itself is attached to the SFC. The SFC is a plate steel box mostly covered with passive thermal protection similar to that of the FS, or otherwise it is painted white (rear-part entrance doors).

The FL consist of carbon fiber painted white with TiO, paint. Recently, the feed legs of the Plateau de Bure telescopes have been wrapped in aluminum foil 3.5 m down from the SR in order to prevent and minimize extreme heating by solar irradiation. The SR consists of carbon fiber and a shiny aluminum front surface protected by a Hostaflon film (like the PA).

The IRAM/SEST 15-m telescopes use passive thermal control, either consisting of an elaborate thermal facade for the FS or low thermal expansion carbon fiber material for the RBS, PA, FL, and SR. The operation of the telescopes is subject to a large sun avoidance zone ( - 30-60").

B. Recorded Thermal Behavior The understanding of the thermal behavior as illus-

trated below requires some knowledge of the interferometer operation on the exposed site Plateau de Bure (French Alps, 2500-m altitude). During adverse weather conditions, for maintenance, etc., the telescopes are occasionally brought into a hangar that, when closed, usually has a stable inside air temperature of 15"-20°C. In case the telescopes remain sufficiently long in the hangar, they attain an equilibrium temperature close to the inside air temperature. When later positioned on an interferometer station, the telescopes may initially not be in thermal equilibrium with the outside environment and the thermal equilibrium is reached only after a characteristic adaptation time t , .

C. Thermal Behavior of the Fork Fig. 4 shows the recorded average temperature of the

steel plates of one fork arm (the other arm behaves similar). The recording illustrates the transition from the higher equilibrium temperature ( - 20°C) attained inside the hangar to the lower equilibrium temperature ( - 10°C)

in the cooler outside environment. This particular temperature adaptation of the steel plates can be expressed by the exponential function

T p ( t ) = TA01 - A T J 1 - exp(-t/to)l (14)

with Tp(0) = 20"C, ATp = 9.5"C, and to = 18.7 hr. The final equilibrium in the outside environment is reached approximately three days after rollout of the telescope. However, no degradation of the pointing occurred since the thermal adaptation was identical for both fork arms. When in equilibrium with the outside environment, the average temperature of the steel plates was significantly higher than the temperature of the ambient air; this situation is generally observed and is probably due to heat generated in the PE and diffusion of the heat into the fork arms. The daily temperature variations of the steel plates ST, = 1" around the equilibrium temperature of N 10°C are due to solar irradiation rather than influences of the ambient air.

The temperature homogenity of the steel plates of one fork arm (and similar for the other fork arm) is also shown in Fig. 4 by the width of the recording, the rms variation is I 0.5-1.0". The temperature equality of the left and right side fork arms is shown in Fig. 5 and is of the order of 1-2". Fig. 4 and 5 illustrate (though only for the particular climatic situation shown, but similar also for many other periods) that the specifications of thermal homogenity and equality are fulfilled. Finally, the efficiency of the thermal protection is demonstrated in Fig. 6(a), which shows for one particular position the temperature profile recorded through the facade of the FS.

v. MODEL CALCULATIONS OF THE FORK In the thermal model calculations, the symmetric me-

chanical structure of the fork was divided into one fork arm, the central box section containing the A Z bearing and the compressor cabin [see Fig. 3(b)l. This part of the steel structure, including the corresponding thermal protection (insulation-air gap-radiation shield), was decomposed into 112 nodes and associated thermal connections, including the corresponding radiative nodes (cf. [19]). The model of the environment consisted of three nodes, i.e., the ambient air, the ground and the sky, and the radiative nodes of the sky and the ground. The recorded ambient air temperature T,(t) was used in the model calculations. The temperature of the ground T,(t) was derived from (9) with 6TG = 7", the calorimetric sky temperature Ts( t ) was derived from (8) with 6Ts = 20". The solar irradiation was calculated from (10) to (11). However, since the telescope was used for astronomical observations during the period shown in Fig. 6(a), the illumination aspect did vary frequently so that an appropriate average solar illumination was chosen in the calculation. We considered in a similar way the shadowing of the FS by the reflector.

