Kees van ‘t Klooster ESA Estec Noordwijk, The Netherlands e-mail: kvtkloos@esaint

Abstract – The paper describes an example of spin-off of an advanced space technology development for application in another high tech terrestrial field, namely for sub-millimetre radio telescope antennas

I                                    Introduction

The scientific satellite XIVIM-Newton of European Space Agency (ESA) was launched in 1999 as one of ESA’s cornerstone missions (large missions) Recently XMM-Newton mission has been extended, because of its very valuable and strong observation capability, providing a very high throughput X-ray spectroscopy The satellite can see more sources in a small area of the sky than one of the first X-ray satelites (UHURU) did see during its total 3 year lifetime several years ago The three X-ray telescopes are crucial for the observations Each telescope has a grazing incidence set of multiple coaxial mirrors which act as a lens at the X-ray wavelength The coaxial mirror-sets of 07 m diameter provide focusing in the 75 m length X-ray telescopes and are dedicated, highly specialised technological developments Descriptive information is found on http://sciesa int/science-e/www/area/indexcfmfareaid=23 The ESA website http://sciesaint provides further interesting information about ESA’s scientific missions in general and about XMM-Newton in more detail

Technological questions have been solved, related to design, manufacturing and testing of the X-ray mirror sets A rich expertise is now available in the dedicated company Media-Lario in Italy for this very specialized field, with various associated material technologies, like precision electroforming and coating techniques The realization of the coaxial mirror configurations for XMM- Newton required electro-forming processes, to allow high accuracy and reproduction capability

Studies have been initiated by ESA after the XMM- launch to explore the application potential of the technology processes in other domains and for other customers Various interesting applications have been found and some have been developed The most promising application has been worked out and resulted into the very interesting and awarding spin-off: the design, manufacturing and testing of highly accurate reflector panels for sub-millimeter radio telescope antennas This spin-off appeared to be very “time-coincidental” and successful, because it has led to installation of panels according to these processes on a prototype sub-millimeter radio telescope antenna for the ALMA project

Realization of reflector panels for antennas, using electroforming has been investigated in the past [1], using polyester materials as supportive material Here quite different approaches have been followed and already the first results indicated very good perspective for mm and sub-mm wave applications [2] Excited by these good results, it has been for us extremely interesting to target to sub-millimeter radio telescope applications and to use the requirements for the ALMA antenna As is indicated in Table 1, the reflector assembly of the ALMA sub-mm wave radio telescope antenna has to be better than 25 ЦТ rms The ALMA antenna design has been ongoing during the same time, which made it even more attractive to investigate such application

Prototype panels have been realized and qualification tests have been carried out with success [3] The continuation of the activities was possible and a complete panel set has been realized for one of the ALMA prototype antennas, as described in [4] below Electroforming technology with its possible processes is very suitable for recurrent production This makes it even more attractive compared to other techniques, like for instance the elaborative numerical controlled machining of separate Aluminum panels (competitive candidate panel technology) Furthermore, the lower (than for Aluminum) thermal expansion coefficient of the electroformed material (Nickel controls this) is a strong advantage compared to Aluminum The difference is ~ a factor 2 Consequently, a panel gap width requirement for a particular temperature range permits to work with larger sized panels: fewer panels (larger in size) are needed for one telescope (~in total indicatively a factor ‘4’ less) This leads obviously to less time needed for a final alignment and precision tuning (iterative exercise exploiting for instance holography) It is important when one has to realize a larger number of antennas in a certain time- schedule

The realization of the ALMA prototype antenna is outlined in [5], when it was almost completed

The investigation of the panel technology continued and efforts have been done under ESA contract for extreme environments This did lead to an installation of panels with integrated heating (for de-icing) on the radio telescope of Plateau de Bure in France [9] This wider application scenario underlines the perspective of this spin-off of a space technology

The spin-off technology has been developed under ESA contracts with Media-Lario [3, 4]

It is an interesting example demonstrating suitability of technology developed for one high tech space development (XMM-Newton) for another different but also high tech application for terrestrial utilization

It also underlines importance to be given to space technologies associated with the potential for spin-off

II        ALMA Sub-Millimeter Radio Telescope Antennas

Today there is an international cooperation in place with USA, Europe and Japan for the realization of a large sub-millimeter wave array of radio telescope antennas for the ALMA project The location of ALIVIA will be in the Atacama Desert in Chili in South America – a climatologically interesting site for sub-millimeter observations ESO (European Southern Observatory – wwwesoorg) operated already other obsenyatories in Chili The ALMA project is unique and a step forward in radio astronomy There are 25 antennas of 12 meter diameter constructed under contract with National Radio Astronomy Obsen/atory (NRAO) in USA Another 25 antennas of 12 meter are realised under contract with ESO in Europe More is coming with collaboration of Japan More information is found on http://wwwesoorg/ projects/alma/ The antennas are for the millimeter and submillimeter regime High-level overall antenna requirements are summarised (Table 1) Three prototype antennas have already been realised and have been evaluated on the site of the Very Large Array in New Mexico (wwwaocnraoedu) One ofthe three prototype antennas distinguishes itself by the fact, that it exploits this described new reflector panel technology, developed under ESA contracts as spin-off of the XMM-NEWTON technology for X-ray mirror sets

