Waiting for Observation Reports, 2008 Dec 07, 09:55 UT
(20000) Varuna has now passed the target star and we are awaiting the observation reports.
These images by EUMETSAT show the cloud cover at 00:00 UT and 03:00 UT respectively at the most promising observing places.
Southern Africa:
Brazil:
Andrew Elliot from U.K. has sent a report to Planoccult. He tried the observation:
“Tonight I attempted the (20000) Varuna occultation ‘just in case…’, and because the sky was totally clear.
The target star (Mr 14.2) was easily visible using a Watec 120N with 16-frame integrations (0.64 sec/frame), + 406mm S-C at f/10.Unfortunately, the recording had to be abandoned around the time of the expected occultation due to ice forming on *both* sides of the Schmidt corrector plate (despite a dew zapper)!At least this was ‘proof of concept’ for future faint star occultations.”
This map is the same as in message “Maps of Potential Observing Stations and Finder Charts“ (Dec 02 at the blog), but with “Red photographic magnitudes” added for several stars.
The prediction (2008 Nov 26) following the results taken from the analysis of 101 frames obtained at the Lowell 42-inch Telescope, du Pont Telescope, SMARTS 0.9m Telescope, and Swope Telescope.
There is no major path shift compared to the predictions by the Rio Group or Raoul Behrend.
This update uses most recent astrometry from Marcelo Assafin (Rio Group).
Steve has set the uncertainty larger than the formal uncertainty.
With the relatively small number of observations and the strong weighting for this recent astrometry he was reluctant to trust the formal uncertainties of the orbit fit.
The Probabilities of Positive Detection of Occultation for Stations at the Southern Hemisphere Based on Latest Predictions
Jean Lecacheux (IMCCE, France) wrote:
“The updated nominal path is shifted by 0″.2 or 6000 km south -unfortunately for all the European observers-, and now is predicted from the south tip of South-Africa (near 02:10 UT in early dawn) to south Brazil (near 02:15 UT at low elevation).
In the same time the “one sigma” uncertainty was lowered to 0.12 Re (i.e. ten times better than before the new astrometry), whilst in consequence the probability of positive detection was boosted greatly.
Now it could reach up to 44 % from Porto Alegre (south Brazil), 40 % from Sao Paulo (Brazil), 37 %from Cape Town (South-Africa), 14 % from Windhoek (Namibia), 8 % fromRecife (nordeste, Brazil), 1.5 % from Belem (Brazil) at the Amazon mouth…, and will be fully negligible from any place in the northern hemisphere.
Never any occultation by a Kuiper body, Pluto excepted, reached a likehood level so high.”
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):
and the following offset for Varuna (from 32 images taken at Pic du Midi T1M):
off_RA = +42masoff_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 48.171sDEC=+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 errorsin (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:
T60:
off_ra = +15mas (16mas)(n=10)
off_de = +27mas (14mas)(n=10)
A12:
off_ra = +20mas (22mas)(n=15)
off_de = +24mas (22mas)(n=15)
947:
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:
global:
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:
Felipe B. Ribas is starting to contact some stations in Brazil from amateurs which could try to observe the possible event (Sao Paulo, Brasilia city regions).
The event is too low for the LNA observatory at Itajuba (IAU 874), with h=19 degrees, but we are in touch with the technical personnel to see if, and how, we can manage to observe the event with their 60cm automated telescope.
Prediction using the UCAC2 star position and NO offset on Varuna ephemeris
The following two predictions using the same data obtained from observations made at Pic du Midi:
2.
Update as of 25 November 2008, using a recent star position derived from observations made at Pic du Midi, reduced by Marcelo Assafin et al. (Rio group) and an offset for Varuna (18 positions from the Pic du Midi observations):
Star position: RA= 07h 29mn 48.1720secDE= +25d 40′ 07.741″(J2000/UCAC2)
Varuna offset: off_RA = +94mas ,off_DE = -279mas
The dotted lines are typical errors on path, roughly +/-2000 km, or about +/-65 mas.
3.
Update 27 November 2008, using a recent star position and an Varuna offset derived from the same observations, but reduced by Raoul Behrend (Geneva):
Star position: RA= 07h 29mn 48.1720secDE= +25d 40′ 07.748″ (J2000/UCAC2)
Varuna offset: off_RA = +23mas,off_DE = -185mas
How to read the maps:
Black lines: Varuna shadow, dotted lines: typical uncertainty band. The dots on the centre line are plotted every min, shadows move from right to left. The larger dot is geocentric closest approach, occurring at the UT time indicated at the bottom of the figure.
Comparing the second and the third maps with the first, the typical errors on the path are much smaller now.
Preparing the Varuna Event – Status Report, 2008 Nov 27, 22:45 U.T.
Steve Preston’s new prediction (Nov 25) is based on a recent star position and the position of (20000) Varuna derived from observations made at Pic du Midi, reduced by Marcelo Assafin et al.
Steve gave strong weighting to the Pic du Midi observations at his calculations.
Bruno Sicardy (Observatoire de Paris, LESIA) states that the Varuna event should be prepared and monitored for several reasons anyway:
- to get a feeling of SNR’s obtained by the various people interested in future events of this kind.
- to have a rehearsal, as many things can go wrong in this kind of time critical observations.
- for the larger telescopes, get a better astrometry when Varuna is very close to the UCAC candidate star. Probably a relative accuracy of 20 mas or better might be obtained. The obtained, accurate offset can then be stored for future monitoring on how Varuna’s offset behaves with time.
- who knows, the astrometry can still change…
At the moment we are searching for potential observers in the southern hemisphere (e.g. Brazil, Namibia, South Africa).
There is still hope to catch the shadow of Varuna…
