Green Bank Convention

This convention was developed at a meeting to discuss standard FITS formats for interchange of single dish radio astronomy data, held at Green Bank WV, in October, 1989. It is designed primarily for the case where only one field is a multidimensional array or where all array fields in a row have the same dimensions.

The extension header keyword

MAXIS (integer) gives the number of dimensions of the array in the table field.

The properties of the array would be given in columns in the table. The column labels would be the names of the keywords used to describe a primary or image data array, except that NAXIS$n$ would be replaced by MAXIS$m$. These labels are given by the values of the TTYPE$n$ keywords in the binary table header. Thus,

• TTYPE$n$= 'MAXIS$m$' means that the number of elements along axis $m$ in the array contained in the table is given in column $n$ of the table. MAXIS1 describes the most rapidly varying axis of the array.

• TTYPE$n$= 'CTYPE$m$' means that the name of the physical coordinate of axis $m$ in the array contained in the table is given in column $n$ of the table.

• TTYPE$n$= 'CRPIX$m$' means that the reference point for axis $m$ in the array contained in the table is given in column $n$ of the table.

• TTYPE$n$= 'CRVAL$m$' means that the value of the physical coordinate for axis $m$ at the reference point in the array contained in the table is given in column $n$ of the table.

• TTYPE$n$= 'CDELT$m$' means that the rate of change of the physical coordinate along axis $m$ per unit change in the counting index at the reference point of the array contained in the table is given in column $n$ of the table.

Because the array field must be the same size in all rows, the values of the MAXIS$m$ will usually be the same for all rows of the table, and they can then be given as header keywords instead of table columns:

MAXIS $m, m=1, \ldots,$ MAXIS (integer) gives the number of elements along axis $m$ in the array contained in the table.

It is possible for array to have the same size in two rows but different numbers of elements along the axes, for example $36 \times 6$ in one row and $24 \times 9$ in another. In that case, the number of elements along each axis must be given in a table column.

One field - call it $k$ - will contain the actual data array. For this row, two keyword-value pairs are required:

TFORMk  = 'DATA    '
TMATXk  =                    T

A value of F is understood for the TMATX$m$ keyword for all other columns and need not be included explicitly.

It is possible to combine this convention with the use of the TDIM$n$ keyword proposed in the binary table paper by setting

TDIMk    = '(MAXIS1,MAXIS2,...)'

where the MAXIS$m$ represent the number of elements along axis $m$ that would otherwise be the values of the MAXIS$m$ keywords. The CLASS package for analysis of radio astronomical data supports the Green Bank convention.