New Prediction by the Rio Group (2008 Nov 29)


Another update calculated by Marcelo Assafin et the web page The possible Varuna stellar occultation of 6/7 December 2008 (Bruno Sicardy)


Update as of 29 November 2008, using a recent star position derived from 62 observations of the star at Pic du Midi (T1M François Colas, T60 Bernard Trégon), St Sulpice (T60, Bernard Christophe) and Sozzago (T40, Frederico Manzini) reduced by Marcelo Assafin et al. (Rio group):


 Prediction by Rio Group, 2008 Nov 29

RA= 07h 29mn 48. 1725sec        DE= +25d 40′ 07.740″       (J2000/UCAC2)

and the following offset for Varuna (from 32 images taken at Pic du Midi T1M):

off_RA = +42mas    off_DE = -194mas.

Formal errors on this prediction, taken from rms and number of observations, are about 25 mas, or +/- 800 km, as shown by the dotted lines.



Details about this prediction:



On Sat, 29 Nov 2008, Marcelo Assafin wrote:


  In the following, I give updated astrometric results for Varuna and the star, from reductions using the PRAIA package. The observational sources are (many thanks to these heroes):


 – François Colas, Pic-du-Midi, France (T1M), run from 20 to 28 November 2008

 – Bernard Trégon at Pic-du-Midi, France (T60), run 21 November 2008

 – Federico Manzini at Sozzago, Italy (A12), run 25 November 2008

 – Bernard Christophe at St-Sulpice, France (947), run 25 November 2008



   Note that all the observations from our earlier report were re-reduced, so that the results reported here include all the observations made so far in the recent effort to derive offsets for Varuna and for the star. Unfortunately I have not yet been able to access the 36cm-telescope images from Jose Luis. Therefore these images are not yet included in our analysis.


   Below we give the details of the astrometric reductions and results for the star and for Varuna.



   Star position


   As before, I take as reference the star position which has been used in the first plots of the occultation by Dave Herald:



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



   Individual CCD frame solutions were obtained using the UCAC2 catalog and a 3rd degree  polynom for the reductions of the large field CCDs. Typically more than 200 stars were used per field. Mean errors  in (RA,Dec) were about 50mas.


  Taking the individual solutions from the (T60), (A12) and (947) sets, the offset in the star positions wrt the reference position above was:




  off_ra = +15mas (16mas)  (n=10)

  off_de = +27mas (14mas)  (n=10)




  off_ra = +20mas (22mas)  (n=15)

  off_de = +24mas (22mas)  (n=15)




  off_ra = +20mas (15mas)  (n=37)

  off_de = +34mas (17mas)  (n=37)


 all telescopes together

  off_ra = +20mas (17mas)  (n=62)

  off_de = +30mas (18mas)  (n=62)




    In the global solution all large CCD frames were mixed and a 3rd degree polynom is used to place positions into the UCAC2 frame in the tangent plane. The star position from the global solution gives:




  off_ra = +19mas (17mas)  (n=62)

  off_de = +30mas (18mas)  (n=62)




       Varuna offsets


   Individual CCD frame solutions (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.


Considering these individual solutions, the offsets wrt the JPL ephemeris were:


T1M: (20-21 November)

  off_ra = +077mas (093mas)  (n=18)

  off_de = -170mas (103mas)  (n=18)



T1M: (26-27 November)

  off_ra = +052mas (169mas)  (n=08)

  off_de = -156mas (112mas)  (n=08)



T1M: (27-28 November)

  off_ra = +089mas (91mas)  (n=06)

  off_de = -191mas (44mas)  (n=06)



T1M: all nights together

  off_ra = +073mas (113mas)  (n=32)

  off_de = -170mas (096mas)  (n=32)



Note that the use of the improved 2MASS and the 3rd degree polynom here is an improvement over the former solutions with 1rst degree polynom and about 5 UCAC2 stars, in the individual solutions.



   In the global solution for Varuna, all the large CCD frames (referred to the UCAC2) were mixed with the T1M individual frames (referred to the improved 2MASS as described above), using a 3rd degree polynom to place the positions into the UCAC2 frame in the tangent plane. The offsets wrt the JPL ephemeris from the global solution gives:




  off_ra = +42mas (82mas)  (n=32)

  off_de = -194mas (69mas) (n=32)

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