Gran Telescopio Canarias: A 10 m telescope for the ORM. J. M. Rodriguez Espinosa & P. Alvarez Martin

Transcription

1 Gran Telescopio Canarias: A 10 m telescope for the ORM J. M. Rodriguez Espinosa & P. Alvarez Martin GTC Project nstituto de Astrofisica de Canarias La Laguna, Tenerife, Spain jreiac.es; pam@iac.es ABSTRACT A project to build a large telescope at the Spanish Observatorio del Roque de los Muchachos (ORM), in the island of La Palma, has been underway for several years. Spanish Astronomy has progressed steadily to the point where gaining access to large telescope time is key to its continued and future growth. Also the technological situation within Spain is such that building a large telescope is regarded as a scientific and advanced technology endeavour worth investing in. The Gran Telescopio Canarias (GTC) project has thus been granted approval in February this year, with an allocation of 51% of the budget. Current activities of the Telescope Project are mainly concentrated in the conceptual design and analysis of the teiscope optics, including plans for the adaptive optics, its mechanical structure and enclosure, and an active campaign of site testing. We are also carrying out detailed programmes aimed at producing and testing prototype models for sensors, actuators and the control system for aligning the primary mirror segments. n the following sections, the current status of the activities being carried out by the Telescope Project Team are described. Keywords: large telescopes: general, large telescopes: scientific drivers, segmented mirrors, site testing. 1.ABTOFHSTORY The idea of building a large telescope at the Spanish Observatorio del Roque de los Muchachos has been recurring over and over since 1987 or so. Since that time a small telescope office was set up at the AC mainly concentrating its efforts into developing a working test bench for the active support of a thin meniscus mirror. n 1995 a proposal was presented to the Spanish authorities seeking funds for the construction of a large telescope at the ORM. That proposal a) recognized the necessity of providing Spanish astronomy with an advanced tool for astronomical observations, and b) set up the conceptual basis for an advanced telescope to be built mainly by Spanish industries. The 1995 proposal was rewieved by a panel of experts who made very valuable recomendation indeed. 2. TOP SCENTFC REQUREMENTS The 1995 proposal also stated the Top Scientific Requirement that should guide the design and building phases of the large telescope. These requirements represent top priorities ofthe Project and can be summarized as follows: 1) To achieve the best optical performance possible. This excellent optical performance is better explained by stating that we will endeavour to a) Preserve a natural seeing of 0.4" by not spoiling it by more than 10%. b) nclude Adaptive Optics in the Project from the design stages. 2) To build a Telescope that caters for a diversity of observational programmes with high throughput for Optical and R work. 3) To produce a highly efficient telescope for astronomical observations that allows: a) Simultaneous availability of various instruments X/971$1O.OQ SPE Vol /69

2 b) Queue as well as classical, service and remote observing modes, and c) High reliability and robustness. 3. TELESCOPE OPTCS A first outline of the telescope optics that allows meeting the above specifications has been adopted as a working reference for defming the rest ofthe telescope. The telescope, which is currently being designed, will carry a segmented primary mirror along the lines pioneered by the Keck project. Deviations from Keck will be kept at a minimum in an effort to build a reliable telescope. Features of this new large telescope project include planing an adaptive secondary mirror from the design phase.these preliminary ideas are summarized in the following: a) the primary mirror will be segmented, 10 metre in diameter. b) the F ratio ofthe primary will be between 1.6 and 1.8 We are presently studying wether to adopt a number of hexagonal segments equal to that of Keck's, i.e. 36 segments, or to increase this number to 60 segments, 1.4m in size each. This may have the advantage of lower blank and polishing costs, although a larger number of segments may results in a larger error budget and perhaps larger maintenance needs. The telescope will have Casegrain and Nasmyth foci. The visible Casegrain focus will be obtained with an Ritchey-Chretien configuration and an FD=15 (±1). This will provide a plate scale of about 1.38"/mm and an unvigneted field of view (FOV) of 20' with a secondary of l.43m in size. The Nasmyth foci will also be FfD=15 and with a tertiary mirror of about im x 1.4m it will alilow an unvigneted field ofview of2o'(±l). The fi 5 beam will not be suitable for the thermal R. Therefore we are proposing to have an JR Gregorian secondary with an FD ratio of 25 (±2), which will produce a plate scale of O.83"/mm. t will employ an undersized O.80m secondary which will allow an unvigneted field of view of about 6', which for thermal JR observations is rather adequate. We are studying the possibility ofmaking this R secondary adaptive. That will have the advantage ofproviding corrected beams at all the foci. We intend to produce a telescope with good JR performance, and therefore it is our goal to keep the JR emissivity at or below 4%. More details about the optical design work currently being done for this telescope can be found in the paper by Escudero et al. in this proceedings. 4. NSTRUMENTATON PLAN As we are still on the preliminary phases of the telescope, and a detailed instrumentation plan is therefore not ready yet. Only some guidelines have been adopted for planning purposes. These guidelines stems from the intended use that the telescope is going have, and hence the plan is to equip the telescope with a reduced set of first light instruments that add sufficient capabilities to carry out a variety of observing programmes both in the optical and in the R. These instruments will have to be defmed in detail but initially we expect that they will provide capabilities for: a) optical imaging and mid to high resolution spectroscopy (R=1O.000 to ) b) infrared (1 to 5 tm) imaging and spectroscopy. 3D mode is desired. c) High resolution (adpative optics) near JR camera. The first light instruments will very likely be built by consorptia. The project will however emphasize the participation of Spanish groups, as it is an aim of the telescope project to secure that enough expertise in astronomical instrumentation is gained by Spanish groups and institutes. 5. CURRENT ACTVTES A number of activities have been undertaken since the decission to adopt a segmented configuration for the primary mirror was taken. Also, after the fmal approval of the project in February this year, some additional activities have been started. We 70 /SPE Vol. 2871

