UCAC4 errors

[Demonperformer]

The UCAC4 download includes a file "u4xtycho", which equates ucac4 numbers with their tycho equivalent. Within this file there are 157 Tycho numbers that are each equated to two or more widely separated stars in the UCAC4 catalogue. A total of 1044 stars are affected by this (including the 157 stars that are correctly identified). This can be demonstrated by by performing a search on one of these tycho numbers in the "Tycho 2" field of of the ucac4 database on vizier. The attached screenprint shows partial results of one such search. Note: if you enter the tycho number into the search field at the top of the page, it only produces one result, because this is deciphering the position of the tycho number from SIMBAD and using that position (rather than the tycho number itself) as the search criterion.

There is one star (UCAC4 198-061713) which appears to have been manually amended on Vizier, but this now shows no tycho number (H=0). As can be seen from my table, there is a star of roughly the right brightness in the tycho catalogue (TYC 8222-00059-1) at a distance of less than 0.1" from the UCAC4 position, which I believe to be the correct cross-identification.

For each of the 1044 stars, the J2000 position was used in the search field on the Tycho2 catalogue on Vizier, using a 2.0" radius. In all but four cases, this yielded a single result, most of these (1025/1044) being within 1.0" of the calculated position.

The attached file "p-uc.txt" provides a fixed-width-column table of these 1044 stars, with suggested correct identifications. The file "readme.txt" provides details of the what the columns represent. I have sent a copy of this data to USNO, so it is always possible that they will correct the info on Vizier. In the mean time, feel free to make any use of this data in any way you wish.

Thanks.

p-uc.txt

readme.txt