A DSBSpecFrame is a specialised form of SpecFrame which represents positions in a
spectrum obtained using a dual sideband instrument. Such an instrument produces a
spectrum in which each point contains contributions from two distinctly different
frequencies, one from the
" lower side band
" (LSB) and one from the
" upper side band
(USB). Corresponding LSB and USB frequencies are connected by the fact that they are an
equal distance on either side of a fixed central frequency known as the
" (LO) frequency.
When quoting a position within such a spectrum, it is necessary to indicate whether the quoted position is the USB position or the corresponding LSB position. The SideBand attribute provides this indication. Another option that the SideBand attribute provides is to represent a spectral position by its topocentric offset from the LO frequency.
In practice, the LO frequency is specified by giving the distance from the LO
frequency to some
" spectral position. Typically this central position
is that of some interesting spectral feature. The distance from this central
position to the LO frequency is known as the
" intermediate frequency
" (IF). The
value supplied for IF can be a signed value in order to indicate whether the LO
frequency is above or below the central position.
"format specifiers identified by
"symbols in the normal way.
"format specifiers, then an optional list of additional arguments may follow it in order to supply values to be substituted for these specifiers. The rules for supplying these are identical to those for the astSet function (and for the C
A null Object pointer (AST__NULL) will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.