Scientists from the Indian Institute of Astrophysics (IIA) have devised a metric system, named the ‘root mean square’, that can help quantify the image quality of the Sun taken with ground-based telescopes.
Scientists Saraswathi Kalyani Subramanian and Sridharan Rengaswamy performed simulations of how an image of the Sun would look when there is no atmospheric turbulence and compared it to the image when there is an atmosphere.
Importance of this discovery:
Dynamic events like solar flares and coronal mass ejections make the Sun the focus of interest among astronomers. To know these features in greater detail, large telescopes are built. India too is building one — two-metre-long National Large Solar Telescope — at Merak in Ladakh. However, there is a big disadvantage when telescopes are built on the ground. The light from the Sun passes through the Earth’s atmosphere, which is not a homogenous medium. This causes the light to bend randomly and can be observed as the variation of intensity and position of the image on the detector. One way to overcome this is to use the AO (adaptive optics) system to correct distortions in real time.
But astronomers can’t quantify the performance of the AO system or quantitatively evaluate the quality of images taken from ground-based telescopes. The IIA scientists have therefore proposed the root mean square granulation contrast to quantify the quality of images taken with solar telescopes.
They considered telescope apertures that reflect the sizes of existing or planned solar telescopes in India and around the world, and determined the Strehl ratio (it is a measure of the quality of optical image formation) 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 the 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.