Investigation of HeI 10 830 angstrom 'dark points' at the Sayan solar observatory and the Baikal Astrophysical Observatory

A new two-bandpass birefringent filter has been produced at ISTP, Irkutsk f or the investigation of the tint structure of the chromosphere. One tilter passband is centered on the He I 10 830 Angstrom line, the second one is ce ntered on H alpha. The FWHM of the He I 10 830 Angstrom passband is 0.46 An gstrom and of the H alpha passband is 0.3 Angstrom. A large number of filte rgrams were obtained with the filter at the Sayan observatory. At the same time. spectral observations with high spatial and spectral resolution were carried out by the large solar vacuum telescope at the Baikal Observatory. We selected 29 'dark point' spectra with sizes from 2" to 13". as well as ' dark points' on the filtergrams. Comparison of spectrograms and filtergrams has shown a good agreement of their size and intensity in relation with th e surrounding chromosphere as well as the absence of primary line-of-sight velocities in both observation types. From spectral observations. the depth of 10 830 Angstrom is over 30% for some 'dark points'. and the FWHM is mor e than 1 Angstrom. He I 10 830 Angstrom line profiles in 'dark points' are more deep and wide than in quiet regions. The optical depth of the chromosp here in 'dark points' is estimated. Comparison with the unperturbed chromos phere showed that 'dark points' in He 1 10 830 A are more optically thin th an the nearby chromosphere.