How far have we peered into the Sun, our nearest star? A new metric proposed by scientists may help quantify the quality of images of the Sun taken from ground-based telescopes.
Dynamic events such as eruptions, bulges and coronal mass ejections occurring on the surface of the Sun have made the Sun the focus of our astronomers’ attention because it is the closest star, it can be studied in great detail and the properties of other stars can be extrapolated by understanding the Sun.
In order to make out even the smallest objects in more detail, large telescopes are built – one of them, the 2 m National Large Solar Telescope (NLST) at Merak, discussed by the Indian Institute of Astrophysics (IIA).
However, there is a big disadvantage when the telescopes are on the ground. Light from the Sun passes through the Earth’s atmosphere, which is not a homogeneous environment. There is a random variation in temperature that leads to a variation in the refractive index.
This causes the light to bend randomly and can be observed as a change in intensity (scintillation/flickering) 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 atmospheric distortions in real time.
Quantify the performance of AO system
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 images obtained from ground-based telescopes cannot be quantified by the Strehl ratio or other metrics used directly for nighttime astronomical telescopes.
Scientists at the IIA, an autonomous institute of the Ministry of Science and Technology, have proposed using a new metric called root-mean-square (rms) graining contrast to quantify the image quality of ground-based solar telescopes.
Using theories that can be used to explain turbulence caused by the atmosphere, scientists Saraswathi Kalyani Subramanian and Sridharan Rengaswamy ran simulations of what the image would look like when there was no atmospheric turbulence (the ideal case) and compared them to an image when there is an atmosphere (disturbed image) and when AO correction is performed.
They considered telescope apertures (D), reflecting the sizes of existing or planned solar telescopes in India and around the world, and determined the Strehl ratio and grain contrast for various combinations of their input parameters. Since this is a simulation, the Strehl ratio can be easily determined, while it cannot be easily determined in a practical system.
By comparing the results of idealistic simulations with practical systems, they calculated 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 characterizing the performance of any solar telescope and associated AO system.
Read Now:Dr. Jitendra Singh reveling that 231 stolen antiquities have been brought back to India
