## 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.