Warren Skidmore

Thirty Meter Telescope Site Testing I: Overview

As part of the conceptual and preliminary design processes of the Thirty Meter Telescope (TMT), the TMT site-testing team has spent the last five years measuring the atmospheric properties of five candidate mountains in North and South America with an unprecedented array of instrumentation. The site-testing period was preceded by several years of analyses selecting the five candidates: Cerros Tolar, Armazones and Tolonchar in northern Chile; San Pedro Martir in Baja California, Mexico; and the 13 North (13N) site on Mauna Kea, Hawaii. Site testing was concluded by the selection of two remaining sites for further consideration, Armazones and Mauna Kea 13N. It showed that all five candidates are excellent sites for an extremely large astronomical observatory and that none of the sites stands out as the obvious and only logical choice based on its combined properties. This is the first article in a series discussing the TMT site-testing project.

Thirty Meter Telescope Site Testing X: Precipitable Water Vapor

The results of the characterization of precipitable water vapor in the atmospheric column carried out in the context of identifying potential sites for the deployment of the Thirty Meter Telescope (TMT) are pre- sented. Prior to starting the dedicated field campaign to look for a suitable site for the TMT, candidate sites were selected based on a climatology report utilizing satellite data that considered water vapor as one of the study vari- ables. These candidate sites are all of tropical or subtropical location at geographic areas dominated by high-pressure systems. The results of the detailed on-site study, spanning a period of 4 yr, from early 2004 until the end of 2007, confirmed the global mean statistics provided in the previous reports based on satellite data, and also confirmed that all the candidate sites are exceptionally good for astronomy research. At the locations of these sites, the atmospheric conditions are such that the higher the elevation of the site, the drier it gets. However, the data analysis shows that during winter, San Pedro Martir, a site about 230 m lower in elevation than Armazones, is drier than the Armazones site. This finding is attributed to the fact that Earths atmosphere is largely unsaturated, leaving room for regional variability; it is useful in illustrating the relevance of in situ atmospheric studies for understanding the global and seasonal variability of potential sites for astronomy research. The results also show that winter and spring are the driest seasons at all of the tested sites, with Mauna Kea (in the northern hemisphere) and Tolonchar (in the southern hemisphere) being the tested sites with the lowest precipitable water vapor in the atmospheric column and the highest atmospheric transmission in the near and mid-infrared bands. This is the tenth article in a series discussing the TMT site-testing project.

Thirty Meter Telescope Site Testing VI: Turbulence Profiles

The results on the vertical distribution of optical turbulence above the five mountains which were investigated by the site testing for the Thirty Meter Telescope (TMT) are reported. On San Pedro Martir in Mexico; the 13 North site on Mauna Kea; and three mountains in northern Chile: Cerro Tolar, Cerro Armazones, and Cerro Tolonchar; MASS-DIMM turbulence profilers have been operated over at least two years. Acoustic turbulence profilers—SODARs—were also operated at these sites. The obtained turbulence profiles indicate that at all sites the lowest 200 m are the main source of the total seeing observed, with the Chilean sites showing a weaker ground layer than the other two sites. The two northern hemisphere sites have weaker turbulence at altitudes above 500 m, with 13N showing the weakest turbulence at 16 km, responsible for the large isoplanatic angle at this site. The influence of the jetstream and wind speeds close to the ground on the clear sky turbulence strength throughout the atmosphere are discussed, as well as seasonal and nocturnal variations. This is the sixth article in a series discussing the TMT site testing project.

