Instrument design for the observation of the sun at near IR wavelengths: (around 1.6 µ)

Abstract

The H minus ions prevalent in the sub-photospheric level are found to have minimum opacity at around 1.63 m which makes this wavelength suitable for sub-photospheric observations . Some previous observations by Foukal et al Wang et al and so on in that wavelength range inferred that some Near IR dark faculae do not show visible light analogs and many Near Infra Red (NIR) dark faculae show substantially lower dark contrasts in visible. So synoptic observations in NIR (1.6 m) is now perceived to be important in order to detect small structure dark regions which are not so prominent in visible, even though angular resolution in NIR is worse than in the visible due to Rayleigh criterion. For this purpose, first the central wavelength and passband of the observation have to be selected which satisfies both the science motive and also instrumentation aspects (such as enough SNR). Then accordingly, NIR filter and detector have to be selected. An InGaAs Linear Detector Array (LDA) has been selected and a camera is being developed as a part of this project. This project is in collaboration with SSIF, ISRO 1. A single board full fledged PCB is targeted which can accommodate the detector, interfacing electronics, and the ADC. The hardware comprises of the interfacing electronics with the detector, signal-conditioning circuits, and an ADC. The sampled values are transferred to the PC via Ethernet. These samples are collected by a software developed in Python. This software also serves as a front-end for controlling the functions of the camera such as the exposure time, speed and so on. So far, the PCB has been designed and the ADC has been characterized. Also, as a part of preliminary testing, the sampled data is transferred to the PC via RS232. The RS232 interface can be used for low speed data transfers or long exposure operations. However, in order to take full advantage of the speed of the detector, data should be transferred via ethernet. Data transfer via ethernet is partially completed and is under development. After the completion of the development, the camera along with the NIR filter can be placed behind a suitable telescope and synoptic observations can be carried out.