We estimate the formation height range contributing to broad-band and
narrow-band filter imaging data in Ca II H to investigate whether spicules can
be detected in such observations at the centre of the solar disc. We apply
spectral filters of FWHMs from 0.03 nm to 1 nm to observed Ca line profiles to
simulate Ca imaging data. We estimate the relative intensity contributions of
off-limb and on-disc structures. We compare the synthetic Ca filter imaging
data with intensity maps of Ca spectra at different wavelengths and temperature
maps at different optical depths. We determine the intensity response function
for the wavelengths covered by the filters of different FWHM.
The intensity emitted off the solar limb is about 5% of the intensity at disc
centre. For a 0.3 nm-wide Ca II H filter, up to about 1/3 of the off-limb
intensity comes from emission in Hepsilon. On the disc, only about 15% of the
intensity transmitted through a broad-band filter comes from the line-core
region. No traces of elongated fibrillar structures are visible in imaging data
at disc centre, opposite to the line-core images of the Ca spectra. The
response function for a 0.3 nm-wide filter peaks at about 200 km. Relative
contributions from atmospheric layers above 800 km are about 10%. The inversion
results suggest that the slightly enhanced emission around the photospheric
magnetic network in broad-band Ca imaging data is caused by a thermal canopy at
a height of about 600 km. Broad-band Ca II H imaging data do not trace upper
chromospheric structures such as spicules in observations at the solar disc
because of the too small relative contribution of the line core to the total
wavelength-integrated filter intensity.Comment: 10 pages, 11 figures, accepted for publication in A&
