Keywords: class-herschel-fits

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## Spectroscopic section

Table: Look-up table between the CLASS Spectroscopic section and the Herschel-HIFI FITS. Depending on the FITS structure (pipeline level), some elements found in a FITS header or in a FITS binary table. FITS names are case sensitive.
CLASS | Type | Value
`line` | C*12 | `DECON_SSB` if Card `OBS_MODE` contains `SScan` and
|  | if Card `CLASS___` contains `Spectrum1d`, else
|  | `XXXXXXXX_YYY` where `XXXXXXXX` is the `lofreq` in GHz and
|  | `YYY` is the metacard `sideband` value (`GHZ` if not defined)
`nchan` | I*4 | Card `NAXIS2` (level 2.5) or
|  | Card `TDIM*` for column `flux_*` (level 2.0)
`restf` | R*8 | Value at `rchan` in column `frequency` or `wave` (level 2.5) or
|  | value at `rchan` in column `lsbfrequency_*` or `usbfrequency_*` (level 2.0)
`image` | R*8 | 2*`lofreq`-`restf` if `lofreq` is defined,
|  | CLASS internal code for undefined value otherwise
`doppler` | R*8 | 0.d0
`rchan` | R*8 | ceiling(`nchan`+1)/2 (i.e. middle of spectrum)
`fres` | R*8 | Absolute of spacing in column `frequency` or `wave` (level 2.5) or
|  | absolute of spacing in column `lsbfrequency_*` or `usbfrequency_*` (level 2.0)
`vres` | R*8 | `fres` / `restf`
`voff` | R*8 | 0.d0
`bad` | R*4 | -1000.0
`vtype` | I*4 | code for LSR referential if Metacard `freqFrame` is `LSRk`,
|  | code for unknown referential if Metacard `freqFrame` is `source`,
|  | error otherwise.
Not in header: |  |
`lofreq` | R*8 | Metacard `LoFrequency` else `LoFrequency_measured` (level 2.5) or
|  | Column `LoFrequency` else `LoFrequency_measured` (level 2.0),
|  | 0.d0 otherwise.

The exact status of the spectroscopic axes after HIFI calibration is available here:
`http://herschel.esac.esa.int/twiki/pub/Public/HifiCalibrationWeb/freq_vel_1.1.pdf`

In the context of CLASS we can note that the frequencies found in the Herschel-HIFI FITS are expressed in the LSR frame2 (hence the null Doppler factor). The source velocity is assumed to be 0 at import time; this is consistent with the null Doppler factor. It is the responsibility of the user to apply the needed `MODIFY VELOCITY` correction to apply the frequency shift implied by the source systemic velocity (see section ), i.e., to get the frequency axis in the source frame.

Moreover, at import time, the default rest frequency of the spectrum is arbitrarily set to the middle of the spectrum bandpass. Here again, it is the responsibility of the user to apply the needed `MODIFY FREQUENCY` correction to center the velocity axis around the line rest frequency under consideration.

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