The observations were carried out at the Cassegrain focus of two
telescopes at the Observatório do Pico dos Dias (OPD) of the
Laboratório Nacional de Astrofísica - Brazil. About 70% of
these
observations were made at the 1.6 m Perkin-Elmer reflector for wich
the scale at the focal plane is 13''/mm and 30% at the 0.6 Zeiss
Jena reflector for wich the scale is 275/mm. The latitude of the
observatory is
and so Neptune's culmination
is near the zenith of those years. For details about the
telescopes we refer to Veiga et al. (1987).
For all of the 229 observation made at the 0.6 m telescope we used
the
CCD EEVP8603A which is an array with 385578 square pixels,
each measuring
m, which corresponds to
. The 530
observations made at the 1.6m telescope used the EEV
CCD-05-20-0-202
which has an array of 770
1152 square pixels with
m
or
(for 319 observations) and the CCD SITe SI003AB wich is a square
with 1024
1024 square pixel with
m corresponding to
(for 211
observations). No filter was used for most of the observations.
However, for 53 observations made in 1997, an Johnston R filter was used.
The exposure time varied from 1 to 5 seconds depending on the meteorological conditions and on the utilized devices.
We use the program ASTROL (Colas & Serrau 1993) to find the
centers of the planet, satellite and stellar images. For the
determination of each center we take a small area containing the
image and a bi-dimensional Gaussian is fit to this image.
The Gaussian is
added to a second degree polynomial as to remove the sky
background. The errors upon the centering procedure were for
Neptune
and
for Triton. For a discussion on the determination of
astrometric
centers see Veiga & Vieira Martins (1995).
For the astrometric calibration we made an adaptation of the method of the secondary catalog to the special conditions of the small CCD fields. This method, which is presented and tested in Vieira Martins et al. (1996) (see also Assafin et al. 1997a,b), consists on the setting of an astrometric catalogue for the stars on the CCD, using their images in the Digitized Sky Survey and the positions of nearby stars from the Guide Star Catalog corrected by the PPM Catalog.
To correct the systematic errors due to the color difference
between Neptune and Triton for the frames took without filter, we
computed the total astronomical refraction separately for Neptune
and Triton following the procedure presented in Paper I. In few
words, we considered a difference of for the planet and the
satellite refraction constants which corresponds to
after
the corrections due to the usual values for the pressure and the
temperature during our observations. This value corresponds to a
difference of
m between the effective wavelength observed
for Neptune and Triton. No correction was made for the observations
taken with the R filter.
In Table 1 (accessible in electronic form) we list the observed
positions of Triton relative to Neptune. The data are presented in
the following form: the first line gives the year, month and day
and decimal fractions of UTC days, corresponding to the mean
instant of the observation. In the next line we list the name of
the satellite followed by and
in arcseconds, referred to Neptune. The
reference system is referred to the equator and equinox of J2000.
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