Thirty Meter Telescope Site Testing V: Seeing and Isoplanatic Angle

In this article we present an analysis of the statistical and temporal properties of seeing and isoplanatic angle measurements obtained with combined Differential Image Motion Monitor (DIMM) and Multi-Aperture Scintillation Sensor (MASS) units at the Thirty Meter Telescope (TMT) candidate sites. For each of the five candidate sites we obtained multiyear, high-cadence, high-quality seeing measurements. These data allow for a broad and detailed analysis, giving us a good understanding of the characteristics of each of the sites. The overall seeing statistics for the five candidate sites are presented, broken into total seeing (measured by the DIMM), free-atmosphere seeing and isoplanatic angle (measured by the MASS), and ground-layer seeing (difference between the total and free-atmosphere seeing). We examine the statistical distributions of seeing measurements and investigate annual and nightly behavior. The properties of the seeing measurements are discussed in terms of the geography and meteorological conditions at each site. The temporal variability of the seeing measurements over timescales of minutes to hours is derived for each site. We find that each of the TMT candidate sites has its own strengths and weaknesses when compared against the other candidate sites. The results presented in this article form part of the full set of results that are used for the TMT site-selection process. This is the fifth article in a series discussing the TMT site-testing project.

High-accuracy differential image motion monitor measurements for the Thirty Meter Telescope site testing program

Differential image motion monitors (DIMMs) have become the industry standard for astronomical site characterization. The calibration of DIMMs is generally considered to be routine, but we show that particular care must be paid to this issue if high-accuracy measurements are to be achieved. In a side by side comparison of several DIMMs, we demonstrate that with proper care we can achieve an agreement between the seeing measurements of two DIMMS operating under the same conditions to better than +/-0.02 arc sec.

Four Years of Optical Turbulence Monitoring at the Cerro Tololo Inter-American Observatory (CTIO)

The optical turbulence conditions as measured between 2004 until end of 2008 above Cerro Tololo, their seasonal as well as nocturnal behavior are presented. A comparison with the MASS-DIMM system of the Thirty Meter Telescope site testing was conducted and identifies an artificially increased seeing component in the data collected by the CTIO DIMM system under northerly winds. Evidence is shown that this increased turbulence is caused by the telescope dome. A correction for this effect is attempted and applied to the CTIO DIMM data. The MASS data of this comparison campaign allow to set constraints on the general assumption of uniform turbulent layers above a site.

Polarimetric analysis of the Thirty Meter Telescope (TMT) for modeling instrumental polarization characteristics

The Thirty Meter Telescope (TMT) will be called upon to support a polarimetric observing capability. Many different observing programs covering a range of different science areas are being considered for the TMT and a model of the overall polarization characteristics is being developed. The instrument development program will provide a means for polarimetric instruments to be developed, however the telescope itself and the AO system must be able to support polarimetric instruments. As a first step to defining the necessary polarimetric technical requirements we have created an international working group to carry out a study in which technical and cost implications will be balanced with scientific impact; new requirements will be generated with supporting science cases. We present here initial results of the instrumental polarization sensitivity of TMT with NFIRAOS, the first-light adaptive optics system.

Study on the precision of the multiaperture scintillation sensor turbulence profiler (MASS) employed in the site testing campaign for the Thirty Meter Telescope

The multiaperture scintillation sensor (MASS) has become a device widely employed to measure the altitude distribution of atmospheric turbulence. An empirical study is reported that investigates the dependence of the MASS results on the knowledge of the instrumental parameters. Also, the results of a side-by-side comparison of two MASS instruments are presented, indicating that MASS instruments permit measurements of the integrated seeing to a precision better than 0.05 arc sec and of the individual turbulence layer strength C(n)(2)(h)dh to better than 10(-14) m(1/3).

FIREBALLS, FLARES, AND FLICKERING: A SEMIANALYTIC, LTE, EXPLOSIVE MODEL FROM ACCRETION DISKS TO SUPERNOVAE

We derive simple analytic expressions for the continuum light curves and spectra of flaring and flickering events that occur over a wide range of astrophysical systems. We compare these results to data taken from the cataclysmic variable SS Cygni and also from SN 1987A, deriving physical parameters for the material involved. Fits to the data indicate a nearly time-independent photospheric temperature arising from the strong temperature dependence of opacity when hydrogen is partially ionized.

Status of the Thirty Meter Telescope site selection program

The Thirty Meter Telescope (TMT) project has been collecting data on five candidate sites since 2003. This paper describes the site testing portion of the TMT site selection program and the process and standards employed by it. This includes descriptions of the candidate sites, the process by which they were identified, the site characterization instrument suite and its calibration and the available results, which will be published shortly.