With the publication of the USNO-A2.0 (Monet et al. 1998), we have been
able to, for the first time, make the astrometric calibration of the CCD
frame directly, without using secondary catalogs. The USNO-A2.0 catalogue
contains more than 526 millions of stars, with magnitudes ranging between
7.5 and 21, referred to the ICRF. Its nominal position error is about 0
20.
The catalogue does not include proper motions and therefore position epochs
are given by the date when the plate was taken.
To do the astrometric calibration, the following procedure was employed
for every CCD. Preliminarly, any star within the CCD field for which the
centering error was larger than 0
05 had been removed. Then, using the
measured positions for the USNO-A2.0 stars in the CCD and the corresponding
catalogue positions for them, a four parameters fitting was adjusted. In
this way, the position of every star measured on the CCD frame was placed on
the ICRF system. As suggested by Monet et al. (1998), the step above was also
attempted using locally corrected portions of the USNO-A2.0 catalogue,
instead of getting the star positions directly from the catalogue. Three
different approaches were used to obtain the local corrections: a simple
translation by average, a first degree complete polynomial, and a third
degree complete polynomial. In all cases the correction was calculated
from the comparison between ACT positions, on the plates epoch, and the
USNO-A2.0 catalogue entries. There was no gain in adopting the corrected
USNO-A2.0 positions, and thus the original catalog positions were used
throughout.
Accordingly, the calculated star positions were checked against the catalog
ones. Also the calculated plate scale was compared with its nominal value,
being found that the difference was smaller than 2%. The calculated minus
catalog residuals in equatorial coordinates were, in general, smaller than
0
8 and the standard deviations
and
were smaller than
0
4. For the frames where one of these values was larger than the above
upper limits, a further reference star discarding procedure was adopted.
Trial reductions were made removing only one of the reference stars.
Therefore as many trial reductions as there were reference stars were made.
The reduction in which the average residual and the standard
deviations resulted largest indicated the reference star to be removed.
In half of the cases this procedure was repeated two or more times, till
the values fall below the upper limits.
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Figure 3:
CCD frame observed at 08/15/1997,
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In Fig. 3 a typical CCD frame with Phoebe is presented, to exemplify the
reduction procedure. The USNO-A2.0 reference stars are marked by a circle.
Those eliminated in the second fitting have cross superimposed. In this frame
there were 22 USNO-A2.0 stars, at the beginning. After the first fitting, the
following values were found:
for the plate scale,
for the
largest star residual and
and
for
and
.
Thus, to comply with the reduction criteria, further rounds
of reference stars discarding were made. After the last fitting, there
were 19 reference stars and the values found were:
per pixel for
the plate scale,
for the largest value of the residuals and
and
for
and
.
The residuals for the Phoebe
positions are (
)
and (
,
),
before and after the discarding of reference stars.
Along the different observing nights for the 5 missions, Phoebe was
imaged on fields of varying star density, as result the number of USNO-A2.0
varied to a large extent in different nights. The number of reference stars
in each frame, used in the final reduction, ranges from 5 to 19.
In Table 1
all observed positions of Phoebe are presented. The positions are referred
to the ICRS, thus tied to the equator and equinox of J2000. The instants of
observation are given in universal time. In Table 2 the normal places for
each of the twelve nights of observations are presented.
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