4.3. UNITS

27

Table 4.4.

Characters and strings allowed to denote mathematical operations.

String

Meaning

Multiplication

str1 str2

Multiplication

str1*str2

Multiplication

str1.str2

Division

str1/str2

Raised to the power expr

str1**expr

Raised to the power expr

str1^expr

Raised to the power expr

str1expr

Common Logarithm (to base 10)

log(str1)

Natural Logarithm

ln(str1)

Exponential (estr1)

exp(str1)

Square root

sqrt(str1)

be subject to misinterpretation. A space character implies multiplication which can

also be conveyed explicitly with an asterisk or a period. Therefore, although spaces are

allowed as symbol separators, their use is discouraged. Note that, per IAU convention,

case is significant throughout. The IAU style manual forbids the use of more than one

solidus (/) character in a units string. However, since normal mathematical precedence

rules apply in this context, more than one solidus may be used but is discouraged.

A unit raised to a power is indicated by the unit string followed, with no intervening

spaces, by the optional symbols ** or ^ followed by the power given as a numeric

expression, called expr in Table 4.4. The power may be a simple integer, with or

without sign, optionally surrounded by parentheses. It may also be a decimal number

(e.g., 1.5, 0.5) or a ratio of two integers (e.g., 7/9), with or without sign, which must

be surrounded by parentheses. Thus meters squared may be indicated by m**(2),

m**+2, m+2, m2, m^2, m^(+2), etc. and per meter cubed may be indicated by m**-3,

m-3, m^(-3), /m3, and so forth. Meters to the three-halves power may be indicated

by m(1.5), m^(1.5), m**(1.5), m(3/2), m**(3/2), and m^(3/2), but not by m^3/2 or

m1.5.

4.3.2

Units in Comment Fields

If the units of the keyword value are specified in the comment of the header keyword, it

is recommended that the units string be enclosed in square brackets (i.e., enclosed by `['

and `]') at the beginning of the comment field, separated from the slash (`/') comment

FITS Standard