Retaining specified columns

If you choose to retain in the output catalogue only some of the columns in the two input catalogues you will be prompted to supply the names of the columns required and hence you must be prepared with this information. If you are not familiar with the details of the columns in your input catalogues you can use catheader (see Section , above) to obtain the necessary information.

Once you have indicated that you are to retain only specified columns (by replying `NO' to prompt ALLCOL) you will be prompted to enter the names of columns to be retained from the primary catalogue. Type the name of the first column required then hit return. For example to retain column X simply type:

X

A corresponding column with the same name and other attributes will be created in the output catalogue. Columns may also be retained with a name in the output catalogue which differs from the name of the corresponding input column. In this case you type: the name of the input column, a right chevron and the name required for the new output column. For example, if the column was called X in the input catalogue and X_PRIM in the output catalogue you would type:

X > X_PRIM

An arbitrary number of spaces may appear on either side of the right chevron. A column with the specified new name will be created in the output catalogue, and all its other attributes will be the same as those of the corresponding column in the input catalogue.

Continue in this fashion until you have entered all the columns required from the primary. Then type:

END

Next you will be prompted for the names of the columns required from the secondary. Proceed exactly as for the primary and again type END when you have finished.

If you are retaining a large number of columns it is inconvenient (and, indeed, error-prone) to have to supply all the column names interactively in response to prompts. In this case it is much more convenient to run catpair from a script, and I strongly recommend that you do so. This option is described in Section , below.

The handling of multiple columns with the same name in the output catalogue is rather different when column names are being specified than when all the columns are being copied automatically. A single column with the specified name is created in the output catalogue and values for all the appropriate columns in the input catalogue are written to the field of this column for the current row. This behaviour is adopted because there there are cases, particularly in MOSAIC and ALLREJ pairing where you might want fields for corresponding columns in the two input catalogues to be written to a single column in the output catalogue. In the case where fields are available from both the primary and secondary catalogues it is always the field from the secondary which is retained.