TMT.AOS.PRE.09.030.DRF01 1

Turbulence and wind speed profiles for simulating TMT AO performance

Tony Travouillon

M. Schoeck, S. Els, R. Riddle, W. Skidmore, B. Ellerbroek, G. Herriot

S. Els

On the menu today

What and how are we measuring.

The data available.

How to read this data.

How we are so far using it for AO simulations.

TMT.AOS.PRE.09.030.DRF01 2

S. Els

Measuring turbulence with a MASS-DIMM

TMT.AOS.PRE.09.030.DRF01 3

A. Tokovinin

La Silla

Las Campanas

AURA

Santiago

ArmazonesParanal

Tolar

ALMA

Tolonchar100 km

Where did we make those measurements

Tolar

Armazones from the air

The little white speck on the summit is the site testing telescope.

Tolonchar

San Pedro Mártir

MASS/DIMM telescope

A special care given to cross-calibration and error management

TMT.AOS.PRE.09.030.DRF01 10

Wind profiles

We use the NCEP/NCAR reanalysis data

Publicly available

Global grid (~250km resolution)

Data available every 6 hours

16 layers from 761m to 25km

Data verified against balloon measurements

We also have ground data taken with SODARs and weather stations

TMT.AOS.PRE.09.030.DRF01 11

Turbulence data available

TMT.AOS.PRE.09.030.DRF01 12

Huge database

Between 150,000 and 285,000 profiles per site.

Will be made public in the future.

How can that much data be useful to AO simulations?

AO simulations that are CPU intensive may not run every individual profiles

The difficulty we are dealing with here is the following: There is no such a thing as an average or typical profile

This difficulty comes from more the statistical nature of turbulence and is an issue on all time scales

“An average profile does not have an average seeing”

TMT has looked at several options

TMT.AOS.PRE.09.030.DRF01 13

Short and long term variations

TMT.AOS.PRE.09.030.DRF01 14

Median profiles

How do you select a median profile?

Median of each layer?

Selected around Median seeing?

Selected around median isoplanatic angle?

See Els et al. (2009) PASP for full details

TMT.AOS.PRE.09.030.DRF01 15

Median profiles

A solution considered for TMT simulation: Averaging profiles around the median open loop wavefront variance due to the combined effects of fitting and servo-lag error:

TMT.AOS.PRE.09.030.DRF01 16

dBG ffrd 33/5

02 //28.0

A new approach…creating a standard night

Auto-regressive model

Generate time series that reproduce the 1st and 2nd order temporal statistics of the log(seeing)

Uses all data to recreate a time series of seeing of manageable size for simulations

Driven by temporal autocorrelation vector and Gaussian Random number generator

TMT.AOS.PRE.09.030.DRF01 17

A new approach…creating a standard night

Keeps statistical characteristics of the site while reducing the number of profiles

Method to be presented at next OSA conference in September by Herriot et al.

TMT.AOS.PRE.09.030.DRF01 18

Conclusion

TMT has collected a high quality and statistically representative sample of turbulence parameters at 5 sites.

Database to be made public. High potential for AO simulations.

Lessons learned: When possible, run the simulation on all data and then calculate the statistics on the results.

For time intensive simulations, it is important to realize that there is no such a thing as an average profile. Simulations give noticeably different results depending on selection criteria.

TMT.AOS.PRE.09.030.DRF01 19