1. Meteorological Site Evaluation and Forecasting needs for the Southern African Large Telescope (SALT) D. A. Erasmus Certified Consulting Meteorologist and C. A. van Staden South African Astronomical Observatory Correspondence: erasmus@saao.ac.za

2. What is SALT? SALT will be the Southern Hemisphere ‘twin’ of The Hobby-Eberly Telescope (HET) in Texas, USA SALT will be the largest single telescope in the Southern Hemisphere SALT is being built by an international consortium at Sutherland Observatory SALT will use a cost-effective and innovative mirror design with 91 hexagonal segments forming an array 11 meters across SALT will observe in the wavelength range: 340 nm to 2500 nm (ultraviolet to near infrared)

3. How is SALT progressing? SALT Webcam picture on: 4 th Sept. 2001 at 1:19 p.m.

4. Transparency: Cloud cover  optical, water vapour  IR (Observing quality: photometric, spectroscopic, unusable) Surface winds (structural considerations, operating thresholds) Temperature (thermal controls: design, operations) Turbulence (Image quality or “seeing”, adaptive optics, building height) Atmospheric Conditions Relevant to SALT Design, Site and Operations

5. Atmospheric Transparency and SALT Design SALT observing wavelengths (UV to near IR) were predetermined Site Predetermined that SALT would be located at Sutherland Observatory Operations SALT will employ a queue scheduling modus operandi Forecasts of observing conditions based on cloud cover and water vapour forecasts will be needed to optimise scheduling

6. ESO PWV and Cloud Cover Forecasts

7. Sample image forecast product

9. Forecast accuracy at Paranal

10. Surface wind speed and SALT Design SALT will operate in wind speeds up to 16.8 m/s Natural ventilation will be used at night to keep the telescope in thermal equilibrium with the environment Site Frequency of occurrence of winds above this threshold at Sutherland Observatory is unknown (only one year of automated weather station data) Operations Wind speed forecasts will be important for SALT operations

11. Surface temperature and SALT Design Mirror alignment is critically dependent on temperature Thermal imbalances inside the dome degrade seeing SALT dome will be air conditioned during day to match the expected temperature at start of observations Site Temperature change during the night at Sutherland Observatory is relatively large for a telescope site Operations Temperature forecasts will be essential to SALT operations 21h – 03h LST1 st QuartileMedian3 rd QuartileMax -dT/dt ( o C/h)0.20.50.81.45

12. Forecast methodology uses ECMWF model output in combination with in situ observations 14 days of in situ data are used to train a Kalman filter Kalman filter corrections are applied to the ECMWF forecasts

13. Kalman corrected ECMWF ParanalLa Silla Temperature ( o C) 0.971.12 Wind speed (m/s) 2.172.80 Mean absolute 12-hour forecast errors

14. Turbulence Telescope Mirror ΔT → Δρ → ΔN Turbulence creates small air pockets of different temperatures, hence densities Rapid small scale fluctuations in the refractive index of light occur and an aberrant light path through the atmosphere results Atmospheric Turbulence and “Seeing” (Image quality) Image motion and blurring occurs

15. Turbulence Telescope Mirror The apparent angular size of the object is a measure of the “seeing” quality Strong turbulence implies greater image motion and blurring, hence a large “seeing” angle Atmospheric Turbulence and “Seeing” (Image quality) Image motion and blurring occurs

16. Note the fast drift pattern from left to right and a slower drift pattern from top-left to lower-right. These are produced by turbulent layers at different altitudes being transported by winds from different directions Atmospheric Turbulence Effects

17. Theory The long-exposure image size (  ), FWHM, depends on the integral along the light path through the atmosphere of the refractive index structure parameter (C N 2 ),  = 5.35 -1/5 [  C N 2 (z) dz ] 3/5 (Radians)[1] where is the optical wavelength(m) and z is height(m). C N 2 is a function of C T 2 as follows: C N 2 = [(7.9x10 -5 P)/T 2 ] 2 C T 2 (m -2/3 )[2] where P is pressure (mb) and T is ambient temperature (K) and, C T 2 = /  x 2/3 ( o C 2 m -2/3 )[3] where x is a position vector, indicates a time average and the - 2 / 3 exponent is an artifact of Kolmogorov turbulence theory. Atmospheric Turbulence and “Seeing” (Image quality)

18. Atmospheric Turbulence and SALT Design Does the SALT image quality error budget match what the atmosphere will allow at Sutherland Observatory and vice versa? Will adaptive optics improve SALT image quality What is the optimal construction height for SALT? Site Are there local variations that would make one site better than others? Operations Forecasts of the C N 2 - height profile, of wind speed at the level of turbulent layers and total “seeing” will help to optimise telescope scheduling and use of adaptive optics systems

19. SALT image quality error budget 0.6 arcsecond for 50% enclosed energy (FWHM) Atmospheric Turbulence and SALT: Design

20. Adaptive optics makes good seeing better it does not make bad seeing good The successful application of adaptive optics depends on which turbulent layers dominate the seeing SCIDAR profiles of turbulent layers at the Sutherland Observatory Jet stream (15-20km) Tradewind/Westerly boundary (~3km) Boundary layer Height (km) → Atmospheric Turbulence and SALT: Design

21. When “seeing” is good, most of the time, the free atmosphere turbulence dominates Atmospheric Turbulence and SALT: Design

22. Turbulence near the ground and SALT construction height The shape of the thermal turbulence profile indicates that little benefit is gained by locating SALT more than 15m above the ground Atmospheric Turbulence and SALT: Design

23. Atmospheric Turbulence and SALT: Site Sutherland Observatory R Candidate SiteSite R S10.970.92 S20.960.95 S31.070.89 Wind direction weighted DIMM seeing at SALT candidate sites (Median in arcsecond)

24. Forecasting the C N 2 - height profile and total seeing This is a challenging undertaking Forecasts are based on simulation schemes that use height profiles of temperature and wind speed to model the C N 2 – height profile Valid simulations require high vertical resolution Synoptic scale forecast models (ECMWF, MRF) are inadequate MM5 run in high vertical resolution mode shows promise (Rick Knabb, see figure) Mauna Kea Observatory Hawaii Atmospheric Turbulence and SALT: Operations

25. Thank You