For bookkeeping purpose, CLASS keeps track of a Scan and a Subscan
number, which can be used as a selection criterium. The Scan number is
attributed at observing time which is carried out in the CLASS data
format. Moreover, two different observing mode are nowdays in common use in
single-dish telescope:
- The pointed observing mode for which the telescope is pointed toward
the source direction during all the integration to obtain only one
spectrum. The scan is made of only one spectrum whose intensity is
accumulated during the scan duration. Hence the Subscan number is always
1.
- The On–The–Fly (OTF) observing mode for which the telescope drift
on source during the integration to make a small map. The scan is here
composed of a collection of spectra dumped regularly (typically every 1
second) during a contiguous portion of time. Each dumped spectrum is also
tagged by a subscan number whose value is incremented for each new OTF
line (both to enable easy selection of a single line inside one OTF scan,
and to ensure consistency with the 30m numbering). This subscan number is
foreseen to always be greater than 1. There is one exception: when
CLASS read data in old format, the subscan number is zero and a
warning is issued.
