Published on: May 24, 2023

New metric- the root mean square (rms) granulation contrast

New metric- the root mean square (rms) granulation contrast

Why in news? Scientists from the Indian Institute of Astrophysics (IIA) have proposed a new metric, which can help quantify image quality of the Sun taken from ground-based telescopes.

Sun as focus of interest

  • According to the Ministry of Science and Technology, dynamic events like flares, prominences, and coronal mass ejections taking place on the surface of the Sun have made the solar body the focus of interest of 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.

Disadvantages of using telescopes on 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.
  • 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.

What can be done to overcome the disadvantage?

  • 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.
  • Scientists from IIA 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.

Simulations and Evaluation

  • To evaluate image quality, scientists conducted simulations comparing ideal conditions (no atmospheric turbulence) with perturbed images affected by atmospheric distortion. They also examined images after AO correction. These comparisons facilitated a thorough understanding of image quality and performance.

Efficiency Factors

  • Through their simulations, scientists computed efficiency factors for two crucial metrics: the Strehl ratio and contrast. The Strehl ratio exhibited an efficiency range of 40-55%, while the contrast achieved a lower bound of 50%. These factors provide valuable insights into the performance of solar telescopes and associated AO systems.