The result of the calculation is shown in Fig. 6(b). There exists good agreement between the measurements [Fig. 6(a)] and the calculation [Fig. 6(b)], in particular in view of the uncertainties of the material constants (for

G R E W et ul.: THERMAL BEHAVIOR OF MILLIMETER WAVELENTH RADIO TELESCOPES 1381

20 -

10 -

0 -

'k s

lii i t . I , , , I , , , I , , , , , , , j

50 52 54 56 TIME (DAYS)

Fig. 4. IRAM 15-m telescope; temperature of the steel plates ( S ) of one fork arm; the width of the line (error bars) is the rms of the temperature deviations. Exponential adaptation to the outside environment: ---- (rollout of the telescope on -day 49.7); A = ambient air temperature.

50 52 54 56 TIME (DAYS)

Fig. 5. IRAM 15-m telescope; temperature equality between the left (-) and right (- - -1 fork arm.

instance the radiative constants of anodized aluminum) and the environmental conditions (solar irradiation). Nonetheless, the thermal behavior of the complex and heavy (- 20 ton) mechanical structure of the fork can be reproduced, and hence predicted, with sufficient accuracy from model calculations. The thermal protection of the FS was designed from similar calculations supported by the results of a scaled-down experiment (171).

VI. REFLECTOR BACKSTRUCTURE The temperature of the RBS, i.e., of the bar members,

is measured at eight equally spaced radial positions approximately halfway between the vertex and rhe rim. The four sensors of the upper section and lower section [in tilt direction, see Fig. 8(b)], respectively, give the temperature TU(upper) and TUlower) of the RBS. The average temperature TR of the RBS and the rms temperature fluctuations are TR = CTJ8 and rms(TRI2 = C(T, - TR)*/8.

Fig. 7 shows the average temperature TR of the RBS and the temperature of the ambient air TA. The displayed period corresponds to that of the FS shown in Fig. 4, i.e., the telescope was placed on an interferometer station at t = 49.7 d. Contrary to the behavior of the compact FS (see Fig. 41, the RBS does not exhibit the exponential adaptation to the outside environment but rather seems to attain quickly a thermal equilibrium. This allows the use of the telescopes soon after rollout from the hangar. The figure shows that during day time TR = TA indicating that only a small amount of solar energy entered the RBS, probably because of the large sun avoidance zone (= 60" at the time of the measurements so that the sun illumi- nated primarily the rear side) and the efficient thermal buffering by the rear-side radiation shield. During night time, the temperature TR falls below the ambient air temperature, probably because of significant radiative energy loss of the reflector surface towards the cool sky.


I . l l l . . . I . I . I . . .

50 52 54 56 TIME (DAYS)

(a)

t ' I " ' I " ' I " ' I " ' 4

50 52 54 56 TIME (DAYS)

(b)

Fig. 6. (a) IRAM 15-m telescope; temperature profile measured through the thermal protection [see Fig. 3(b)]: S = steel plate; I = insulation (front side towards air gap); RS = radiation shield (inner side towards air gap). Dashed line represents ambient air temperature; (b) IRAM 15-m telescope; result of model calculations of the condition shown in Fig. 6(a). Steel: -, insulation: air in gap: ---, radiation shield: ambient air: - - - - -.

10 - d

w 5 - 2 4 r,

0 -

-5 -

-10 50 52 54 56

TIME (DAYS)

Fig. 7. IRAM 15-m telescope; average temperature (R) of the reflector backstructure (measurements: 0) and temperature of the ambient air ( A ) , dashed line.

3 -

1383

4 . , , . , , , , , , . , , 1 . ~ .

