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Lessons learned from the Thirty Meter Telescope site testing
Tony Travouillon Sebastian Els, Angel Otarola, Reed Riddle, Matthias Schoeck, Warren Skidmore

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Introduction TMT and its Site testing. Important choice before going on site. Lesson learned from on-site work. Considerations during analysis of the data.

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Some Background about the TMT Site Testing
TMT is a 30m, segmented, RitcheyChretien telescope design. Field of view of 20' (15' unvignetted) MCAO with 2 deformable mirrors 3 first light instruments

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Some Background about the TMT Site Testing
Site testing started in 2004 and was concluded by the selection of Mauna Kea 13N in 2009 Minimum of 2 years of data at each site. Comprehensive measurement of turbulence, weather conditions, PWV and more. Data available for download at: sitedata.tmt.org
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Instruments & Parameters
Weather stations DIMM seeing monitors MASS turbulence profilers SODAR acoustic sounders IRMA mid-infrared radiometers ASCA Allsky cameras Particle sensors Sonic anemometers Simulations, satellite analysis temp, hum, wind, press, sol.rad, heat flux seeing, coh. time, basic photometry high-el. profiles, isopl. angle, coh. time 20 800m turb/wind profiles, coh.time PWV, atm. transparency Cloud statistics (incl. cirrus), light pollution Ground level dust particle count 7m wind, temperature, turbulence

Turbulence, weather, long baseline

Other considerations: Location, elevation, geology, access, cost of construction and operation, operation model, ...

TMT's candidate sites

Cerro Tolar (Coastal site, Chile) Cerro Armazones (Coastal site, Chile) Cerro Tolonchar (Isolated mountain, Chile) San Pedro Martir (Coastal site, Baja California) Mauna Kea 13N (Volcanic Island, Hawaii)

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Chile Sites - Locations

Tolar
La Silla Las Campanas AURA

ALMA
Santiago

100 km
Paranal
7

Tolonchar

Armazones

Tolar

Armazones

Tolonchar

San Pedro MАrtir

13 North Site

Mauna Kea 13N

Early choices 1: Strong pre-selection
Satellite study commissioned to narrow down the study to 5 sites All sites confirmed by in-situ study to be good site After confirmation from the ground, data useful for longer time scale study.
Clear (photometric) fraction for the observing night

A Satellite Survey of Cloud Cover and Water Vapor in the Southwesthern U.S.A. and Northern Mexico D Andre Erasmus, Ph.D.
Certified Consulting Meteorologist

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Early choices 2: Not an Engineering project.

"Buy, don't build" philosophy Focus on gathering data and build up statistics Verification of instrument specifications done in the field.

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Early choices 3: Standardization

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Early choices 4: Redundancy
Spares of all instrumentation: Allows for quick replacement and minimizes down time. Instrumentation with overlapping use: Opportunity to cross check data and complete knowledge gaps. Applies to people too: 2 people with expertise of each instrument and analysis.

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Spend time on a cross-calibration campaign

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Cross calibrate everything

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Simultaneity of measurements

Multi-year campaign to avoid measuring in atypical weather conditions (el nino vs la nina, Bolivian winter, etc..) Maximize overlapping time of campaigns between sites (understand local effects and removes potential trends)

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Telescope choice

Open dome Design: minimize local turbulence Open tube: No tube seeing, smaller footprint Robust telescope: Observe in most wind conditions Stiff tower: Keep vibrations to a minimum Active focus: Allows better and remote alignment checks
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Hardware: Telescope choice

Open dome Design: minimize local turbulence Open tube: No tube seeing, smaller footprint Robust telescope: Observe in most wind conditions Stiff tower: Keep vibrations to a minimum Active focus: Allows better and remote alignment checks
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Hardware: Telescope choice

Open dome Design: minimize local turbulence Open tube: No tube seeing, smaller footprint Robust telescope: Observe in most wind conditions Stiff tower: Keep vibrations to a minimum Active focus: Allows better and remote alignment checks
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Hardware: Wind speed sensors

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Software
Fully automatic, robotic system, but constantly connected via Internet. Full manual remote operation and a lot of remote troubleshooting and control are possible

Example of the Site Status Overview web page

Software management and data access
Software updated on the central server and pushed to all sites to avoid discrepancies between sites Daily backup of data and logs to central server and backup server Database accessible online, formatted for quick re- analysis

Site 1 Central server Backup server
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Site 2 Site 3 Etc...
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Analysis coherence and verification

Analysis routine started early in the program. Each analysis done by two people. Allows verification of code and results but also highlighted different methods for the analysis and filtering of the data. Reports written with Matlab...

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Cloud Cover Definition
First idea: Use geometrical cloud coverage, averaged in both space and time. Problem: Not a useful quantity for TMT as illustrated using the following examples (see also the images on the next slides) Sky entirely covered first half of the night, entirely clear second half: 50% coverage Bands of clouds moving through every 15 minutes, with equally long clear gaps in between: 50% coverage Small broken clouds covering half of the sky at any time during the night: 50% coverage All these cases give the same geometrical coverage, but are very different in their usefulness for TMT. This is not sufficient. Need other definition of cloud cover.

Cloud Cover Definition
Geometrical coverage is not a useful definition. Other possibilities: Cut night into one (half, two, ...?) hour segments. Class segment as: Photometric: no clouds within 65o of zenith during entire hour Spectroscopic: thin or broken clouds present Unusable: continuous thick clouds Define a "cloud coherence time": Average time during which conditions remain constant Problem: obtaining a quantitative estimate Others?

ASCA Cloud Cover Analysis
So how do we do it in practice? Options: Visual analysis of ASCA images/movies to get cloud cover Pros: "Simple" method; easy to implement Cons: How quantitative is it? Somebody has to watch all the movies Automated photometry software package Pros: Quantitative analysis of cloud cover, time constant, transparency High time and spatial resolution opacity measurements Cons: Huge software development effort needed (not possible for us and LSST effort will probably not be useful for TMT)

Fin...

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Acknowledgement
The TMT Project gratefully acknowledges the support of the TMT partner institutions. They are the Association of Canadian Universities for Research in Astronomy (ACURA), the California Institute of Technology and the University of California. This work was supported as well by the Gordon and Betty Moore Foundation, the Canada Foundation for Innovation, the Ontario Ministry of Research and Innovation, the National Research Council of Canada, the Natural Sciences and Engineering Research Council of Canada, the British Columbia Knowledge Development Fund, the Association of Universities for Research in Astronomy (AURA) and the U.S. National Science Foundation.

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