TIC2 - Tycho-2 - Hipparcos Look-Up-Table
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This file describes the format and contents of the look-up-table (LUT)
that connects TIC2 with Tycho-2 and with Hipparcos.

The original file has 11 columns (described below) and 3,332,391 rows
(one header and 3,332,390 data lines). The file is a plain, ASCII CSV
file with comma as field separator. Missing data is denoted by
blanks. Note that sources exist, for instance IDkey = 28640 (TIC2ID =
2864) or IDkey = 28650 (TIC2ID = 2865), that seemingly have no
information, except that these sources have Tycho counts available.
Note also that some records refer to the same target, e.g., TYC2
9504-366-1 and 9504-366-2 (IDkey = 3021 and 3022) both refer to TIC2ID
302.

During ingestion on the TAP server, the fields TIC2 and TYC have been
added, leading to 13 columns.

The columns denote:

"IDkey" [integer]: a unique key composed of ten times the TIC2ID (see
below) plus zero if no Tycho-2 star was derived; plus one for the
first Tycho-2 star derived; and plus two for the second Tycho-2
star. Each Tycho-2 star will have its own record.

"TIC2" [string]: TIC2_1-3 (TIC2 number).

"TIC2ID" [integer]: the running number [1,4351323] in the Second Tycho
Input Catalogue (TIC2). Only the 3,325,148 targets for which sorted
counts are preserved, are included.

"TIC2_1" [integer]: region number [1,9537] if the target is from TYC
or GSC, else blank.

"TIC2_2" [integer]: running number in region [1,12119] if the target
is from TYC or GSC, else blank.

"TIC2_3" [integer]: component number [0,2] if the target is from TYC
or GSC, else blank.

"TYC" [string]: TYC1-3 (TYC number).

"TYC1" [integer]: region number [1,9537] from the Tycho-2 Catalogue
for the star derived from this target. TYC1 will coincide with TIC2_1
if TIC2_1 is present.

"TYC2" [integer]: running number in region [1,12121] from the Tycho-2
Catalogue. TYC2 will differ from TIC2_2 in a few cases.

"TYC3" [integer]: component number [1,3] from the Tycho-2
Catalogue. TYC3 will often differ from TIC2_3.

"HIP" [integer]: The Hipparcos Catalogue identifier [1,120404]
according to the Tycho-2 Catalogue. Hipparcos stars that did not make
it to Tycho-2 will be present in this table but not identified as
such.

"CCDM" [string]: The component identifier from the Hipparcos Catalogue
according to the Tycho-2 Catalogue.

"Tyc2_srcs" [integer]: number of Tycho-2 stars derived from this
target.
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