The temperature homogenity of the RBS is illustrated by the rms temperature fluctuation, rms(TR), shown in Fig. Na). There appears to exist a characteristic daily behavior: at nighttime rms(TR) = 0.8-1.0"; during daytime the rms values increase by a factor of = 2-3 so that rms(TR) = 2-3". The larger inhomogenity rms(TR) during daytime was due to a significant vertical temperature gradient in the RBS of the tilted telescope. Fig. 8(b) shows the temperature gradients TRU = TU - TR and TRL =

TL - TR in the upper and lower sections of the RBS with respect to the average temperature TR. The full vertical gradient AT across the RBS is AT = TRU - TRL, which is equivalent to the separation of the two curves shown in Fig. 8(b). The gradient AT shows a characteristic daily behavior, during night time AT = 2", during day time AT = 10". The upper part of the RBS was always warmer, and this behavior was also found in measurements at other periods.

5 -

a i? 2 3

a s aw 0 - 3

a

d E I?

-5

VII. MODEL CALCULATIONS OF THE BACKSTRUCTURE

I ' ' I I ' ,

0

0

UPPER

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09" $9# e

a . " :d 0 0 1 '?: 7-\! LOWER 8 ,

-

I I , , l . . , l . I , l . I L

50 52 54 56

Static thermal load cases of the RBS were investigated by Raffin (cf. [9]) from finite element calculations; a dynamic model calculation using the ESACAP program was made by Delannoy [9].

Following Fig. 8(a), the temperature homogenity of the RBS is rms(TR) I 3" and typically rms(TR) s 1" during nighttime. For carbon fiber members of the RBS of typical length L, = 3 m, the differential dilatations calculated from (1) are ~ 0 . 0 1 mm; the dilatations of the shorter steel members are of similar order. These deformations are acceptable in the surface error budget of the IRAM 15-m telescopes [91.

There seems to exist (always), with the reflector in tilted position, a vertical temperature gradient AT in the RBS [see Fig. 8(b)] that, in particular, affects the steel bar members; during daytime AT = lo", during nighttime AT


= 2". The finite element calculations (Raffin, cf. [9]) indicate that a temperature gradient of lo" across the RBS will introduce a surface error of only a few micrometers.

Thus, although the recorded temperatures indicate the existence of a temperature inhomogenity (gradient and rms fluctuations) of the RBS, the application of carbon fiber material in combination with the vertical steel bar connections allow to support these temperature distributions without degradation of the radio performance.

VIII. THERMAL DESIGN, BEHAVIOR, AND MODEL CALCULATION OF THE IRAM 30-M TELESCOPE

(PICO VELETA, SPAIN) A. Thermal Design

A picture of the IRAM 30-m telescope is shown in Fig. 9(a) and (b); a description of the telescope and of the thermal design and thermal behavior is given in [12] and [81.

The telescope may be divided into the following thermal substructures: pedestal (PE); yoke (YO); secondary focus cabin (SFC); reflector backstructure (RBS); panels (PA); feedlegs (FL) and primary focus cabin, which in model calculations are again subdivided into smaller components of thermal nodes.

The concrete PE is covered with insulation (compressed stone wool), which is white (ordinary paint) in order to reduce the influence of solar radiation. The thermal coupling (conduction) of the PE and the telescope steel structure (at the A Z bearing) is small and may be ne- glected. Some air is circulating between the PE and the SFC.

The YO consists of two box-type arms (see Fig. 9) and the roof of the YO. The lower sections of the YO arms contain the counterweight of the reflector. The YO is thermally protected by insulation (4-cm polyurethane) cased in thin aluminum plates that are painted white with TiO, at the outer surfaces. The YO is separated from the RBS by a steel membrane as indicated in Fig. 9(b).

The RBS is the network of steel bars supporting the reflector surface (PA). The network is completely covered by insulation (a "very close dome") forming a closed air volume. The rear side insulation (4-cm polyurethane) is cased in thin aluminum plates that are painted white with TiO, at the outer surfaces. The front side are the PA separated from the RBS by similar insulation.