3 are working toward producing a conceptuallpreliminary design for the telescope project before the summer of Among the ongoing activitites it is worth mentioning the following: a) The optical configuration is being studied in detail as mentioned above. (see e.g. Escudero et al., this conference). b) The fmal size of the segments for the primarry mirror is also under study. Our goal is to defme an optimal size for the segments that considers technical and budgetary issues like fabrication and polishing of the segments, optical testing, handling aspects during operation and maintenance, and costs associated to each ofthese issues. c) A programme to produce prototypes sensors and actuartors for the segments is also being carried out by a team that includes personnel from research institutes and from aerospace industry. This team will fabricate prototypes of the sensors and actuators to be tested individually in the lab. A second phase will see the production of a test bench with two segments in place that will allow a test of the actual sensors and actuators and of the strategy for the controlling them. d) An adaptive optics plan that defmes the requirements to be met by the telescope design is also under study (see e.g. Devaney, and Rodriguez Ramos & Fuensalida; this conference) e) Another area of activity is the study of several approaches for the mechanical structure of the telescope, including the use of non traditional materials. Our starting point is however the Keck's structural design. 0 The enclosure is also under carefull study, including the development of an aero-thermodynamical model that will allow an understanding of the influence of various dome geometries and ventilation schemes in efficiently reaching the desired thermal stability between telescope and ambient. (See Pescador & Castro's paper; this conference). g) Preliminary ideas for the telescope control system have already been laid down. More work will be needed at a later stage. h) Finally, an operation plan that covers queue scheduling is under study (e.g. Hammersley, this conference). 6. STE TESTNG An intensive campaign of site testing has been ongoing for more than one year. This is to be added to previous seeing monitoring campaings"2 that have been carried out at the Observatorio del Roque de los Muchachos (ORM) for a number of years, and specially in the last two years at the site of the Telescopio Nationale Galileo, a site relatively close to the two sites that we have earmarked for further studies for as possible sites for the GTC. All these campaigns, plus the actual performance of several telescopes, ranging in size from 1 m to 4.2 m (the Williams Herschell telescope, WHT), confirm the excellent quality of the skies above the ORM. n particular, it is worth noting the astronomical images obtained by the NOT (Nordic Optical Telescope) in which seeing on less than 0.3" have been reported. Also on this telscope with active correction of the primary seeing as low as 0.2" have been measured. Our own campaign extends one year back, and has concentrated on measuring several relevant parameters on the two different sites within the ORM that have been preselected. For these two sites, one located by the rim of the Caldera, the highest part of the observatory, and another located some 200 meter downhill, we have collected and are still collecting seeing and meteo data (relative humidity, temperature and wind speed and direction). The seeing data are being collected with the AC-Nice seeing monitor, a DMM type of device that was jointly developped by the AC and the University of Nice3. Two ofthose monitors are operating simultaneously at each ofthe two sites. A further seeing monitor ofthe same type is being operated near the WHT. These monitors have been properly calibrated against similar instruments belonging to ESO. Advantages of having three identical seeing monitors simultaneously collecting seeing data at three different positions within the observatory are a) errors in one monitor are easy to spot and track down; b) differential measurements are obtained which are useful to point out differences between the sites due to local perturbances. And c) the combination of the three measures provide a good source for studying the night to night variations of the seeing at the observatory as a whole. Preliminary data obtained with these monitors are very encouraging, even if this winter has been one the worst of the last 40 years in La Palma, and actually in many parts of Europe and the north of Africa. Median seeing values of 0.6" for more than 50% of the time have been obtained, as well as minima of 0.2" during non negligible fractions of time. SPE Vol /71

