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6.1. KEYWORDS
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The total number of bits in the random groups records exclusive of the fill described
in §6.2 is given by the following expression:
Nbits  = |BITPIX| × GCOUNT ×
(PCOUNT + NAXIS2 × NAXIS3 × · · · × NAXISm),
(6.1)
where Nbits is non-negative and the number of bits excluding fill, m is the value of NAXIS,
and BITPIX, GCOUNT, PCOUNT, and the NAXISn represent the values associated with those
keywords.
6.1.2
Reserved Keywords
PTYPEn Keywords  The value field shall contain a character string giving the name
of parameter n. If the PTYPEn keywords for more than one value of n have the same
associated name in the value field, then the data value for the parameter of that name
is to be obtained by adding the derived data values of the corresponding parameters.
This rule provides a mechanism by which a random parameter may have more precision
than the accompanying data array elements; for example, by summing two 16-bit values
with the first scaled relative to the other such that the sum forms a number of up to
32-bit precision.
PSCALn Keywords  This keyword shall be used, along with the PZEROn keyword, when
the nth FITS group parameter value is not the true physical value, to transform the
group parameter value to the true physical values it represents, using Eq. 6.2. The value
field shall contain a floating-point number representing the coefficient of the linear term
in Eq. 6.2, the scaling factor between true values and group parameter values at zero
offset. The default value for this keyword is 1.0.
PZEROn Keywords  This keyword shall be used, along with the PSCALn keyword, when
the nth FITS group parameter value is not the true physical value, to transform the
group parameter value to the physical value. The value field shall contain a floating-
point number, representing the true value corresponding to a group parameter value
of zero. The default value for this keyword is 0.0. The transformation equation is as
follows:
physical_value = PZEROn + PSCALn × group_parameter_value
(6.2)
FITS Standard