name:: apply ICRS corrections
query::
SELECT 
 u.raj2000+d_alpha+d_pmalpha/cos(radians(u.dej2000))*(u.epoch-2000) AS ra_icrs,
 u.dej2000+d_delta+d_pmdelta*(u.epoch-2000) AS de_cicrs,
 u.pmra+d_pmalpha AS pmra_icrs,
 u.pmde+d_pmdelta AS pmde_icrs,
 u.*
FROM
  TAP_UPLOAD.T1 AS u
  JOIN ucac3.icrscorr AS c
  ON (c.alpha=FLOOR(u.raj2000)+0.5 and c.delta=FLOOR(u.dej2000)+0.5)
description:: The tables :taptable:`ppmxl.usnocorr` and :taptable:`ucac3.icrscorr`
give mean corrections in 1-degree-sized squares in RA and Dec each to bring
positions (and possibly proper motions) based on USNO-B1 and UCAC3 (closer)
to ICRS.  This query shows how this is done (approximately; in reality, the
corrections would need to be applied in the tangetial plane).  The scenario
is that you have a table you can upload (i.e., a VOTable or something you
can load into TOPCAT).  As you can see, we assume the positions and proper
motions in your table are in the ``raj2000``, ``dej2000``, ``pmra``, and 
``pmde`` columns, and ``epoch`` contains the epoch the data has been reduced to
using the proper motions, in Julian years.

You will also have to adjust the string after ``TAP_UPLOAD`` according to
whatever name you give to your upload.  The query given would work when your
table is the first table within TOPCAT.

The equations are taken from the `service info`_; again, they are not exact
and will fail miserably near the poles, for epochs far away from 
J2000., and they do not take into account crossing the RA=0 line.

.. _service info: /ucac3/q/corr/info
..