Development of SHABAR for estimating atmospheric scintillation

Abstract

Electromagnetic radiation from Sun and stars suffers from the turbulence in earth atmosphere. The turbulence changes the complex refractive index of air which in turn modifies the electromagnetic signal. This condition is known as Seeing . Due to this, the images get blurred and the telescopes cannot achieve their maximum resolution.

SHABAR stands for SHAdow Band Ranger. It is well established that the scintillation of starlight is a measure of optical seeing in the atmosphere. Seykora in his 1993 paper demonstrated that the scintillation of sunlight is also closely correlated with seeing. Beckers (1993, 1997) showed that the pattern of decreasing cross-correlation with increasing separation measured by a linear array of scintillometers contains information of the atmospheric refractive index structure function C2n(h). This can be further used to calculate the Freid's parameter, leading to the estimation of seeing at the site which is used to characterize the astronomical site.

National Solar Observatory (NSO), Sacramento Peak, USA developed a SHABAR instrument which was later used by IIA for site characterization at Hanle. There is a need to develop more such units so that simultaneous measurements at different locations can be taken up.

The NSO SHABAR uses Measurement Computing’s PCI-DAS 1000 data acquisition board for digital conversion of the photo-diode signals. Therefore, in the NSO SHABAR instrument, the signal pre-processing and digital conversions are performed by separate cards. This adds not only to the cost, but also to the complexity of the hardware, as the PCI-DAS is proprietary. In our design, we have replaced this system with a Microchip PIC microcontroller. Similarly, in the software, the data acquisition is performed by PCI-DAS software. The data analysis for the calculation of Fried’s parameter is performed in IDL. His increases the software licensing costs as both the data acquisition and analysis softwares are proprietary.

The aim of this project is to study the NSO SHABAR instrument, understand its operation, and develop a new, less expensive SHABAR instrument. It has been found in this study, that the free and open-source softwares offer a feasible alternative to the proprietary software used in the instrument.