Specifications of Frequency Switching Observational Mode
From December 2000, we have released frequency switching
observational mode in addition to the conventional position switching
mode. The merits and demerits of frequency switching are as follows.
- Merits: (1) OFF position is not necessary. For sources with difficulty
to find a good OFF position, this method may be worth trying.
(2) Becuase of the merit (1), the observing efficiency is higher than
that of position switching.
- Demerits: (1) The baseline ripples generally appear to some extent.
They may disturb or confuse line observations.
(2) Due to the nature of this switching, an absorption-like feature(s)
is generated at the frequency determined by "Frequency throw" below.
This may also disturb line observations.
The specifications are as follows.
- Spectrometers
- AOSs can be used with S100, S80, S40, H28/32, and H22 receivers
(BEARS, T100V, and T100H are not supported).
- ACs can be used only with BEARS.
When ACs are connected to the single-beam receivers
listed above, frequency switching is not supported.
- Switching rate
- AOSs = 0.25 Hz
- ACs = 0.25, 0.5, or 1.0 Hz
- Frequency throw
- Quality of baselines can be worse than that of the position
switching. The quality strongly depends on the value of frequency throw:
Baseline ripples are generally prominent for frequency throw of 1
- 2 MHz, but ripples are not so prominent if the throw is less than 1 MHz.
It may be necessary to do some trial until acceptable baselines are obtained.
When the line width is narrow, please employ frequency
throw as small as possible to minimize the baseline ripples.
- For ACs with BEARS, several MHz is possible. Frequency throw of 7-8 MHz is
often used successfully for BEARS.
- Mode
- Two modes "single" and "double" are available, where
"single" is a switching between two frequencies
f and f +
Δf, and
"double" is a switching among three frequencies
f, f +
Δf,
and f - Δf.
- Simultaneous observation
- [S100 and S80], or [S100 and S40] receivers can be used simulatanously
(same as position switching).
- Preparation of OBSTABLE (scan table)
-
Please specify the above setting (switching rate, frequency throw, etc.) in the
scan table
(Parameters
for Frequency Switch).
-
Integration time for each scan must be set to the mutiple of one switching period.
-
In the "Sequence Pattern" only ON position (*) is needed, because there is no OFF position.
- During observations
- Please ask your assistant to check whether the phase-lock loop
(PLL) is stable or not
by checking the beat signal on a spectrum analyzer in the observing room. If it is unstable,
please consider to employ smaller value of the frequency throw.
- Data reduction
- After observations, a folding of the spectra is possible in
the process of data reduction. A task "fold sp." is available in
the reduction software NEWSTAR.
Examples of observational spectra with frequency switching
LEFT : A spectrum of 13CO J=1-0 transiton obtained with
frequency throw of about 7.7 MHz (with BEARS, double mode, before folding)
RIGHT: Same as the left figure but after folding
Revised 2008-04-18
2008-04-18 nro45mrt @ NRO
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