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
program (either GUI or script) to obtain the accurate velocity axis.
The observation made in the non-SpW mode is not affected.
The shift is 4 channels or less.
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.
Director, Nobeyama Radio Observatory