Meteorological Site Evaluationand Forecasting needs for the

Southern African Large Telescope (SALT)

D. A. ErasmusCertified Consulting Meteorologist

and

C. A. van Staden

South African Astronomical Observatory

Correspondence: erasmus@saao.ac.za

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)

How is SALT progressing?

SALT Webcam picture on: 4th Sept. 2001 at 1:19 p.m.

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

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

ESO PWV and Cloud Cover Forecasts

Sample image forecast product

HUMIDITY AND CLOUD COVER FORECAST FOR LA SILLA, CHILE.

Forecast period starting at 09:00 on 2001.09.04 (day 247 of 2001). Forecast Period | HUMIDITY | CLOUD COVER (UTC) | Avg UTH(%) Sig UTH PWV(mm) | %Total %Opaque %Transp. T.Index 2001.09.04 09h | 20.2 1.8 4.58 | 0 0 0 1.00 2001.09.04 09h-12h | 19.0 2.0 4.44 | 0 0 0 1.00 2001.09.04 12h-15h | 22.5 3.2 4.66 | 0 0 0 1.00 2001.09.04 15h-18h | 34.0 5.0 5.54 | 0 0 0 1.00 2001.09.04 18h-21h | 37.8 3.0 5.86 | 0 0 0 1.00 2001.09.04 21h-00h | 33.8 3.1 5.68 | 0 0 0 1.00 2001.09.05 00h-03h | 41.7 2.7 6.47 | 0 0 0 1.00 2001.09.05 03h-06h | 46.2 2.0 6.97 | 7 0 7 1.00 2001.09.05 06h-09h | 47.4 1.5 7.19 | 31 0 31 0.99

Cloud Cover Category % Total cloud Clear < 25 Mostly Clear 25-50 Mostly Cloudy 50-75 Cloudy >75

Forecast Period %Hit %Neutral %Miss 00-03hrs 96.5 2.9 0.6 03-06hrs 94.6 3.4 2.0 06-09hrs 93.6 3.0 3.4 09-12hrs 92.8 3.0 4.2 12-15hrs 92.5 2.9 4.6 15-18hrs 92.0 2.8 5.2 18-21hrs 91.7 2.7 5.6 21-24hrs 91.8 2.5 5.7

0-24hrs Avg. 93.6 3.0 3.4

Forecast Period UTH (%) PWV (mm)

00-03hrs 2.4 0.34 03-06hrs 4.7 0.61 06-09hrs 6.2 0.68 09-12hrs 7.3 0.81 12-15hrs 8.1 0.90 15-18hrs 9.1 0.94 18-21hrs 9.8 1.10 21-24hrs 10.7 1.06

Forecast accuracy at Paranal

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

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 LST 1st Quartile Median 3rd Quartile Max

-dT/dt (oC/h) 0.2 0.5 0.8 1.45

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

Kalman corrected ECMWF

Paranal La Silla

Temperature (oC)

0.97 1.12

Wind speed (m/s)

2.17 2.80

Mean absolute 12-hour forecast errors

Turbulence

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

Turbulence

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

• 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

Theory

The long-exposure image size ( ), FWHM, depends on the integral along the light path through the atmosphere of the refractive index structure parameter (CN

2), = 5.35 -1/5 [ CN

2 (z) dz ]3/5 (Radians)[1]

where is the optical wavelength(m) and z is height(m). CN2 is a function of

CT2 as follows:

CN2 = [(7.9x10-5P)/T2]2 CT

2 (m-2/3)[2]

where P is pressure (mb) and T is ambient temperature (K) and,

CT2 = < [(T(x) - T(x+ x)]2> / x 2/3 (oC2m-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)

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 CN

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

Distribution of DIMM seeing values for the site survey period and the

period of the long-term record at Sutherland Observatory

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0.3 - 0.6 0.6 - 0.9 0.9 - 1.2 1.2 - 1.5 > 1.5

arcsecond

% F

requ

ency

Period: 4/94-2/98

Period: 1/99-3/00

SALT image quality error budget0.6 arcsecond for 50% enclosed energy (FWHM)

Atmospheric Turbulence and SALT: Design

• 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 layersat the Sutherland Observatory

Jet stream (15-20km)

Tradewind/Westerly boundary (~3km)

Boundary layer

Height (km) →

Atmospheric Turbulence and SALT: Design

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

Total Seeing (FWHM) (arcsecond)

Bou

ndar

y La

yer

See

ing

Con

trib

utio

n(a

rcse

cond

)

When “seeing” is good, most of the time, the free atmosphere turbulence dominates

Atmospheric Turbulence and SALT: Design

0

5

10

15

20

25

30

35

1.0E-05 1.0E-04 1.0E-03 1.0E-02

CT2

Hei

ght

(m)

30 min. Averages

30 min. Minima30 min. Maxima

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

Atmospheric Turbulence and SALT: Site

Sutherland Observatory

RCandidate Site Site R

S1 0.97 0.92

S2 0.96 0.95

S3 1.07 0.89

Wind direction weighted DIMM seeing at SALT candidate sites

(Median in arcsecond)

Forecasting the CN2 - 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 CN

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

HUMIDITY AND CLOUD COVER FORECAST FOR LA SILLA, CHILE.

Forecast period starting at 09:00 on 2001.09.04 (day 247 of 2001). Forecast Period | HUMIDITY | CLOUD COVER (UTC) | Avg UTH(%) Sig UTH PWV(mm) | %Total %Opaque %Transp. T.Index 2001.09.04 09h | 20.2 1.8 4.58 | 0 0 0 1.00 2001.09.04 09h-12h | 19.0 2.0 4.44 | 0 0 0 1.00 2001.09.04 12h-15h | 22.5 3.2 4.66 | 0 0 0 1.00 2001.09.04 15h-18h | 34.0 5.0 5.54 | 0 0 0 1.00 2001.09.04 18h-21h | 37.8 3.0 5.86 | 0 0 0 1.00 2001.09.04 21h-00h | 33.8 3.1 5.68 | 0 0 0 1.00 2001.09.05 00h-03h | 41.7 2.7 6.47 | 0 0 0 1.00 2001.09.05 03h-06h | 46.2 2.0 6.97 | 7 0 7 1.00 2001.09.05 06h-09h | 47.4 1.5 7.19 | 31 0 31 0.99

Thank You