Table 1 ALMA Principal Performance Requirements

Configuration

Elevation over Azimuth

Frequency

30 – 950 GHz

Performance

Conditions

Night: 9 m/s wind Day: 6 m/s Sun at any anqle

Reflector Surface

20 ЦТ rms goal,

25 ЦТ rms specification

Pointing

06 arcsec

2 arcsec absolute pointing

Fast Switching

Move 15 ° in position in 15 seconds

Phase Stability

15 ЦТ rms

Close Packing

1,25 dish diameter (15 m)

Solar

Observing

Allowed

Transportable

Transportable on special rubber-tire vehicle

The radio telescopes are configured as array elements on the site in a configuration, which has flexibility The relative position of the antennas can be changed, using an antenna transportation system (http://wwwesoorg/ projects/alma/) Both compact and wider spaced configurations within some 18 kilometer are to be possible The resulting interferometer configurations have been investigated for their imaging capabilities

The panel assembly for the ALMA telescope antenna according to the Alcatel design consists of 5 rings of panels as described in [4] and is obvious from the pictures in [5]

Ill             Reflector Panel Technology

The application of electroformed panels for ALMA is indeed interesting A continued effort led to another application, which deserves attention as well It shows a further advancement in electroforming technology A heating system has been designed and integrated in electroformed panels for de-icing of a millimeter wave radio telescope A number of such prototype panels has been installed on a radio telescope in Plateau de Bure in France Information and results are described in [9] The reader is recommended to consult [9] for this advanced application, which shows the capability of the spin-off panel technology to operate under more severe meteorological conditions, than envisaged for ALMA

The reflector panels for the ALMA antenna have to perform under various environmental conditions, but do no contain such de-icing capability The site in Chili has more favorable meteorological conditions than the site in Plateau de Bure in France For the panel design and development it is needed, that the process considers several properties in a combined manner, like RF reflection

thermal and structural aspects This is usual the case for antenna realization for space applications Advantage was obviously taken from such design practices during development and manufacturing and test [3, 4]

The qualification testing showed good results Subsequently a contract was awarded to Media-Lario for a panel set for an ALMA prototype antenna

An overall accuracy requirement for the reflector surface of 25 ЦТ rms led to a target requirement at panel level of <10 ЦТ rms. Reflector panels have been verified against such requirement, using a dedicated test facility. Numerous tests have been carried out as is described in

[4]  and dedicated solutions have been implemented to diffuse the visual light, whilst reflecting the RF signal to be observed The benefit ofthe electroforming or replica technique has been exploited clearly, with Nickel as a main material, capable to handle various environmental conditions The electroforming solution permits also operation ofthe antenna in the direction ofthe Sun, as required, using simple replicating techniques

Dedicated diffusing techniques are needed for the competitive panel designs like micro-machined Aluminium panels The latter approach implies individual micromachining ofthe Aluminium panels with a special pattern to diffuse the visual light and that has to be repeated for every panel Furthermore, there is a temperature range on the site (cold nights, hot days) Taking the thermal expansion coefficient into account, the number of panels is larger when Aluminium panels are used, because of the larger thermal expansion and allowed maximum gap between panels (within the operational temperature range)

Electroformed nickel has not so good thermal absorption and emission coefficients as Aluminium, therefore a dedicated coating of Rhodium has been applied (picture in [5]) giving an even more durable surface

The lower thermal expansion coefficient of Nickel compared to Aluminium permits to use larger panel size (factor ~4 in area) for the operational temperature range and the same maximum gap distance between the panels The Rhodium coating furthermore improves the absorption and emission properties ofthe Nickel panel Nickel alone would perform electrically well, even although it has somewhat higher reflectivity loss compared to Aluminium RF reflectivity loss has been measured at a special facility in Applied Physics Institute in Nizhny Novgorod in the frequency range 100-200 GHz and recently at higher frequencies Various samples have been investigated [7, 8]

The facility at Applied Physics Institute is rather sensitive Such type of testing is very important for applications, in which low noise is desirable The testing of new composite materials with whatever coating is – I would say – even mandatory in such millimeter and sub-millimeter frequency ranges, because the parameters related to skin- depth, roughness, precise material properties and fiber directions cannot feed well enough accurate multilayered electromagnetic models – “ with great respect for the electromagnetic modeler “ In consequence samples have to be tested, moreover such sample testing assist designers

–    depending on applications – to use other coatings which can be acceptable from thermal, mechanical and electrical point of view, taking also polarization properties into account [8]

Further alignment and verification of the complete antenna was carried out after the realization [5] Good results have been obtained

