Plane grating as polar heliostat in eclipse observations of the chromosphere and prominences with special application to studies of Doppler-widened Thomson-scattered Fraunhofer lines

Attention is drawn to a very simple instrumental arrangement for eclipse observations of the spectra of the chromosphere and prominences using a plane grating as polar heliostat and a short focus camera. A discussion is made of an infrared chromospheric spectrum showing traces of Thomson-scattered Ca ii absorption lines in the continuous spectrum. If the effect is real, it indicates a temperature of the order of 5000 K. A simple vector formula is presented describing various types of Doppler effects appearing in radiation scattered by moving particles.