The PA are aluminum honeycomb sandwiches of several centimeter thickness covered by an aluminum skin painted with TiO, . Because of their small thermal time constant of - 0.5 hr, the PA follow quickly the variations of the ambient thermal environment [7]. The exposed PA can support the environmental influences without degradation of the surface accuracy.

The SFC is painted white (TiO,) and has only weak direct thermal coupling with the other parts of the telescope and the environment, except some radiative coupling with opposing surfaces of the YO. The conductive coupling to the YO via the EL bearings is small. The vertex tunnel is insulated against the RBS and closed at

(b)

Fig. 9. (a) Picture of the IRAM 30-m telescope (Pic0 Veleta, Spain). Note the cladding with insulation panels; (b) Structural components of the IRAM 30-m telescope with 1 = yoke; 2 = reflector backstructure; 3 = secondary focus cabin; 4 = feedlegs. The position of the temperature sensors (.) is indicated, with R being the position of the sensor for the reference temperature (in the YO). M is the membrane separating the YO and the RBS. 1(1) is the insulation of the RBS, 1(2) is the insulation of the PE. This figure also gives a schematic representation of the environmental influences with the thermal nodes N(SK) = sky; N ( G ) = ground; N ( A ) = ambient air; CD = conductive coupling; CV = convective coupling; RC = radiative coupling; 0 = external heat source (sun).

G R E W et al.: THERMAL BEHAVIOR OF MILLIMETER WAVELENTH RADIO TELESCOPES 1385

IT CONTROL ON 4

10 - B A LL

(L

U-

0 > d 0 - B

- 1 0 1 ' ' " 5 ' ' ' ' ' ' ' ' I ' ' ' 0 10 20

TIME (OAYS)

Fig. 10. IRAM 30-m telescope. Temperatures of the telescope without thermal control (bracketed by the vertical lines). For the period of NO thermal control the temperatures of the yoke, reflector backstructure, and the feedlegs show oscillations (maximum amplitude for the FL, minimum amplitude for the YO). For the period of thermal control the temperatures of the YO, RES, and FL are equal. The temperature of the ambient air is shown for comparison (not on scale).

the top with a radio transparent cloth. This cover is occasionally removed for high-frequency observations so that then the receiver room of the SFC is in direct contact with the environment. The data presented here refer to a closed vertex tunnel. The heat from the electronic equip- ment in the SFC produces a fairly constant temperature of 15-20°C inside the SFC, when closed. Part of this heat may diffuse into the nearby telescope structure.

The feedlegs are steel tubes protected by insulation and painted with TiO,. The subreflector (profile sheet) consists of carbon fiber.

The telescope is built of steel. The temperatures of the YO: T(YO), RBS: T(RBS), and FL T(FL) are regulated by a combined passiue-actiue thermal control system. The passive control consists of complete insulation with polyurethane foam of 4 cm thickness and white TiO, paint on the outer surfaces. The active control consists of forced ventilation of the RBS, i.e., a tangential air flow in the RBS of several ms-' speed with the air either being heated or cooled (climatized) in order to establish a specific and homogeneous temperature distribution of the RBS. A liquid (freon) is pumped around the FL, either heating or cooling them. The temperature of a representative component of the YO: TR [see Fig. 9(b)] is taken as reference to which the temperature of the RBS and FL are regulated within approximately 0.5" rms, as required to guarantee negligible thermal degradation of the radio performance [8]. The specification of the required thermal stability and homogenity may thus be summarized as

T(RBS) = T(FL) = TR( = T ( Y 0 ) )

and rms(T(RBS)) = rms(T(FL)) 5 0.5"

During adverse weather conditions the PA, the rear side of the RBS and the FL can be deiced [see Fig. 12(a)], using approximately 100-200 W/m2.

