SITE selection for a m telescope (ELT)

Transcription

1 SITE selection for a m telescope (ELT) Interim Report, June Alistair R. Walker (awalker@noao.edu) Sites WWW site: NIO WWW site: 1. Status of CY2000 and CY2001 deliverables 1.1 Cloud Cover Analysis (Chile) An analysis of Meteosat and Goes satellite (6 and 10 micron channels) data by Dr D.A. Erasmus is in progress, covering , and latitudes 20.5 to 30.5 S. This includes all the major Chile sites. This project has been facilitated by a data-sharing collaboration with ESO, and a data and results sharing collaboration with U. Tokyo, the latter who are proposing to build a 6-m class IR-optimized telescope near Chajnantor. The analysis will provide an 8-year baseline, long enough to cover the most recent El Niño and La Niña phenomena as well as several normal years. The study will identify the best sites based on lack of cloud, and on low water vapor, and compare them to the existing sites. The actual sites were chosen based on the Topographical analysis (3.2). STATUS: Contract completed 30 April 2001, presented in La Serena May Document A Satellite Survey of Cloud Cover and Water Vapor in Northern Chile, plus a CD-ROM with the individual plots, were the deliverables. Copies of the document were provided, with warnings not to make it publically available, to NIO, U. Tokyo, ESO, Cornell-Texas, and CELT. (pdf version on intranet site soon) 1.2 Topographical analysis (Chile) A set of maps of Northern Chile has been purchased in CY1999 from the Instituto Geographica Militar. These maps are at a scale of 1:250000, most areas also have 1:50000 available which will be purchased for potential sites, as will digital data from radar imagery. The maps will be studied in order to provide a list of possible mountains, using as criteria: distance from city lights and mining operations, altitude, relief, isolation from the surrounding landscape, and summit area size. Previous site survey information from the AURA, MMA/LSA, and ESO surveys will be gathered. A number of sites on the Coast range, the Domeyko range, and the Andes have been surveyed. Many of these sites have had a small amount of weather data that would be valuable for an initial idea of local wind speeds and diurnal temperature variations. STATUS: Completed by September See Ground to Boundary layer wind-modeling (Chile and Mauna Kea)

2 A NOAO-funded wind-flow modeling analysis of Chajnantor sites for Cornell. by Dr D. De Young (NOAO) will serve to characterize the wind flow patterns for the high altitude sites chosen by Cornell as possible locations for a large IR-optimized telescope. These analyses, based on radar imagery, will be invaluable in our own evaluation of these sites for the ELT, and as a model for analyses of other sites. See De Young & Charles (A.J., 110, 3107, 1995). STATUS: This work has gone very slowly due to other calls on D De Young's time. After discussions in April 2000, the initial analysis of the Cornell-specified regions was re-scheduled to be completed by July- August An analysis of the Gemini site on Mauna Kea is contained in the above reference. A similar analysis could be carried out for the Mauna Kea ELT site.. Due to the large amounts of human and computer time needed for these analyses, it is feasible to analyze only a few sites in this way. STATUS: Dave De Young has indicated that he would be able to do an analysis of this site in September The work should go quickly since the top of MK is already gridded at satisfactory resolution. 1.4 Initial Survey (Chile) A ground and/or air visual survey of the final sites needs to be made, to ascertain access, structure, and proximity to any mining activity. STATUS: This activity is being led by Bob Blum and consist of two parts: (a) A weather station has been installed on the Honar ridge at 5400m altitude in the Chajnantor science preserve, in October This work is in collaboration with Cornell, and also serves as a test-bed for our site-testing equipment. During the initial set-up, a DIMM was successfully operated. See (b) Via AOSS, Asesoria y Servicios de Ingenieria (ASI) have been asked to investigate the legal property and mineral rights status of two large regions containing six of the peaks determined to be the most promising according to PWV and lack of cloud. See Atmosphere structure The ELT will be a telescope critically dependent on Adaptive Optics in order to achieve its science goals. This will require detailed knowledge of the heights and strengths of the turbulent layers in the atmosphere, and their variability on time-scales ranging from hours to months. The vertical structure of the atmosphere can be directly measured using SCIDAR (Vernin et al. 1990, Radio Sci. 25, 953). Generalized SCIDAR (gscidar, Fuchs et al. 1994, Proc. SPIE Vol. 2222, 682) has extended the technique to also provide structure information below 1000-m. SCIDAR characterization is normally run in campaign mode, generally by the U. Nice group, since it requires use of a special camera on an already existing telescope of at least 1-m aperture. This is inappropriate for an extensive site-testing campaign. It appears possible to built a compact and relatively inexpensive instrument that achieves similar results, and we will initiate such a project in CY2001. ESO is also procuring an identical instrument, designed by A. Tokovinin, now at CTIO. It would be ideal to test such an instrument on MK, since the MM5 model meteorological predictions for MK include CN2. Whether a SCIDAR run, and/or direct measurements

