User manual

Positional cross-match
The positional cross-match board
lets you give two catalogues, as
well as the corresponding
locations (ra, dec). If there is no
ambiguity, column names will
have been selected automatically
by Aladin. It is then necessary to
give the upper limit on the offset,
or even the lower limit offset
between sources to be cross-
matched. It is given in arc
seconds. Finally, the selector gives
3 choices:
1. When several objects are in
the cross-match circle, only
the closest sources will be
cross-matched;
2. On the contrary, all
possibilities will be kept;
3. Another possibility, all the sources with no counterparts will be kept.
Field cross-match (cross-ID)
The cross-ID (or field cross-match) consists in the reunion of sources for which a value
of a field is identical in both catalogues. This cross-match type is dedicated to a field that
identifies each sources in a unique manner, e.g. an identification, a serial number, … In
data bases, this is called “joint”.
Ellipses cross-match
The board to control the ellipse cross-match is similar to the one for simple positional
correlation. The main difference is the possibility to specify not only the columns for the
position, but also the one for the errors on the position. This error should be expressed
with 3 fields that define an ellipse: major axis, minor axis, and position angle with
respect to the north.
Choosing the fields to keep
When Aladin performs a cross-match, a new catalogue plane is created for the results.
Each cross-match pair will correspond to a source in the resulting plane and by default;
all the measures for the two corresponding sources will be kept. To avoid possible
confusions on column names, they are automatically labelled with “_tab1” and “_tab2”
respectively.
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