On measurements of continuum flux in solar flares. Instrument and first results

For a broad-band diagnostics of chromospheric flare plasma is highly desirable to analyze spectra covering many spectral lines and various continuum features. The flare spectra in UV are well detected on the weak background radiation of the solar disk. Detection of flare continuum from the Sun-as-a-star in optical is much more difficult due to a strong background radiation. Knowledge of flare continuum flux or continuum intensity could be a clue parameter for contemporary models and role of accelerated particles. Supposing the flare/background radiation contrast is strong enough to be detected, we developed a device for measuring spectral flux from a selected part of the flaring region. Here we present a technical description of the post-focus instrument installed at the horizontal solar telescope HSFA2 of the Ondřejov observatory. It consists of a system of diaphragms, imaging Hα telescope and a fast spectrometer for measuring flux in 350 - 430 nm with dispersion of 3 px per Angstroem but with cadence reaching up to 50 frames per second. To measure both Balmer and Paschen continua simultaneously, a new spectrometer with even a lower dispersion was installed recently. The first solar flares observed by this novel technique provide quite interesting results which are presented.