  Accurate knowledge of the spectral transmission profile of a Lyot filter is
important, in particular in comparing observations with simulated data. The
paper summarizes available facts about the transmission profile of the DOT
H$\alpha$ Lyot filter pointing to a discrepancy between sidelobe-free
Gaussian-like profile measured spectroscopically and signatures of possible
leakage of parasitic continuum light in DOT H$\alpha$ images. We compute
wing-to-center intensity ratios resulting from convolutions of Gaussian and
square of the sinc function with the H$\alpha$ atlas profile and compare them
with the ratios derived from observations of the quiet Sun chromosphere at disk
center. We interpret discrepancies between the anticipated and observed ratios
and the sharp limb visible in the DOT H$\alpha$ image as an indication of
possible leakage of parasitic continuum light. A method suggested here can be
applied also to indirect testing of transmission profiles of other Lyot
filters. We suggest two theoretical transmission profiles of the DOT H$\alpha$
Lyot filter which should be considered as the best available approximations.
Conclusive answer can only be given by spectroscopic re-measurement of the
filter.
