The two-dimensional velocity field of NGC 6181 was obtained at the 6m
telescope on September 24, 1993. The scanning Perot-Fabry
interferometer was installed inside of the pupil plane of a focal
reducer which was attached to the F/4 prime focus of the telescope.
An intensified photon counting system (IPCS) 
was used as
the detector. Instrument parameters are given in Table 2 (click here).
Table 2: Scanning Perot-Fabry observations parameters
The ``Image-interferometry method" was described in detail earlier
(Boulesteix et al. 1983; Amram et al. 1991). Observational data
(galaxy and wavelength calibration data) were converted into cubes of
32 images (
), with the linear scale being 0.70 arcsec/px
and a spectral resolution of about 2-2.5 channels (
).
A reduction of observational data (correction for phase shifting,
subtraction of night-sky emission spectrum, construction of velocity
map, a.s.o.) was done by using standard methods, and the Perot-Fabry
reducing software ADHOC developed at Marseille Observatory
(Boulesteix 1993) was used.
The continuum subtracted H
image of NGC 6181 (Fig. 1 (click here)) reveals that
the H
emission is rather strong over the whole galactic disk.
So the high quality of the two-dimensional velocity distribution obtained
allows to get more detailed information about gas motions in the disk
of this galaxy. A sub-cube (
),
containing the main fraction of H
emission from the galaxy,
was extracted from a reduced data cube. This smaller cube was used for
the analysis of the velocity field.
Figure 1: Monochromatic H
emission map of NCC 6181. The cross
marks the position of the nucleus
Table 3: Direct image observations
Direct images of NGC 6181 were obtained at the telescope Zeiss-1000
of SAO RAN. Eleven frames of the galaxy have been derived with a CCD
camera through the B, V, R and I filters of Johnson's system
(the log of the observations is given in Tab. 3 (click here)). The seeing quality
ranged from 2.1
(R images) to 2.8
(V and I
images).
Figure 2: The I and B images of NGC 6181. The frame
is 
, north is up and east is
to the left
Gray-scaled sky-subtracted I and B images are presented in
Fig. 2 (click here)a and 2b; flux-calibrated isophotes in B (
and 
for the innermost and the outermost isophotes
respectively with the step of 
) are shown in
Fig. 2c. The B and V frames were calibrated by using 14 aperture
photoelectric measurements of NGC 6181 from Burstein et al. (1987).
The range of aperture radii is from 16
to 48
.
Zero-point magnitudes are obtained with accuracy better than 0.01 mag,
color terms are found to be negligible. The sky brightnesses are
estimated as 
in V and 
in
B; these values coincide with mean sky brightnesses measured fifteen
years ago in the
Special Astrophysical Observatory at 
by Neizvestny (1981).
To check our BV calibration, we compared multi-aperture
photoelectric data for NGC 6181 taken from the catalogue of Longo &
Vaucouleurs (1983, 1985) (18 entities excluding old data of PET-54 and
BIG-51) with values simulated for the same apertures from our CCD
frames. In Fig. 3 (click here) one can see a rather good consistency with the
photoelectric data, even for large apertures. The calibration of R
and I frames was indirect and less precise because standard stars
were not observed. Instead B-V colors for five faint stars in the
field of the galaxy were measured, and adopting them to be dwarfs we
ascribe them mean V-R and V-I colors in accordance with their
spectral types (Straizys 1977). The formal accuracy of the
calibration constants determined in such a way is 0.12 mag, but we
admit a possible systematic shift of our R and I magnitudes by up
to 0.3 mag.
Figure 3: Comparison of published V and B multi-aperture photoelectric
data on NGC 6181 and present CCD observations