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Astron. Astrophys. 318, 631-638 (1997)

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2. Observations

The 1992 and 1993 observing campaigns (hereafter UESAC'92 and UESAC'93) were carried out in similar, but not identical, fashions. Each campaign covered a [FORMULA] region, centred on Jupiter. Nine plates, each covering 5°4 [FORMULA] 5°4, were exposed on each of three nights during one week in March 1992 and March 1993. Follow-up observations were carried out during the next new moon period in April of each year. The centres of the April fields were shifted about [FORMULA] to the west of the March fields to compensate for one month's motion of the main-belt objects. The 1992 March and 1993 March plates were exposed for 90 and 105 minutes, respectively. In the 1993 campaign the shutter was closed for 15 minutes after 60 minutes exposure, producing two trails of each object. This approach helped to distinguish between asteroids and stars since the short end of a star trail almost always points in the opposite direction from an asteroid trail. To optimize the limiting magnitude for objects in the neighbourhood of Jupiter, the plates were tracked at the apparent motion of the planet. The April plates and films were tracked at sidereal rate and exposed for 45 minutes. Seeing conditions were considerably better during the 1993 campaign, a fact which is partly reflected in the greater number of asteroids detected in that survey. We have adopted mean seeing disks of 1[FORMULA]5 for 1992 and 1[FORMULA]0 for 1993 (L95). Table 1 gives information on the number of detected trails in each campaign.


Table 1. Number of trails detected on the plates and films.

2.1. Plate reductions

The deduced J2000.0 topocentric astrometric positions are based on a reference frame defined by PPM and SAO reference stars (Röser and Bastian 1991). The uncertainties given in Table 2 are standard deviations of the comparison-star residuals (least-squares plate-solution position minus catalogued position). The three different sections refers to three concentric regions on the plates; the reductions were performed separately for each region. The number of included reference stars in the reductions are [FORMULA]. A more thorough description of the astrometry may be found in L95.


Table 2. Average quality of the plate reductions.

2.2. Linkage of moving objects

Initial linkage of objects observed on the March plates was performed by extrapolation of the motion vectors derived from the trails. Extension of the linkages to the April objects was more problematic: secure linkage of the March and April objects required observations on pairs of nights in each month. The 1993 campaign was planned with this requirement, but due to a malfunction of the plate vault at ESO, which destroyed most of the unexposed photographic plates, this could not be achieved. As a result, many of the month-to-month linkages were uncertain. Although great caution was exercised in making these linkages, a number have subsequently been shown to be incorrect, through the identification of observations of specific objects at other oppositions. In some cases, confirmation of the UESAC April observations was obtained from observations made by other observers-notably the Spacewatch telescope at Kitt Peak.

Failure to make linkages from March to April was also caused by objects moving out of the field covered in April or by objects being too faint to be observed one month past opposition. Table 3 gives the number of linked UESAC observations. The numbers in the [FORMULA]... [FORMULA] columns refers to objects with at least N linked UESAC observations. The numbers in the M-A column refers to observations of objects linked from March to April. The numbers in the [FORMULA] column do not include the unlinked April positions. The observations were reported to the Minor Planet Center where they were checked for identifications with known numbered or multiple-opposition unnumbered asteroids, as well as with new one-opposition objects. In addition, observations belonging to the outer Jovian satellites were identified (Hernius et al. 1996). The 2956 unidentified objects that had been observed on more than one night were assigned provisional designations.


Table 3. Number of linked UESAC observations.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998