4 Measurements of the water vapour content have been started recently with two identical water vapour monitors that measure the depth of a water band in O.8im. These data are very important as they will dictate how much emphasis the Project devotes to thermal R instrumentation. We expect to combine these data with JR satellite data. Finally, ground tests have been performed at the two preselected sites. These are important to determine the type and composition of the underground soil, and are therefore very valuable for defming the design criteria of the concrete foundations ofboth the telescope and enclosure. A decision, combining all the available data, as to which site is chosen for construction of the Telescope is expected by the end of this year. t is the intention of the project to start with the fmal design of the enclosure and preliminary ground works as soon as a decision on the site is made. Construction work should therefore start by spring 98 if the civil works are not to delay the entire project. 7. PROJECT BUDGET The total cost of the telescope project is estimated at M$ in '96 U.S. dollars. A breakdown of the budget in major packages is given below: GTC Budget M$'96 Telescope optics and control 31.3 Telescope structure and drives 13.4 Telescope Control system 2.2 Enclosure and equipments 25.4 First light instruments 17.6 Management 11.5 Contingency (10%) 10.1 TOTAL Note that a high fraction (28%) of the budget is devoted to the optics including its alignment and phasing control. The enclosure, including dome and ancillary equipment follows (23%), and third in the list of expenditures is the budget for astronomical instrumentation(16%). A contingency of 10% has been added to produce the the overall budget. The above budget has been prepared with actual figures given by manufacturers or by colleagues from other large telescope project that are in a more advanced stage than ours. Operations after first light are estimated at : M$ 4.5 per annum, starting in This operations budget does not include provisions for new developments or new instrumentation. This extra budget is estimated at M$ 3.1 per annum, also starting in This is required if new instrument developments and if telescope upgrades are not to be stopped after telescope completion. 8. STATUS OF THE GTC PROJECT As mentioned above, the project was approved in February 96. Although the approval was with only 5 1% of the funds, permission to start moving ahead and making expenditures was indeed obtained. Theses funds are sufficient to carry on with the project till 1999, even if no new funds are found in a short period of time. The Project is therefore starting to manage the activities mentioned before, and others that will come on line as the year progresses. A preliminary design document is to be ready by the summer 97. A breakdown in work packages of the work needed to get to that preliminary design document is presently being done. The project office is being staffed and it will start full time operations in September this year. By that time the work packages will be well defmed and assigned to the different members of the Project Office for inmediate action. A preliminary design review (PDR) will be held soon after the preliminary design document is delivered. The diagramme below shows the Project schedule. Note that the schedule is dictated by the civil Engineering and dome installation, as well as by the fabrication, including polishing, of the primary mirror optics. First light is planned for year SP/F Vo/. 2871

5 Preliminary design and prototypes Civil engineering..: :. "!' - jjjj Dome Primaryoptics L j U Primary mirror cell and support ii : ii :..:::..: Telescope :.:: Control nstrumentation Operation Desn J' con$truc] Tests nstalation We expect to gather the rest of the funds through international partners interested in participating in the Telescope project. The 5 1% granted so far for the projects has come from the Central Government of Spain and the Local Government of the Canary slands. Potential partners are invited to join in the project where they will be expected to participate in all phases of the project, from the defmition stages through construction and operation of the telescope. This participation will also include participation in planning the actual instrumentation to be build and in instrument building itself. The partners will of course become integrated in the Project Office and in all decision making committees, through ways and means that will need to be defmed. 9. REFERENCES 1. J. Vernin and C. Mufioz-TuiiOn, "Optical seeing at La Palma Observatory.. General guidelines and preliminary results at the Nordic Optical Telescope", Astronomy andastrophysics, 257, (1992). 2. J. Vernin and C. Mufloz-Tuflón, "Optical seeing at La Palma Observatory.. ntensive site testing campaign at the Nordic Optical Telescope", Astronomy andastrophysics, 284, (1994). 3. J. Vernin and C. Mufloz-Tuflón, "Measuring astronomical seeing", Pub.Astron. Soc. Pacfc, 107, 265 (1995). SPE Vol