Archiving; and Astrometric Positions

1. Archiving; and Astrometric Positions

2. Archiving • Important that our observations are Archived – data retention & availability • Currently Archived at NASA’s Small Bodies Node • Intend to also Archive at Centre de Données astronomiques de Strasbourg

3. Archiving Issues • Strict data consistency. The checks at archiving usually identify some data issues • One constraint with PDS – only use ASCII characters. So observer/location names with diacritical accents get rejected. • New version of Occult hopefully ensures smooth archiving

4. Archiving • Intend to also Archive at CDS • Last year, archived the Computer-readable version of the Watts Charts at CDS [cat. VI/122]. While the charts were digitised in the late ’60’s/early 70’s, they had never been archived! XZ80Q is also archived at CDS.

5. Astrometric Positions • Have been involved with reporting astrometry of comets to the MPC since 1972. • Several years ago, recognised that an occultation could give an astrometric position better than typical CCD positions • Changed Occult to generate an astrometric position

6. Astrometric Positions • Raised issue with Gareth Williams at the Minor Planet Workshop following the 2003 Sydney IAU General Assembly – very interested. • MPC created routines to read the data generated by Occult, to feed into their general routines

7. Astrometric Positions • MPC checks for errors by comparing against their other data. A number (>10) of events in initial batch that we considered reliable were clearly inconsistent with other data. This included the purported ‘first’ occultation observed in Malmo, Sweden. • As a result, I apply a policy of – if any doubt, astrometric position not reported. At this stage, this approach is critical to the acceptance of our results.

8. Astrometric Positions • MPC has since allocated Observatory Code 244 for Astrometry from Asteroidal occultations • MPC has established a reporting format for such results, based on the OCCULT output • Occultation positions are referenced to a single star. They have a HUGE advantage over CCD positions (which are effectively referenced to many stars) as they can be immediately updated when the GAIA results become available for each of the single stars.

9. Astrometric Positions • Well observed occultations – precision of around 100 to 200 µ-asec. [At this precision, relativistic bending of light by the sun is quite significant at 90 deg solar elongation!] • Observed by just a few observers, accuracy is typically a few masec. • Single chord observations – error is set as the apparent diameter of the asteroid

10. Astrometric Positions • Single-chord observations have the highest inherent uncertainty – no corroborative observations • Need to be able to confidently exclude other explanations for the star’s disappearance. Default position is to not report a position. But great majority of single chord observations are in fact reported.

11. Questions ?