Figure 5: Equivalent V surface brightness distribution.
It can be fitted by exponential law for disk, and
by gaussian law for central region (are shown by dashed line). The results
of fitting are shown by solid line.
Besides the detailed stellar photometry we have performed the surface B,V,R,I photometry of the galaxy. We determined the surface brightness profiles twice. First round, we calculated azimuthally averaged equivalent brightness profiles using the surface photometry routines developed at the Potsdam Astrophysical Institute. In this way we reduce the surface photometry to one-dimensional photometry. The equivalent light profile was used to extract the isophotal radii, mean surface brightnesses and to determine the best-fitting parameters of the particular density distribution models. In addition to the differential radial surface brightness profiles we calculated the growth curve of the galaxy by summing up the pixel values from center outwards in successive isophotes. The total magnitudes were estimated by asymptotic extrapolation of this radial growth curve.
In order to recover some information about the two-dimensional structure such as ellipticities and position angles of isophotes, in a second round of calculations we used an ellipse fitting algorithm, which is based on the formulas given in Bender & Mölenhoff (1987) in its realization in the SURPHOT package running within MIDAS.
As a result of the ellipse fitting we have obtained a set of radial profiles: surface brightness (SB), minor-to-major axis ratio (b/a), position angle (PA) - in B, V, R and I colours. Combining the B and V, and R and I surface brightness profiles, which were obtained with the one set of fitting ellipses applied to corresponding frames, the colour profile was constructed.
The resulting equivalent surface brightness distribution is shown in
Figure 5.
In both the V and I bands, the surface brightness distribution is
falls off as gaussian from 5" to a radius of
20", and
fairly flat from 20".
The equivalent V band surface
brightness can be well fit by a gaussian with central surface brightness
deviation 
in the inner regions,
and exponential profile with
central surface brightness and exponential scale
length 
at larger radii.
This gives an angular diameter at the 
isophote of
; this corresponds to an isophote of
.
Given the distance of the galaxy, the break in
the surface brightness profile occurs at 
pc.
Integrating the surface brightness profiles within the V and I
bands out to the 
isophote, we find
, and , 
uncorrected for
internal or galactic extinction. The quoted errors are an upper limit
computed assuming that every point in Fig. 3 is
systematically off by 1 
.
After correction for extinction (see §3.1) and considering the
error in the distance modulus, the total absolute magnitude of NGC 6789 is
and 
.
The integrated colour index (V-I)0 increases smoothly from
0.67 in the center of NGC 6789 up to 0.90
within the largest visible radii, which is comparable to the
colours of galaxy types Scd & Im ((V-I)=0.82) for the
Coleman et al. (1980) composite spectral energy distributions.