User:Søren Bertil F. Dorch/Proposed/High resolution groundbased observations of the sun

• Dr. Luc Rouppe van der Voort, Institute of Theoretical Astrophysics, Oslo

Through High resolution ground-based observations of the Sun detailed information on the solar atmosphere is obtained. Most commonly, high resolution refers to high spatial resolution, i.e., the ability to observe small-scale structures on the Sun. But also in other domains, like for example the temporal and spectral domains, high resolution is often required in order to obtain a deeper understanding of the physical processes in the solar atmosphere. Due to its proximity, the Sun is the only star for which the surface can be observed in reasonable detail. Therefore, high-resolution solar observations provide the unique possibility to study fundamental astrophysical processes like for example magnetic field generation, energy conversion and particle acceleration.

Achieving high spatial resolution

The main obstacle that impedes high spatial resolution observations from the ground is the disturbing effect of turbulence in the Earth atmosphere. This effect is commonly referred to as seeing. Several measures can be taken in order to reduce the effects of seeing: site selection, telescope design, adaptive optics and post-processing techniques. High-altitude sites are often found to be favorable in terms of seeing conditions mainly because of reduced air mass and therefore shorter path length through the atmosphere. A detailed study of the local weather conditions is required for any site selection. Volcanic mountain tops on isolated oceanic islands, like in the Hawaiian and Canarian archipelagos, are found to be particularly favorable because of the stable atmospheric conditions that are associated with the prevailing wind conditions. Solar telescopes are often built on towers with typical heights of more than 15 meters above the ground. Such towers raise the telescope above the layer that causes ground seeing, a low-lying layer with vivid turbulence that is induced by ground heating by the Sun. Further measures in telescope design include constructions that ensures an undisturbed air-flow around the telescope and painting the building and immediate surroundings with high-reflective paint to avoid heating of the parts that are exposed to sun light. Adaptive optics (AO) systems measure the amount of seeing-induced image deformation in real-time and operate fast enough in order to apply corrections to decrease the level of degradation.

Figure 1: AO-corrected (left) and uncorrected granulation images. This movie [[1]] shows the clear advantage of using adaptive optics. Observations obtained with the R.B. Dunn Solar Telescope of the National Solar Observatory at Sacramento Peak (NM). Image courtesy of Dr. T. Rimmele.

AO systems use active mirrors to provide corrections to the disturbed wavefront. Flat mirrors can be used to compensate for image motion ("tip-tilt mirrors"). Mirrors made of deformable materials can compensate for higher-order wavefront deformations. These corrections need to be performed at high rates as the seeing changes on a millisecond time-scale. Even though impressive results have been achieved using AO, the current systems often do not have sufficient bandwidth and only correct over a rather limited field of view.

In order to achieve high spatial resolution over larger areas and extended periods of time, image post-processing techniques are required. Typically, fast cameras are employed to acquire bursts of multiple images in short time. Later, numerical techniques are used to combine these images into a single image of superior quality.

Figure 2: High resolution image of the solar photosphere. Each black and white bar in the lower left corner represent 1000 km. The larger cells are convection cells or granulation, the small bright features between the granules are concentrations of strong magnetic field. The dark feature just above the center is a larger concentration of strong magnetic field and is called a micro-pore. For the acquisition of this image, adaptive optics and image post-processing were used. This image was obtained with the Swedish 1-meter Solar Telescope on La Palma (Spain).

All these measures combined enable present-day solar telescopes to reach the diffraction limit in spatial resolution, i.e, the maximum possible resolution that can be achieved with a telescope. For the current generation of telescopes, with apertures up to one meter, this means that an angular resolution of 0.1 arc second can be achieved, on the solar surface this translates to structures of smaller than 100 km that can be resolved.