© ESO - September 2003 37

AAT THE TIME OF WRITING

the French-Italian Con-sortium responsible forone of the two ALMAAntenna prototypes is

completing the assembly phase of the an-tenna at the ALMA test facility (ATF) lo-cated at the site of the Very Large Array(VLA) in New Mexico.

The assembly activities are performedby a team led by ALCATEL Space ofFrance, including engineers of EuropeanIndustrial Engineering (EIE) of Italy, theother member of the Consortium, and re-sponsible for the antenna design.

ALCATEL Space Industries joinedthe original Italian Consortium, formedby EIE and Costamasnaga in December2001, when an amendment to the originalContract was signed by ESO allowing theenlargement of the Consortium. With theamendment ALCATEL Space becamethe Consortium leader.

In the months following the signatureof the Contract amendment, the existingantenna design was brought to the manu-facturing stage.This process was the occa-sion for further optimising a design thathad been considered to be very promisingsince the beginning of the project. Directdrives (similar to those used on the VLTmain structure) are used on both eleva-tion and azimuth axes. The receiver cabinis entirely manufactured of Carbon FiberReinforced Plastic (CFRP). This allowsconsiderable mass saving compared to asteel cabin. Together with the primary re-flector backup structure (BUS), alsomade of CFRP, it provides a thermallystable elevation structure. Both the directdrives and the CFRP cabin are expectedto be advantageous for the dynamicalperformance demanded of the ALMAantennas.

Another new design feature was theuse for the reflector surface of replicatedNickel panels developed by Media Lario

under a Contract from the EuropeanSpace Agency (ESA) and ESO. The repli-ca technique has allowed a fast produc-tion process and may prove to be advan-tageous for the serial production of the 64ALMA antennas.

The replicated panels are Rhodiumcoated for protection against the harshChajnantor environment. The perform-ance of these panels for the ALMA an-tenna has been the object of extensive

studies and tests. One major test hasmade use of a setup constituted by onepanel, mounted by its adjusters on aCFRP slice of the BUS in a climaticchamber to prove some elements of thesurface stability budget of the reflectingsurface.

In the tradition of ESO all new designcharacteristics were subjected to a formalreview process by the team in charge ofthe follow-up of the Contract1. This re-

THETHE ALCALCAATEL/EIE ALMA ATEL/EIE ALMA ANTENNANTENNAPPROROTTOOTT YPEYPE APPROAPPROAACHESCHES

COMPLETIONCOMPLETION ININ NNEWEW MMEXICOEXICOSSTEFTEFANOANO SSTTANGHELLINIANGHELLINI , , ALMA DIVISION, ESO

The ALCATEL/EIE antenna in the final phase of assembly at the ATF.

1The “core” team is composed of J. Baars, F. Biancat Marchet, C. Dichirico, B. Gustafsson,F. Koch, M. Kraus, J. Strasser, A. van Kesteren and the author.

The Messenger 11338

view process was also applied to the newdesign of the internal metrology systemof the antenna. Such a system will beused to guarantee the very demandingpointing and stability performances inthe open conditions of the Chajnantorsite, characterized by strong winds, signif-icant temperature gradients, and strongsolar radiation.

The pre-erection activities in Europewere limited by the short time availableto the Consortium and by the default, inMay 2002, of one of the Consortiummembers (Costamasnaga), whose origi-nal scope of work within the Consortiumhad been the mechanical manufacturingand pre-assembly. A replacement me-chanical shop (Galbiati) was found forthose activities, and EIE took over thetasks linked to the antenna mount manu-facturing (antenna base, yoke, motors,electrical subsystems etc.). At the Gal-biati assembly hall the major parts of theantenna mount were installed andchecked in November 2002, to be finallyshipped to the ATF in January 2003.

The assembly of the CFRP cabin andBUS, performed in parallel, and extend-ing up to spring 2003, proved to be a del-icate task, given the dimensions of theparts to be joined together and the strictdimensional tolerances on the final as-semblies. The cabin and BUS were man-ufactured by two specialized ItalianCFRP manufacturers (Plyform andATR) based on the EIE design, undercontract from ALCATEL. The varioussub-assemblies had to be glued and bolt-ed together under dimensional control bymeans of a laser tracker. For this taskALCATEL made use of a French sub-contractor (SETAT) specialized in themeasurement of large structures2. A set-back was encountered when two of thesixteen sectors of the BUS structure weredamaged in a road accident while beingtransported from ATR to the Galbiati in-tegration hall. Nevertheless in April 2003the cabin and BUS were finished, paint-ed, specially packed and shipped by aircargo to the testing site. (See ESO Mes-senger No. 112. p. 2) .

In the meantime, in order to be able towork in the difficult conditions of theATF (strong winds in late spring, burningsun and thunderstorms in summer) theConsortium had erected a temporaryshelter for the assembly of the antenna.This shelter has proven valuable for the

2The company is known to ESO for having per-formed the dimensional measurement of theprimary mirror cells of the VLT.

The machining of the CFRP cabin interfaces at the Galbiati factory. The machininghead is working on the elevation motor attachment point.

View of the replicated Nickel Panel at the Media Lario factory. The panels at the lefthave been coated with Rhodium and have a brighter appearance than those on theright (bare Nickel).

The BUS being assembled at the Galbiati factory.

delicate task of mounting and aligningthe antenna. All major mounting opera-tions were performed under control ofthe laser tracker. In June the cabin waspositioned and aligned onto the antennamount, and on 11 July the important op-eration of mounting the BUS on the an-tenna was performed without major dif-ficulties. All major interfaces, althoughnot tested at the factory, mated correctly.

In the following weeks the 600 panelsadjusters and the 120 replicated panelswere mounted and aligned on the BUS.A verification of the surface accuracywith the laser tracker has resulted in apanel map with the excellent accuracy of40 micrometers. (It will be a later task ofthe joint ESO-NRAO Antenna Evalua-tion Group (AEG) to obtain the finalspecified accuracy of 25 micrometers.)

On 18 August the roof of the shelterwas removed and the quadrupod withthe sub-reflector and its mechanism wereinstalled. A branch of a tree (later re-moved) had been put on top of the an-tenna to respect an old French buildingtradition. The long-awaited dismountingof the shelter walls revealed for the firsttime the antenna in its entirety and finalconfiguration.

In the entire manufacturing and as-sembly process the close collaborationbetween the ALCATEL integrationteam, and the EIE engineering team, au-thor of the ALMA antenna design, hasbeen fundamental for solving the unex-pected problems and various glitches typ-ical of a prototype. At the ATF site evenminor problems can impact the scheduledue to the transport and import time.Close communication between the siteteam and the design office is essential.

At the time of writing the technicalcommissioning of the antenna has start-ed, in parallel with the finishing activities.The team on site is working round theclock to give the final touches to the an-tenna and to start the verification phasewith ESO. Part of the verification activi-ties is linked to the commissioning of themetrology system, involving a set of deli-cate performance measurements, someof which will be performed on the sky.Following acceptance of the antenna byESO, the final evaluation of the antennacharacteristics will be done by the An-tenna Evaluation Group of the ALMAproject after equipping the antenna witha receiver.

© ESO - September 2003 39Stanghellini S., ALMA Prototype near completion

View of the top of the antenna, after completion of the reflecting surface and installation of quadri-pod and subreflector.

The mounting of the BUS onto the antenna at the ATF site.