The field of the galaxy M81 has been photographically observed
since the early times of stellar photography.
However, there exist not as many early plates for this field as
were found in the case of M51 (Paper I), and some of the oldest
M81 plates were not available to us for measurement.
The oldest observations that we use in this study originate from
the Greenwich part of the Carte du Ciel/Astrographic Catalogue project
(CdC/AC). We selected a sample of 5 fully or partly overlapping plates
(3 from AC, 2 from CdC) with epochs from 1894 to 1900
.
Other early observations come from a pair of plates taken with the
Heidelberg Bruce refractor in the year 1910. We also collected a
number of plates from intermediary epochs. They were taken at the
Observatories of Kiev, Herstmonceux, Tautenburg and Hoher List
and cover the time interval from 1957 to 1980. New plates were
taken in 1993 and 1994 at Tautenburg and Hoher List.
A full account of the plate material is given in Table 1 (click here).
Apart from the plates of the Tautenburg Schmidt, only plates
from telescopes of astrograph type are used.
The plate scale ranges from 40
to 100
per mm.
The field of view varies between 
and 
,
but the central field in which most of the plates overlap has
a size of about 
.
The limiting magnitude also differs among the plates.
In general, the short exposures have a limit of 120 while
the longer exposures reach at least 145. The short exposures
were included because they are important for establishing a
link to bright catalogue stars, in particular those of the Hipparcos
catalogue.
The older plates in our series (epochs before 1960) have been taken
without filter, yet the emulsions that were used are known to be
blue-sensitive. The latter is confirmed by the results of our photometry
(see Sect. 3.2). In order to avoid as far as possible systematic errors
from differences in the colours of the observations,
we chose only exposures in the blue passband also for later epochs.
The two V plates listed at the bottom of Table 1 (click here) served to accomplish
the photometry, but were not used for astrometric purposes.
All plates were measured on a PDS2020GM1.05explus microdensitometer at the Astronomical
Institute Münster in a similar procedure as described in
Paper I. The target list (total of 383 objects) was defined by objects
visible on the plates of the first epoch, that is the Greenwich
AC and CdC plates, and by a number of additional stars in the surroundings
of the central field, which are members of either PPM (Röser &
Bastian 1991) or HIC (Turon et al. 1992).
In each scan control measurements on the positional stability
of the machine were made. Also, some of the plates were measured twice
in different orientation and the results compared. These tests
confirmed that the measuring accuracy of the machine is at the level
of 1
or slightly below.