IX. THERMAL BEHAVIOR WITHOUT TEMPERATURE CONTROL

A two week break-down of the active temperature control system (NO active regulation of the RBS and FL) gave the opportunity to record the noncontrolled thermal behavior of the telescope. The corresponding temperature measurements provide a view of the direct interaction of the telescope with the ambient thermal environment. Fig. 10 shows the temperatures of the YO, RBS, and FL recorded during this period. The recordings exhibit 24-hr periodic oscillations of the temperatures induced by periodic variations of the solar irradiation (day-night variation) and, to a lesser degree, by periodic variations of the ambient air temperature. The temperature homogenity of the RBS, characterized by the rms fluctuation of the temperature, during the time of NO thermal control was occasionally rms(TR) = 2" (during daytime) and thus outside the specification of rms(TR) I 0.5". For further analysis of this behavior see [7].

X. THERMAL BEHAVIOR WITH TEMPERATURE CONTROL The excellent long-term thermal stability and homogen-

ity of the temperature-controlled IRAM 30-m telescope is best illustrated in several graphs showing the behavior for the period of the year 1988, which, however, was not specific in the operation of the telescope. The location of the temperature sensors is shown in Fig. 9(b). In the following figures, (YOKE) is the average temperature of the four sensors installed in the YO, (REFL.) is the average temperature of the 14 sensors installed in the RBS in tilt direction (in contact with the steel members), and (FEEDLEG 1 ) is the average temperature of one feedleg (the others are similar). rms(REFLECT0R) is the rms value describing the temperature homogenity of the RBS.

The RBS and the FL are temperature-controlled against the YO. Fig. l l(a) and (b) show the temperature differ-


2

A w Y

2.

A J L L

LL V

I O

-20. 100 DAY OF YEAR 200 (1 988) 300

(a)

2

A w Y

I

A 0

n w V

9

8

-2

0 100 200 300 DAY OF YEAR (1988)

(b)

0 I . . . , I , . . . , . . .

0 100 200 300 DAY OF YEAR (1 988)

(C)

Fig. 11. (a) Average RBS temperature-average YO temperature (thermal control working); (b) Average FL temperature -average YO temperature (thermal control working); (c) temperature homogenity of the RBS (thermal control working).

1387 GREVE er al.: THERMAL BEHAVIOR OF MILLIMETER WAVELENTH RADIO TELESCOPES

ences (REFL.) - (YOKE) and (FEEDLEG 1) - (YOKE), which illustrate the efficient and precise temperature equalization to within = 1". As demanded, the active temperature control system provides also a good temperature homogenity to within = 0.5" as illustrated by the rms value rms(REFL.1 shown in Fig. ll(c). The temperature homogenity of the feedlegs is as good.

XI. MODEL CALCULATIONS In an earlier publication [8] we have presented a model

calculation of the complete (excluding the PE) temperature-controlled telescope under an extreme climatic condition. This result is reproduced in Fig. 12 since it is the most complicated thermal model and environment we have treated. The telescope, of - 300 tons weight (YO + RBS + SFC), was decomposed into 180 thermal nodes. As input parameters we have used the recorded temperature TA of the ambient air; we approximated the calorimetric sky temperature T, by (8) with ST, = 20°, and for the ground we used (9) with ST, = 5". The solar irradiation was introduced as an average, variable heat source by using (10)-(11) and an average solar illumination aspect of the telescope. There appeared a drastic climatic change on January 12, which required the application of the deicing, This additional energy input unbalanced the otherwise perfect thermal control and thermal homogenity. The agreement between the calculation [Fig. 12(b)l and the measurement [Fig. 12(a)] demonstrates the feasibility that a sophisticated model can handle with sufficient accuracy a very complex structure under extreme environmental conditions.

XII. CONCLUSION The success of the IRAM/SEST 15-m and 30-m tele-

scopes for observations at millimeter wavelengths is based on several technological innovations, of which one is the sophisticated passive and/or active thermal control of the major telescope components. We have explained the thermal design and the thermal behavior of these telescopes and the presented temperature recordings give a fair example of thermal load cases to be considered in structural calculations. For two examples, the fork structure of the 15-m telescope and the complete 30-m telescope, we have demonstrated that dynamic model calculations are capable to reproduce the observed thermal behavior of significant structural components with sufficient precision and thus to allow accurate predictions of the thermal behavior. Such model calculations are able to support the thermal design of large, high-precision radio telescopes, as was done for the IRAM telescopes.

