Last Update on Varuna Astrometry from the Rio Group

 

Marcelo Assafin (Rio group) gives the last updated astrometric results for Varuna and its star, from a new global reduction solution which incorporated those last 30 images observed at the T1M at Pic du Midi in December 7 to 8, kindly provided by Francois Colas.

Reductions were made using the PRAIA package. The relevant output files from the PRAIA reductions are attached for study. The star and Varuna results are in separated files.

PRAIA output file format

 

Result – Star

 

Result – Varuna

 

In general, some T1M observations are centered in Varuna/star, others are shifted in a corner-in-center fashion to allow for a larger field coverage for the global solution. The large field-of-view images come from the T60 (pic du Midi) and 40cm (A12, Federico) observations. Note that both the star and Varuna appeared together in the same CCD frame for a few fields in the December 8 images. One can easily see which are these images by taking a look at the Julian Date.

The offsets in the first 2 columns of the output file are given wrt the respective reference positions. For Varuna, it is the JPL ephemeris (targets file) and for the star, it is the usual position below:

 

RA =  7h 29m 48.171s
DEC=  +25dg 40′ 7.71″

As usual, as a first step before the global solution, the Individual CCD frame reductions (from T1M observations only) were made using as reference frame the 2MASS catalog transformed into the UCAC2 frame by the tangent plane technique (with a 3rd degree polynom). Using this improved 2MASS, the small CCD fields from the T1M observations could be reduced with a 3rd degree polynom. Typically about 15 reference stars were present, which is not ideal for 3rd degree reductions (N=20 or higher), but is nevertheless acceptable. Mean errors in (RA,Dec) were about 70mas. The individual reductions for the large field images were made as before, using the UCAC2 stars and a 3rd degree polynom.

In the global solution, as usual, all the individual T1M and large field frames are mixed, again using a 3rd degree polynom to place the positions into the UCAC2 frame in the tangent plane.

From the global solution, the offsets wrt the reference positions in the sense „PRAIA – reference position“ were:

Star:

DacosD = +30mas  (36mas)
Dd         = +37mas (22mas)
N= 82 frames

Varuna:

DacosD = +48mas (79mas)
Dd         = -191mas (67mas)
N = 37 frames

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