Star Catalog Comparison 2016 IOTA Annual Meeting Steve Preston

Generic Star Catalog Six parameter solution for a star: Inputs Current survey positions (RA,DE) Historical positions from other catalogs (input catalogs) Outputs Mean position (RA, DE) Proper motion (pmRA, pmDE) Parallax Radial Velocity Uncertainty/sigma for each parameter Flags for issues (e.g. double stars)

Different Catalogs => Different Solutions Different Inputs Different survey equipment Resolution/FOV CCD attributes (e.g. saturation of bright stars). Ground based vs Satellite Different reference frame for individual image positions Different source catalogs for historical data Different Algorithms Different algorithms for “solving” inputs to yield a solution Different matching algorithms for source catalogs Different algorithms for handling double star situations Different algorithms for handling other issue (e.g. saturation)

Different Catalogs are NOT Independent Some catalogs use other catalogs as an input PPMXL uses UCAC2 as an input catalog The same input catalog (historical) may be used in many output catalogs Significant errors in one input catalog can effect multiple output catalogs

Comparing Star catalogs Know the general strengths and weaknesses of each catalog Magnitude range (UCAC becomes unreliable for stars brighter than m8) URAT1 proper motions are not very accurate. Normally I combine the UCAC4 proper motion with the URAT1 mean position. Occult’s Star Catalog Comparsion dialog does NOT provide this option. The Star Catalog Comparison shows only the URAT1 position based on the URAT1 proper motion. Know the specific attributes of each catalog What flags indicate issues? What other ancillary catalog data is important (quality of fit, # of images, etc.)

Comparing Star catalogs Primary goal – Pick the ONE best catalog for a star Formal error/uncertainty is key Watch for issues: Magnitude out of range for a catalog Quality of fit Possible double stars HIP – watch for large differences in proper motion when compared to UCAC4 Number of images. Some URAT1 entries have very few images.

Comparing Star catalogs AVOID “averaging” catalogs Don’t offset from the default track because several catalogs indicate an offset The “offset” catalogs may all have very large uncertainty The “offset” catalogs may all be “polluted” by poor data from any early input catalog RARELY If two catalog both contain equally good positions, averaging may be a good idea. As an example: URAT1 and HIP2 positions for a mag 8 star could yield rough

Comparing Star catalogs RARELY If two catalog both contain equally good positions, averaging may be a good idea. Example: URAT1 and HIP2 positions for a mag 8 star For a “clean” star in HIP2, the uncertainty can still be quite low at current epoch (time). URAT1 proper motions are high uncertainty. But current epoch position can be quite good when combined with UCAC4 proper motion. If HIP2 indicates a small parallax and the total uncertainties are similar, HIP2 and URAT1 could be considered equally good INDEPENDENT positions. In this case, it could make sense to average these two positions (HIP and URAT1+UCAC4).

Gaia Sep 2016 Gaia Data Release 1 catalog : ONLY Mean positions Excludes double stars and other “difficult” stars TGAS (Tycho-Gaia Astrometric Solution) Five parameter solution (no radial velocity) Many stars down to mag 12 Dave Herald’s Gaia catalog Gaia DR1 + UCAC4 proper motions Mag 14 limit

Gaia Benefits Star positions Hopefully Gaia will include a large percentage of the stars for asteroid occultations. Asteroid positions Short term Gaia data should yield significant improvement the accuracy of asteroid astrometry. Current epoch relative astrometry should yield significant improvement in path accuracy for a small number of events. Long term Gaia reference frame observations should improve overall accuracy of orbit fits. But… there is work to be done with the orbit fitting software.