The night of April 26 was chosen for the astrometric
part of this work, due to the good seeing value at Cerro La Silla
of far below one arcsecond.
Good seeing produces stable star traces of high quality, which enable
an accurate determination of the right ascension direction.
As it has been already mentioned, the angular separation (
) as
well as the position angle (
) for every binary was calculated
in the V filter only on frames taken in this night.
In order to define the right ascension direction on the
CCD chip, several trails of stars in M 16 open cluster were
taken during this night using the V filter.
Obviously, all the CCD surface was used for taking these traces.
In an earlier work,
Sinachopoulos (1988) presented the technique of star
traces for CCD cameras. Using several star trails per frame on six
different frames taken one after the other, we determined the zero point
of the position angle with an accuracy of 
degrees.
In order to estimate the scale of the CCD we used several stars on
images of M 16 taken in the same night of April 26
through the V filter as well.
A detailed astrometric work on this cluster has been published in
the paper of Tucholke et al. (1986).
According to it the typical internal
errors of 
and 
positions are both 
.
The minimum accuracy (for 
) in the position of one star,
is
Since the error in the angular separation of two stars of the
M 16 cluster depends on the accuracy in the position of each one of
them, the standard deviation will be
According to the same paper, the mean error in the
proper motion for the stars observed is 
in both
components. Since the difference of time between the epoch of the
observations and the epoch to which the paper refers is 
, the error in both components
is
and as we are interested in the angular separation of
two stars, the error due to the proper motion uncertainty
will be
Moreover the error in the difference in the position of two stars does
not depend on the error in the proper motion, the standard
deviation in the angular separation will be
We used several star pairs in M16 having
angular separation around 600 pixels on the CCD chip,
in order to determine the
telescope scale with high accuracy.
The error we derived for the scale factor in that way is
By comparing the positions of the
stars detected by DAOPHOT with the values presented in the paper
mentioned above we calculated the scale factor of our optical system,
which was 
(
). This
agrees with the scale calculated for this telescope by van Dessel
& Sinachopoulos (1993).
For the error of the angular separation 
due to the atmospheric refraction we took
. However, this is a rather poor assumption, since
this error can be up to a few hundredths of an arcsecond for
observations made at airmasses 
if the V filter is used and even higher for the B and U ones.
Nevertheless, 
was
never the case of our astrometric observations, as they
carried out near minimum possible airmasses (usually 
).
A discussion of the calculation of the final accuracy achieved in the angular separation was presented by Nakos et al. (1995) and a detailed one by Sinachopoulos & Seggewiss (1990).
Although the method of star traces was used for the determination of the right ascencion direction, the good quality of the M 16 astrometry would enable us to use it for this purpose as well. A comparison of the two methods would be very interesting.
Table 1 contains the astrometric results. The first column is an
enumeration of the visual binaries. The next four columns contain the
coordinates of the primaries for epoch 2000. Next column contains the
Durchmusterung designation of the
primaries. The next four columns contain the angular separation
(
), its accuracy (
) in arcsecs and the position
angle (
) with its accuracy (
) in degrees. The
last column contains the epoch of the observations.
It must be taken into account that the very small values of
errors are due to the assumption of 
. Realistic values of 
are probably one
order of magnitude higher.