No. 98 – December 1999 First Scientific Results with the VLT in Visitor and Service Modes Starting with this issue, The Messenger will regularly include scientific results obtained with the VLT. One of the results reported in this issue is a study of NGC 3603, the most massive visible H II region in the Galaxy, with VLT/ISAAC in the near-infrared Js, H, and Ks-bands and HST/WFPC2 at Hα and [N II] wavelengths. These VLT observations are the most sensitive near-infrared observations made to date of a dense starburst region, allowing one to in- vestigate with unprecedented quality its low-mass stellar population. The sensitivity limit to stars detected in all three bands corresponds to 0.1 M0 for a pre-main-sequence star of age 0.7 Myr. The observations clearly show that sub-solar-mass stars down to at least 0.1 M0 do form in massive starbursts (from B. Brandl, W. Brandner, E.K. Grebel and H. Zinnecker, page 46). Js versus Js–Ks colour-magnitude diagrams of NGC 3603. The left-hand panel contains all stars detected in all three wave- bands in the entire field of view (3.4′×3.4′, or 6 pc × 6 pc); the centre panel shows the field stars at r > 75″ (2.25 pc) around the cluster, and the right-hand panel shows the cluster population within r < 33″ (1pc) with the field stars statistically subtracted. The dashed horizontal line (left-hand panel) indicates the detection limit of the previous most sensitive NIR study (Eisenhauer et al. 1998). The right-hand panel also shows the theoretical isochrones of pre-main-sequence stars of different ages from Palla & Stahler (1999) and the main sequence for dwarfs. For comparison, some corresponding stellar masses have been plotted next to the isochrones. 1 TELESCOPES AND INSTRUMENTATION The VLTI – The Observatory of the 21st Century A. GLINDEMANN, R. ABUTER, F. CARBOGNANI, F. DELPLANCKE, F. DERIE, A. GENNAI, P. GITTON, P. KERVELLA, B. KOEHLER, S. LÉVÊQUE, G. DE MARCHI, S. MENARDI, A. MICHEL, F. PARESCE, T. PHAN DUC, M. SCHÖLLER, M. TARENGHI and R. WILHELM European Southern Observatory 1. Introduction servatory and with institutes and institu- smaller contracts for the Test Siderostats, tions from other European countries (Italy, for the Coudé Optical Trains of the four After several years on a bumpy road, Switzerland and Belgium) on the verge Unit Telescopes (UTs), and for a Feasi- the support for the VLT Interferometer of joining the project in one way or an- bility Study of the dual-feed facility PRI- project has increased dramatically over other. MA were also placed. the last year with the appointment of Major contracts for the Delay Line Sys- We are now planning for first fringes eight new staff members, with the foun- tems and for the Auxiliary Telescope Sys- with the commissioning instrument and dation of the NOVA1-ESO VLTI Expertise tems (ATs) were signed in 1997 and 1998 the Siderostats before the beginning of Centre (NEVEC) at the Leiden Ob- respectively, the work on the science in- the new Millenium. struments MIDI and AMBER has pro- In this article we will give a report on gressed very well in the last two years the status of the project and we will de- 1NOVA is the Netherlands Research School for and the commissioning instrument VIN- scribe the strategy for making the VLTI Astronomy. CI is coming closer to reality. Several the observatory of the 21st century. Figure 1: The optical layout of the VLTI with two telescopes. The telescopes represent both UTs and ATs that have the same optical design. The Coudé Optical Trains are the mirrors after the tertiary mirror up to the Coudé Focus. Two Delay Lines are shown to demonstrate the prin- ciple of operation. The VLTI laboratory is represented by the beam-combining lens forming fringes. 2 2. The Sub-Systems of the VLTI The layout of the VLTI is displayed in Figure 1, for the sake of simplicity with only two telescopes. A star at infinity il- luminates the apertures in the two tele- scopes with a plane wave that is guided through the Coudé Optical Trains into the Delay Line Tunnel. The delay in arrival of the light at telescope 1 with respect to telescope 2 is compensated by the de- lay lines so that the beams have zero Op- tical Path Difference (OPD) when they in- terfere on the detector in the VLTI labo- ratory. The field of view of the VLTI is 2 arcsec. However, the dual-feed facility PRIMA will allow picking two stars at the Coudé focus of ATs or UTs each in a 2 arcsec field of view and separated by up to 1 arcmin. The measurements of the contrast of the fringe pattern and of its phase, i.e. the