We emphasize that continued monitoring of the thermal behavior (either by sensors or also by infrared images [7]) may lead to further improvements of the performance of a telescope. For instance, an empirical adjustment of the focus position with residual temperature difference between the YO and the RBS has been successfully implemented in the IRAM 30-m telescope [8].

~ ' " " ' ' " " ' " ' " " " ' 1 DE-ICING ON

MODEL (ESAW)

2o 1

0 5 10 15 20

Fig. 12. (a) Recorded temperatures of the IRAM 30-m telescope; and (b) model calculations. Y = yoke; R = reflector backstructure; L = feedlegs. In Fig. 12(b) the AIR is taken as input in the model calculations (taken from [SI).

DAYS (JM 1987)

ACKNOWLEDGMENT

The authors acknowledge the many discussions with J. Delannoy (IRAM, France), B. G. Hooghoudt (Leiden, The Netherlands), and J. W. M. Baars (MPIfR, Germany). They are especially grateful to ESA (Noordwijk, The Netherlands) for allowing the use of the ESACAP program of P. Stangerup; J. L. Casse (Dwingeloo, The Netherlands) gave us a careful introduction in the application of this program.

REFERENCES [l] J. W. M. Baars, "Technology of large radio telescopes for millime-

ter and submillimeter wavelengths," in Infrared and Millimeter Waues. New York Academic, p. 241, 1984.

[2] S . von Hoerner, "Design of large steerable telescopes," Astron. J., vol. 72, p. 35, 1967.

[3] -, "Homologous deformation of tiltable telescopes," J . Struct. Diu., ASCE, p. 461, Oct. 1967.

[4] D. Morris, J. W. M. Baars, H. Hein, H. Steppe, C. Thum, and R. Wohlleben, "Radio holography measurement of the 30-m millimeter radio telescope at 22 GHz with a cosmic source," Astron. Astrophys., vol. 203, p. 399, 1988.

[5] D. Morris, "Phase retrieval in the radio holography of reflector antennas and radio telescopes," IEEE Trans. Antennas Propagat.,

[6] J. Schraml, W. Brunswig, and G. h e n , "Design and software aspect for the control system of the 30 m MRT," in Aduanced Technology Optical Telescopes 11, SPIE 444, 1984, p. 122.

vol. AP-33, p. 749, 1985.


[7] A. Greve, “Thermal design and thermal behaviour of radio telescope structures,” IRAM, Int. Report., 1992.

181 J. W. M. Baars, A. Greve, B. G. Hooghoudt, and J. Penalver, “Thermal control of the IRAM 30-m millimeter radio telescope,” Astron. Astrophys., vol. 195, p. 364, 1988.

[9] J. Delannoy, “Status of IRAM 15 m antennas and SEST telescope in 1985,” in Proc. ESO-IRAM-ONSALA Workshop on (Sub-)Milli- metre Astronomv. ESO Conf. and Workshoo Proc. 22. 1985. D. 25.

[25] P. Stangerup, “ESACAP, a minicomputer-oriented network analysis program,” ESA J., vol. 6 , p. 301, 1982.

[26] P. Stangerup, IEEE Circuits and Deuices Magazine, vol. 4, p. 20, 1988.

[27] P. Raffin, Private Communication.

