Velocity shift problem in SpW mode observation

In the SpW (Spectral Window) mode observations since the 2017-2018 season,
the line rest frequency in the header of the observation data was shifted.
We confirmed that this problem occurs not only in the OTF observation
but also in the position switch observation.

The line rest frequency in the observing data is rounded inappropriately
for the SpW (Spectral Window) mode observation.
You can correct this by manually setting the rest frequency in NOSTAR
(either GUI or script) and NEWSTAR programs to obtain the accurate velocity axis.
The observation made in the non-SpW mode is not affected.
The shift is 4 channels or less.

For example:
channel separation:     15.26kHz    30.52kHz
frequency shift:            <+/-61      <+/-122 kHz
velocity shift (80GHz)     <+/-0.22   <+/-0.42km/s
velocity shift (115GHz)   <+/-0.16   <+/-0.3km/s

The cause of this problem is that the observing preparation software
"nobs" slightly changes the rest frequency with inappropriate rounding.
Because users can correct this problem in the data reduction,
we continue to use the same version of "nobs" in this observing season,
avoiding a risk of the unexpected derivative bug.
For season 2021-2022 and beyond, we plan to release the new version
"nobs" after careful commissioning.
We apologize for any inconvenience resulting from this issue.

Ken Tatematsu
Director, Nobeyama Radio Observatory