3 using balloons are also needed for calibration is tbd, this could be carried out either at MK or Cerro Tololo, in principle. The anticipated improvements to the model resulting from this direct comparison should then give confidence that it can predict the atmospheric structure realistically. The model, in the low verticalresolution mode used for forecasting, provides an estimate for the integrated vertical turbulence, converted to a fwhm of a star image. The best way to compare this prediction to that actually observed is by use of a DIMM (Differential Image Motion Monitor, see STATUS: Procurement of Turbulence Profilometer, initiated Feb 2001, delivery Dec 2001 estimated. Project leader Andre Tokovinin. See Sodium Layer measurements The Imperial College group is conducting a campaign, jointly funded by NOAO ($25K) and Gemini, on Cerro Tololo in CTIO is providing scientific and technical support, CTIO lead is Brooke Gregory. STATUS: Underway, final run scheduled for Feb Site Test equipment We have used the opportunity to upgrade or install equipment (such as a DIMM) that are useful for site monitoring work. The basic workhorse equipment is a well-equipped weather station, and a DIMM. A turbulence profilometer is discussed above. We are also procuring micro thermal sensors (also useful inside domes!), a dust particle meter, etc. Equipment purchase began in CY1999. STATUS: This work is being supervised by Hugo Schwarz. Three stand-alone weather stations with wind, humidity, solar flux and temperature sensors have been assembled and tested. In October 2000 one of the stations was installed on Cerro Honar, in the Chajnantor region. Five portable DIMMs have been assembled. One is for KPNO. These are based on the ESO design, but use Meade telescopes and software ported and extended from that developed by U. Washington for the DIMM at the ARC telescope. One DIMM has been installed at Cerro Tololo, replacing the old Carnegie Seeing Monitor. A second, capable of operating completely remotely, is being tested on Tololo during winter 2000, and will be installed on Cerro Pachon in August-Spetember 2001, depending on weather. Various groups on MK have expressed interest on operating DIMMs on MK. There are at least three drivers for this: testing of extant facilities, eg performance of the Gemini dome under a variety of conditions; comparing existing sites; and testing the possible ELT site. STATUS: Two DIMMs have been purchased by CTIO and sent to MK (nominally these are SOAR DIMMS, on loan). Two lens caps are being fabricated, and two new wedge lenses will be diverted to MK, scheduled for early-july On MK the effort is being led by Mark Chun. 1.8 Other Collaborative Work

4 Brooke Gregory, Steve Strom and Alistair Walker will visit UNAM (Mexico City) to discuss collaborative efforts in site testing. The UNAM group have strong links to the U. Nice SCIDAR group, and have been characterizing the San Pedro Martir site in Baja California, as a prospective site for a large telescope. The CELT site group (chair, George Djorgovsky) desire to characterize a backup site for the CELT telescope; and are interested in analyzing SW USA and Mexico using the cloud data owned by CTIO/U. Tokyo. They have been quoted ($54K) by Andre Erasmus for this work, and intend to go-ahead. An accelerated study (9 months instead of 18) is an option for an extra $12.5K. A request for proposal to do an equivalent analysis for Mauna Kea has been made. CELT are interested in cost-sharing this with NIO. Contacts, formal and informal, with ESO, Cornell, U. Toyko, CELT, etc. STATUS: on-going. 2. Studies to be carried Out in CY Basic meteorological characterization (Chile) Weather stations will be installed at the sites for a few weeks or months.. The weather stations are standalone, with data downloaded every few weeks. Seeing measurements will either be carried out in campaign mode, or fully remotely, depending on the results of the 2001 tests on this equipment on Cerro Pachon.. The data will be compared to the telescope requirements and the remote-sensing predictions. STATUS: Preparatory Work, see 1.4 and 1.7 above 2.2 Climatology analysis (Chile and Mauna Kea) An analysis should be made high altitude wind (jet stream) behavior and other remote sensing products to determine general trends as function of latitude and longitude. The jet stream can play an important role by setting the sampling frequency of the AO system. This information is presently being archived for Chile and Mauna Kea.. (NB : Analysis of this data now regularly being done for MK?) STATUS: Andre Erasmus to prepare a proposal and quotation for this work. 2.3 Site quality and ownership (Chile) A preliminary geological survey will be carried out, in order to eliminate sites with unacceptable faults or rock quality. An initial title search for ownership and mining rights will be made, together with an evaluation of any mining activity in the vicinity of the site STATUS: See 1.4 above 2.4 Modeling (Chile)