IV                    Associated Technologies

Another spin-off technology deserves to be mentioned here It has application potential in millimetre and sub-millimetre radio telescope antennas The deformation of the main reflector, for instance due to gravity or due to thermal distortions, can lead to a slow-varying surface error distribution If one would have available a correcting mechanism, one could potentially correct the focussing properties of the radio telescope antenna A deformable sub-reflector could be used for such purpose If one would derive the error distribution on the main reflector, one could synthesize the corresponding corrective surface shape of the sub-reflector For this reason we have developed a deformable (“massage- able”) sub-reflector Such a correcting principle can also be used in the secondary focus or even inside a potential quasi optics arrangement near the receiver As described in [10], the technique as such is not new The new aspect is the utilisation of a thin Nickel electroformed membrane Excursions and possibilities of the membrane have been investigated and a frequency range near to 500 GHz was targeted, because ofthe potential excursions The surface geometry description had been derived, exploiting a de-composition in Zernike polynomials The reader is referred to [10] for the results

V                                     Conclusion

Spin-off applications for technologies, developed within a large ESA space project have been found and realised for millimetre and sub-millimetre radio telescope antennas The proof is found in New Mexico, Socorro, USA, where an ALMA prototype antenna has been realised with a highly accurate new reflector panel technology Mentioned is also the use of thin Nickel membrane for corrective (sub) reflector application in the same wavelength domain with an example The realisation ofthe interesting radio-astronomy project ALMA has taken a start

VI Final Remarks: Radio Astronomy

In the meantime, investigations are carried out in radio astronomy for sophisticated observation instruments, worthwhile to mention At the longer wavelengths there is an effort devoted to new, more sensitive instruments (SKA

–    Square Kilometer Array, see wwwskatelescopeorg) Also Very Long Baseline Interferometry (VLBI) with one radio telescope antenna in Space, or Space VLBI deserves attention – see wwwascrssiru and wwwjvsop isasacjp

Millimeter wavelength VLBI is moving, with very long baselines – see also wwwjivenl and wwwiramfr A lot of effort is devoted to (near) real-time inter-ferometry with high-speed data transmission between observatories and the correlator wwwradionet-euorg Clearly very interesting developments are going on, with very interesting projects, of which the realization of the ALMA sub-millimeter wave array of telescopes is a very important one

Recent VLBI observation of the transmission of the Huygens signal (few Watt at Saturn distance) required the organization of 17 radio telescopes and to elaborate and adapt the appropriate interfaces, in fact a show case, of demonstrating what is possible with state of art technology and Very Long Baseline Interferometry [6] The latter observation aimed at high angular resolution The ALMA interferometric array configuration will provide a wide field of view, with high resolution capability and should come into operation in a few years from now Perhaps we may see in the future even combined (interferometric) observations, with a space based radio telescope co-observing with ALMA

VI Literature

1 Поляк В С, Бервалдс Е Я Прецизійні Конструкції Дзеркальних радіотелескопів, Академія Наук Латвійської ССРБ Рига, 1990, С166

2  Some Satellite Antenna Applications for Remote Sensing and Scientific Satellites, C G M van’t Klooster, In «Technologies for Large Antenna Arrays», edited by A B Smolders and M P van Haarlem, Netherlands Foundation for Research n Astronomy, 1999 http://wwwastron nl/documents/conf/technology/tech17wpdf

3  High Precision Electroformed Nickel Panel Technology for Sub-Millimeter Radio Telescope Antennas, G Valsecchi,

J Eder, G Grisoni, Keesvant Klooster, L Fanchi, IEEE AP Symposium, Columbus Ohio, USA, 2003

4  A Spin-off Space Technology: Highly Accurate Reflector Panels for a Prototype ALMA Radio Telescope, Kees van ‘t Klooster, Giuseppe Valsecchi, Josef Eder, ICATT Conference 2003, Sevastopol

The Aicatei/EiE ALMA Antenna Prototype Approaches Completion in New Mexico, ESO Newsletter, S Stanghellini, http://www eso org/genfa c/pubs/messenger/archive/no 113-sep03/m113-37_39pdf

5  17 Radio Telescopes Observing the Huygens Signal Coming from 1200000000 Kilometres Distance», Keesvan ‘t Klooster, ICATT Conference, Kiev, 24-27 May 2005

6  Reffecftwfy of Antenna and Mirror Reflectors at 110-200 GHz

S      E Myasnikova, V V Parshin, Keesvan ‘t Klooster,

G Valsecchi, ICATT Conference 2003, Sevastopol

7  Reffecftwfy of Antenna Reflectors at Frequency Range 100 – 380 GHz, S Myasnikova, V Parshin, K van ‘t Klooster, 28*Antenna-Workshop, WPP-247 N’wijk, The Netherlands

8  M Bremer, A Greve, K van ‘t Klooster, IVI Grewing, J Eder,

G     Valsecchi, Front and Rear-side Heated Prototype Panels for the IRAM 15-m Telescopes, 28&quot’ ESA Estec Antenna Workshop, WPP-247, 31 May-3 June 2005

9  A Reconformable Reflector for a Sub-MM Wave Reflector Antenna», F Zocchi, P Binda, H H Viskum, Keesvan’t Klooster, IEEE AP Symposium, Columbus Ohio, 2003

Джерело: Матеріали Міжнародної Кримської конференції «СВЧ-техніка і телекомунікаційні технології», 2006р