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26
SECTION 4. HEADERS
Table 4.3.
Prefixes for multiples and submultiples.
Submult
Prefix
Char
Mult
Prefix
Char
10-1
deci
10
deca
d
da
10-2
102
centi
hecto
c
h
10-3
103
milli
kilo
m
k
10-6
106
micro
mega
u
M
10-9
109
nano
giga
n
G
10-12
1012
pico
tera
p
T
10-15
1015
femto
peta
f
P
10-18
1018
atto
exa
a
E
10-21
1021
zepto
zetta
z
Z
10-24
1024
yocto
yotta
y
Y
Table 4.2. The recommended plain text form for the IAU-recognized base units are given
in column 2 of both tables.1 All base units strings may be preceded, with no intervening
spaces, by a single character (two for deca) taken from Table 4.3 and representing scale
factors mostly in steps of 103. Compound prefixes (e.g., ZYeV for 1045 eV) must not be
used.
4.3.1
Construction of Units Strings
Compound units strings may be formed by combining strings of base units (including
prefixes, if any) with the recommended syntax described in Table 4.4. Two or more
base units strings (called str1 and str2 in Table 4.4) may be combined using the re-
stricted set of (explicit or implicit) operators that provide for multiplication, division,
exponentiation, raising arguments to powers, or taking the logarithm or square-root of
an argument. Note that functions such as log actually require dimensionless arguments,
so that log(Hz), for example, actually means log(x/1 Hz). The final units string is the
compound string, or a compound of compounds, preceded by an optional numeric mul-
tiplier of the form 10**k, 10^k, or 10±k where k is an integer, optionally surrounded by
parentheses with the sign character required in the third form in the absence of paren-
theses. Creators of FITS files are encouraged to use the numeric multiplier only when
the available standard scale factors of Table 4.3 will not suffice. Parentheses are used for
symbol grouping and are strongly recommended whenever the order of operations might
1
These tables are reproduced from the first in a series of papers on world coordinate systems [11]
which provides examples and expanded discussion.
FITS Standard