5 Digital maps for each site will be obtained and wind model studies undertaken. These will be compared and calibrated with actual measurements. The same maps will be used to calculate sky brightness from the nearest population centers, with extrapolation into the future using growth models. In addition, detailed meteorological modeling of potential sites will be used to identify sites of low cloud cover, low water vapor, low winds, and potentially good seeing.. STATUS: Andre Erasmus to prepare quotation. UNAM group also have expertise in this area evidently. Await Profilometer Instrument will be tested in CY2002. Field version constructed if need be. 2.6 Collaborative Work Still to be defined; see 1.8 above. This could have a significant financial implication. 2.6 Deliverables at end of CY2002 By the end of CY2002 the following milestones will be completed:. a. Obtain in-situ weather measurements at candidate sites in Chile. b. Complete an initial geological survey of candidate Chilean sites. c. Refine candidate site list based on meteorological and climatological modeling of sites. d. Profilometer competed, other test equipment proven. 3. Studies to be carried out in CY2003+ With the results of the CY2002 studies, together with the scientific, structural, operational and financial constraints, the number of possible ELT sites will be pared down to 1-3. These will be tested extensively and compared to the already established Chilean sites and MK. Ideally tests should extend until construction of the facility commences. 3.1 Nighttime campaign of seeing measurements This will use the DIMMs. Results will be compared directly with identical or near-identical DIMMs on Cerro Tololo, Cerro Pachon, Las Campanas, La Silla, Cerro Paranal, and MK Simultaneous meteorological monitoring will continue. 3.2 Detailed analysis of the atmosphere We will characterize the vertical structure of the atmosphere in detail, in order to evaluate sites for AO. This has been described in some detail in 1.6 above, and we plan that by the time we are testing the prime Chile site(s) we have available equivalent measurements for Mauna Kea so that critical comparisons can be made. We will use the profilometer, balloon lauches, etc. The outer scale of the turbulence is anticipated to be of order the diameter of the telescope. This will play an important role in the design of the active and adaptive optics systems. For the primary site in Chile we will

6 monitor the outer scale of the turbulence either using a wavefront sensor on the end of the portable telescope, by coupling two DIMMs or by contracting with a group with a Generalized Seeing Monitor (Martin et al. 1994, A&AS, 108, 1). 3.3 Modeling of site wind-flow characteristics in detail (Chile and Mauna Kea) A detailed analysis will be made of the sites with particular attention on the choice(s) of building structure and specific location. See De Young, AJ 112, 2896, CELT have interest in the MK study. 3.4 Property and Environmental issues (Chile and Mauna Kea) A detailed and discreet study of the property and mining rights will need to be made of these final Chilean sites. A full geological survey of the final site will be conducted. Site purchase may have to be made. The major issue on Mauna Kea is the environmental impact of an ELT. 3.5 Sodium layer monitoring The latitude dependence of the sodium column density is well known so we do not anticipate that sodium monitoring is required for many sites. Depending on the outcome of the sodium monitor design study in CY2001, we may need to monitor the sodium column density throughout the nighttime seeing campaign. 3.6 Collaborative Work To be defined 3.7 Deliverables at end of CY2003 By the end of CY2003 the following milestones will be completed: a. One year of night-time seeing campaign at the prime Chilean site(s) will have been completed, and analyzed b. Detailed wind flow modeling for building placement at sites. 4. External Inputs on Site Selection. 4.1 Facility Survivability wind, snow load, earthquakes Wind operational constraints Site size, geology 4.2 Costs Ownership, mining rights Summit accessibility Accessibility in general Altitude 4.3 Adaptive Optics

7 Laser guide stars and cloud cover Atmosphere turbulence profile 4.4 Science PWV, how important? Cloud cover Seeing, clarity Sky brightness