[ I l l

, I

S. Guilloteau, j. Delannoy,‘ D. Downes, A. Greve, M. Guelin, R. Lucas, D. Morris, S. J. E. Radford, J. Wink, J. Cernicharo, S. Garcia-Burillo, R. Neri, J. Blondel, A. Perrigouard, D. Plathner, and M. Torres, “The IRAM interferometer on Plateau de Bure,” Astron. Astrophys., vol. 262, p. 624, 1992. R. S. Booth, G. Delgado, M. Hagstrom, L. E. B. Johansson, D. Murphy, M. Olberg, N. D. Whyborn, A. Greve, B. Hansson, C. 0. Lindstrom, and A. Rydberg, “The Swedish ESO submillimeter telescope (SEST),” Astron. Astrophys., vol. 216, p. 315, 1989. J. W. M. Baars, B. G. Hooghoudt, P. G. Mezger, and M. J. de Jonge, “The IRAM 30-m millimeter radio telescope on Pic0 Veleta, Spain,” Astron. Astrophys., vol. 175, p. 319, 1987. J. Ruze, “Antenna tolerance theory-A review,” Proc. IEEE, vol. 54, p. 633, 1966.

[14] J. D. Bregman and J. L. Casse, “A simulation of the thermal behaviour of the UK-NL mm wave telescoDe,” Intern. J . Infrared MM Waues, vol. 6, p. 25, 1985. K. Akabane, “A large millimeter wave antenna,” Intern. J . Infrared MM Waues, vol. 4, p. 793, 1983. R. B. Baimarov, I. V. Baum, A. M. Vorobev, M. A. Gurbanyazov, I. N. Knyazev, Ju.L. Mazuev, and V. G. Fokin, Climatic Influences on Antenna Systems. Ashkhabad, U.S.S.R., ILIM, 1988. V. S. Polyak and E.Ya. Bervalds, Precision Construction of Reflector Radiotelescopes. Riga, U.S.S.R., Zinatne, 1990. J. W. Mar and H. Liebowitz, Structural Technology for Large Radio and Radar Telescope Systems. Cambridge, M A MIT Press, 1969. A. J. Chapman, Heat Transfer. R. W. Bliss, “Atmospheric radiation near the surface of the ground; a summary for engineers,” Solar Energv, vol. 5 , p. 103, 1961. C. G. Granqvist, “Radiative heating and cooling with spectrially selective surfaces,” Appl. Optics, vol. 20, p. 2606, 1981. T. S. Eriksson and C. G. Granqvist, “Radiative cooling computed for model atmospheres,” Appl. Opt., vol. 21, p. 4381, 1982. W. C. Swinbank, “Long wave radiation from clear skies, Q.J.R. Meteor. Soc., vol. 89, p. 339, 1963. I. Hamberg, J. S. E. M. Svenson, T. S. Eriksson, C . G. Granqvist, P. Arrenius, and F. Norin, “Radiative cooling and frost formation on surfaces with different thermal emittance: Theoretical analysis and practical experience,” Appl. Optics, vol. 26, p. 2131, 1987.

New York MacMillan, 1974.

Presently he works at wavelength radio astr

Albert Greve was born in Germany in 1938. He studied astronomy at Leiden and received the Ph.D. degree at Utrecht, Holland.

He worked at Culham Laboratory, England, Belfast University, Ireland, the Max Planck In- stitut for Radioastronomy, Germany, and IRAM-France. He was involved in the construction of several radio telescopes, in particular the surface adjustments, but also in the thermal design of modem millimeter-wavelength telescopes (and several optical telescopes).

IRAM-Granada, Spain, in the field of millimeter- onomy, and administration.

Michel Dan was born in France in 1956. He received the degree “maitrise” at the University of Grenoble, France, in 1979.

He worked in robitics and joined IRAM- France in 1985 in the group of operators of the Plateau de Bure interferometer.

Juan Penalver was born in Spain in 1955. He received the degree from University of Madrid in 1982 in telecommunications engineering.

He worked one year at the Centro Astro- nomico at Yebes, Spain, on the calibration of antennas. In 1983 he joined IRAM-Spain working first as operator, later in the computer area, and, since 1988, as the chief engineer of the 30-m telescope.

thermal behavior of millimeter wavelength radio telescopes

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