1 Introduction

A filter magnetograph is the more suitable instrument for observing rapid changes in the magnetic fields, associated with the solar dynamic phenomena compared to the spectrograph based magnetographs (Zirin 1995). The pre-flare syndrome and the flare activity is best observed by using the chromospheric H$\alpha$ line. The emergence of new magnetic flux is believed to be important for triggering most of the large solar flares (Heyvaerts et al. 1977; Canfield et al. 1993; Debi et al. 1998). These fluxes appear as pores and bipolar sunspots in photoheliograms and arch filament system (AFS) in H$\alpha$ filtergrams. Therefore it is important to have simultaneous or near simultaneous observations of the magnetic field, chromospheric and photospheric activities. The filter magnetographs operating at Marshall Space Flight center, Big Bear Solar Observatory (U.S.A.) and Huairou solar station (China) have shown the importance of these instruments in flare study. However, the birefringent filters used in these magnetographs can not be tuned rapidly to different wavelengths. Keeping in view of the above factors, we have designed and fabricated a voltage tunable LiNbO₃ Fabry-Perot filter based video magnetograph, which can obtain near simultaneous photospheric magnetograms, H$\alpha$ filtergrams and continuum images of an active region. Since these images are made by using the same imaging set-up, the problems of registration and inter calibration are minimized.

The Fabry-Perot (FP) etalons are extensively used in astronomy (Meaburn 1970; Vaughan 1989). The availability of high finesse FP etalons with large free spectral range made it possible to use them in solar observations (Rust et al. 1987; Bonaccini & Stauffer 1990; Debi et al. 1998). Compared to the birefringent Lyot filters which are extensively used in solar observations, the LiNbO₃ FP etalons have the added advantage of fast voltage tunability which is very convenient for multi-wavelength observations.

The USO video magnetograph (VMG) is the only such instrument operational in the Indian longitude and fills an important gap for continuous coverage of solar magnetic field observations. The solar video magnetograph operating at USO could provide valuable data for synoptic studies of evolution of active regions. In this paper we present the details of the instrument and examples of observations.