Velocity shift problem in SpW mode observation


(2017-2018シーズン以降のSpWモード観測において、観測データのヘッダーの
ライン静止周波数がずれていました.またこの問題はOTF観測でもPosition switch
観測でも生じます)

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.

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


2021-02-17