How far have we peered into the Sun, our closest star? A new metric proposed by scientists can help quantify image quality of the Sun taken from ground-based telescopes.

Dynamic events like flares, prominences, and Coronal Mass Ejections taking place on the surface of the Sun have made the Sun the focus of interest of our astronomers, being the closest star, it can be studied in great detail, and properties of other stars may be extrapolated by the understanding of the Sun. To resolve even the smallest features in greater detail, large telescopes are built-- one of them, the 2 m National Large Solar Telescope (NLST) at Merak, being deliberated by the Indian Institute of Astrophysics (IIA). 

However, there is a major disadvantage when the telescopes are on the ground. The light from the Sun passes through the Earth’s atmosphere, which is not a homogenous medium. There are random temperature fluctuations that lead to refractive index fluctuations. This causes the light to bend randomly and can be observed as the variation of intensity (scintillation/twinkling) and position of the image on the detector. One way to overcome this is to use an adaptive optics (AO) system to measure and correct for the distortions introduced by the atmosphere in real time.

But, how do we quantify the performance of our AO system or quantitatively evaluate the quality of images from ground-based telescopes? The quality of the images obtained from ground-based telescopes cannot be quantified with the Strehl ratio or other metrics used directly for nighttime astronomical telescopes.

Scientists from IIA, an autonomous institute of the Department of Science and Technology, have proposed to use a novel metric called the root mean square (rms) granulation contrast to quantify the image quality of ground-based solar telescopes.

Using theories that can be used to explain the turbulence introduced by the atmosphere, the scientists Saraswathi Kalyani Subramanian and Sridharan Rengaswamy performed simulations of how an image would look when there is no atmospheric turbulence (ideal case) and compared to the image when there is an atmosphere (perturbed image) and when AO correction is done.

They considered telescope apertures (D) that reflect the sizes of existing or planned Solar telescopes in India and around the world and determined the Strehl ratio and contrast of the granulation for various combinations of their input parameters. Since it is a simulation, the Strehl ratio can be easily determined, while in a practical system, it cannot be determined easily.

Comparing the results of the idealistic simulations to practical systems, they computed an efficiency factor deriving an efficiency of about 40 to 55% for Strehl ratio and about 50% as a lower bound for contrast. Their results will be useful in characterising the performance of any solar telescope and associated AO system.

Publication link: https://doi.org/10.1007/s11207-022-02105-2

Figure 1: Rms granulation contrast (in %) versus Strehl ratio for perturbed case. Markers represent the values of telescope diameter (in cm). Arrow on the left indicates better atmospheric conditions.

Figure 2: Rms contrast (in %) versus Strehl ratio after compensating for atmospheric turbulence. Markers represent the number of Zernike terms for which correction is done and the different colours represent the different telescope diameters (in cm) as shown right of the plot.