## Scan and Subscan Number

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.