position of the white light fringe with respect to the nominal zero OPD position are the tasks of the VLT Inter- ferometer. Doing these measurements Figure 2: The carriage for the cat’s-eye of the Delay Line System in the Clean Room at Fokker for many different baselines (different in Leiden. The carriage has three wheels running on the 65-m steel rails. The diameter of the in length and orientation) allows re- wheels is about 40 cm and the length of the carriage 2.25 m. constructing an image with the angular resolution of a single telescope with a di- ameter equal to the longest baseline. Since the longest baselines are 130 m for readily manufactured and they will be de- three telescopes and thus three baselines the Unit Telescopes and 200 m for the livered to Paranal in January 2000. The at the same time will allow the applica- Auxiliary Telescopes and since the spec- implementation of the Siderostat control tion of closure phase techniques elimi- ified precision for the measurements of software will be finished by May 2000. It nating the influence of atmospheric tur- the fringe contrast is very high, the re- is planned to have first fringes with the bulence on fringe position. Each AT will quirements e.g. for the Delay Line sys- Siderostats and VINCI in December be equipped with a tip-tilt system cor- tems are very tough. Here, the beam tilt 2000. recting for the fast image motion induced accuracy has to be better than 1.5 arc- by atmospheric turbulence. Under the seconds (15 milliarcsec on the sky) and Delay Line Systems seeing conditions at Paranal, tip-tilt cor- the absolute position accuracy is 1µm rection on a 1.8-m telescope in the near over 65 m of travel in the tunnel. Similar Three Delay Lines have been ordered; infrared means almost diffraction limited accuracy values are required for all mir- the first two will be installed in the De- image quality. One should note that the rors and mirror mounts of the VLTI, in- lay Line Tunnel in July 2000, the third one ATs are available exclusively for the VLTI, cluding the 8-m primary mirrors. Also, the in December 2000. The systems consist forming an observatory that is operated environmental conditions have to be very of the cat’s-eye mirror system retro-re- independently of the Unit Telescopes. stable in order to minimise internal tur- flecting the incoming parallel beam, and bulence. of the carriage transporting the cat’s-eye Coudé Optical Trains and Transfer It is the advantage of the VLTI that along the delay lines. Figure 2 shows the Optics these considerations have been driv- carriage on its three wheels in the clean- ing the design of the Unit Telescopes room at the manufacturer. The three-mir- The five mirrors after the tertiary mir- and of the Auxiliary Telescopes, as ror optical design of the cat’s-eye has a ror of the Unit Telescopes form the Coudé well as the construction of the infra- variable curvature mirror (VCM, see [4]) Optical Train with a field of view of structure. Several measurement cam- in the focus of the cat’s eye in order to 2 arcmin (see Fig. 1). The last mir- paigns have confirmed that both opto- re-image the telescope pupil into a fixed ror before the Coudé Focus is very close mechanical and environmental specifi- position in the VLTI laboratory while the to the telescope exit pupil and can be re- cations are met. Delay Line System is tracking. The placed by the deformable mirror of the The status of the VLTI sub-systems is cat’s-eye can handle two input beams as adaptive optics system. The image qual- as follows: required for a dual feed system. A total ity at 2.2 µm is diffraction limited on of eight Delay Line Systems can be host- the optical axis and better than 97% over Test Siderostats ed in the Delay Line Tunnel. the full field of view. The Coudé Optical Trains for all four UTs have been or- For tests with the VLTI and the in- Auxiliary Telescope Systems dered and will be installed at Paranal in struments, two Test Siderostats were de- October 2000. signed and built (for details see [3]). The In June 1998, two 1.8-m Auxiliary Tele- The transfer optics between the Coudé free aperture of 400 mm allows for about scopes were ordered, and in June 1999, Focus and the Delay Lines are current- 20 stars to be observed in the N-band at the option for the third AT was exerted. ly being designed. The concept is to avoid 10 µm. Since for the VLTI Control Sys- A model of the telescope is displayed in the need of human intervention in the de- tem the Siderostats will ‘look’ the same Figure 3.