V. Andretta 1ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

An assessment of the potential of Dome C for solar observations

Vincenzo Andretta

INAF/Osservatorio Astronomico di Capodimonte

Naples, Italy

V. Andretta 2ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Sources

A personal assessment, with material (and help...) from several sources, for instance:

– ARENA WG6 activities;

– Aristidi et al. (2009): TGL (Turbulent Ground Layer) properties;

Plus some info from:– Arnaud et al. (2009): SBM (Sky Brightness Monitor)

results.

V. Andretta 3ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

First things first: The seeing

DIMM data (Nice group):– Long data series (2003 → present)

– Taken at various heights above ground:● 3 m (ground level)● 8 m (Concordiastro platform)● 20 m (top of one of Concordia buildings)

– Results (in short): median seeing

@ 3 m @ 8 m

2.4" 1.7" Winter

0.95" 0.57" Summer

V. Andretta 4ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

First things first: The seeing

V. Andretta 5ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

What about solar elevation?

● Elevation <40° (z>50°);

V. Andretta 6ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

What about solar elevation?

● Elevation <40° (z>50°);● Duty cycle depends primarily on the miminum

acceptable elevation:

Min elev.: 15 deg.

Cou

rtes

y: C

. Den

ker

(AIP

)

V. Andretta 7ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

What about solar elevation?

DIMM measurements:– Canopus (α Car) → Zenith → Sun

– Model: one-parameter, scale-free Kolmogorov turbulence → ε ~ cos(z)^(-3/5):

V. Andretta 8ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

What about solar elevation?

V. Andretta 9ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Mimimum acceptable altitude

V. Andretta 10ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Comparison with other sites

However: These values are of little significance if not compared with other sites.

An example: The site chosen for the ATST (Advanced Technology Solar Telescope):

Haleakala, Hawaii

– Location: 20° 42' 17" N, 156° 10' 36" W

– Elevation: 3084 m

– Lake Area: None

– Distance to ocean: 11 km

– Estimated sunshine: 2800 hrs/year

V. Andretta 11ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Comparison with other sites

Goals set for the chosen site (from the ATST Site Survey Working Group Final Report, 2004):

● Clear daytime fraction of 70%, 3000 hours annual sunshine.

● 1800 annual hours with r0 (500 nm) > 7 cm (measured at the telescope aperture),

including at least 100 continuous 2-hr periods.

● 200 annual hours with r0 (500 nm) > 12 cm (measured at the telescope aperture)

including at least 10 continuous 4-hour periods.

● Large isoplanatic angle, i.e., good atmospheric conditions at high altitudes.

● 480 annual hours with a sky brightness less than or equal to 25 ± 10 millionths at 1.1 radii at 1 micron with a radial profile equal to or steeper than R-0.8, including at least 40 continuous 4-hour periods.

● 600 annual hours with the precipitable water vapor below 5 mm, including at least 40 continuous 4-hour periods.

V. Andretta 12ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Comparison with other sites

V. Andretta 13ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Comparison with other sites

Notes:– Best seeing in winter (October – February), and in

the morning (HA: -6 – -4 hrs) → z < 25°!

– And... at 28 m above ground...

V. Andretta 14ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

More on summer DIMM data

V. Andretta 15ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

The Turbulent Ground Layer

● The bulk contribution to seeing comes from the TGL

● Estimates of the thickness of the TGL:– (Winter) ~ 20 – 30 m

– (Summer) Up to 400 m?

● Above the TGL: – 0.36'' (possibly free-atmosphere seeing)

V. Andretta 16ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

The Turbulent Ground LayerT

rinquet et al. 2008 (SP

IE)

V. Andretta 17ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

A simple model

Assume:– Free-atmosphere seeing: ε=0.36'';

– A time-varying TGL, with ε=∞, practically;

– Telescope at height h above ground; h is such that instrument is above the TGL for 70% of the time;

– Cloud coverage probability of 15%.

V. Andretta 18ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

A simple model

Results:

V. Andretta 19ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

'Seeing' is not everything

● Isoplanatic angle, θ:– Degrades much more easily than seeing (θ ~

cos(z)^(8/5));

– Thus: θ may be not that good towards the Sun, even above the TGL;

– May seriously impact perspectives for AO (Adaptive Optics) systems.

V. Andretta 20ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

'Seeing' is not everything

● Isoplanatic angle, θ:

V. Andretta 21ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Sky brightness

● Very low, if first indications from SBM measurements are confirmed;

● Compared with other sites (again: Haleakala)...

V. Andretta 22ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Sky brightness

V. Andretta 23ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Science cases (examples)

● High angular resolution:– Long duration observations (compared to the

minutes/less than 1 hour in other sites);

– High duty cycle (can be as high as 700 hrs/year for r

0>10 cm);

– Small contamination from telluric absorption● High spatial/spectral resolution 2D spectroscopy →

atmospheric small-scale dynamics

V. Andretta 24ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Science cases (examples)

● Coronal observations:– Low sky brightness;

– Good/moderate seeing (even within TGL)

– Easy access to IR● high S/N line polarimetry → coronal magnetic fields

V. Andretta 25ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

About infrastuctures

● Small-size coronagraph, no AO;– Note: competitive even on a low-height platform

(e.g.: Concordiastro platform).

● Medium-size telescope with AO;– Note: high duty cicles comparable or greater than in

other sites, but only if telescope is placed fairly high above ground (above TGL, or almost...).

V. Andretta 26ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Still to be done

● A more quantitative characterization of the TGL:– Its time dependence (height, strength of turbulence);

● Answer for instance to a question: there are two reversal of the temperature gradient every day, in summer; only one of these reversal (at about 17 o'clock local time) corresponds to an actual dip on DIMM data.

● Dependence of seeing, isoplanatic angle, etc., with time, AT SOLAR ELEVATIONS ABOVE HORIZON.

V. Andretta 27ARENA Workshop: Progresses on site testing (Nice, Dec. 2009)

Still to be done

● To convince the solar community (and Polar Agencies...) that Dome C is worth the effort.

– A possible strategy: to proceed with installing a small(-ish) telescope (a coronagraph?) for science and to show off site characteristics...

● Historically: already proposed – CASTEL project: INAF/OAC, 2001, but not good enough for PNRA nor for INAF...