Last Update: 1st August 2017
How to Plan an Observation using the Nobeyama 45-m telescope
In making your observing plan, please note that targets fulfil following conditions:
- 1. Elevation
- The observable elevation range of the telescope is 11-80 degrees.
Since the telescope is located at 138d 28m 21.2s (E), 35d 56m 40.9s (N), objects
(ON and OFF positions, pointing sources, and standard sources)
within the declination range of 26 < D < 46 degrees
will reach elevations higher than 80 degrees (upper limit).
- 2. Avoid the sun
- In sunny days, when you observe at less than 35 degrees from the Sun,
the pointing accuracy of the telescope is degraded due to the Solar heating of the antenna.
If you note the period which your target is near the Sun in the Submission Form, the observatory allocates observing time in order to be out of such period.
The Solar position is shown in the following link [Table of Sun Position].
- 3. Skyline
- The skyline of the Nobeyama 45-m telescope is mainly limited by the observation building and nearby trees.
These obstacles above the elevation limit (EL > 11 degrees) are mostly located on the north side of the Nobeyama 45-m telescope.
Fig. 1 illustrates the skyline of the Nobeyama 45-m telescope in 2016.
Fig. 1: The skyline of the Nobeyama 45-m telescope
- 4. Target Duplication Check
- Large observation projects led by the observatory have been carrying out.
Applicants are required to check their targets in the NRO 45-m legacy projects before submission.
If the objects have already been in the list and target lines are as same as your objects of interest,
you must justify why you would like to re-observe the same target in Science Justification.
- 5. LST Range
- Applicants are required to choose the LST range for their observations and can determine the LST range from Fig. 2.
For example, an object of (R.A., Dec.) = (12h, 10d) can be observed above an elevation of 30 degrees for LST = 8h-16h.
The LST range can be determined by two conditions:
(1) Since the system temperature gets worse at a lower elevation,
it is better to observe the target during high elevation (generally saying, > 30 degrees except for objects at low declination).
(2) Since receiver tuning and observation of the standard source should be done every day, shorter observing time per day cause lower observing efficiency (also see, Observing Plan for Line Observations).
Fig. 2: Elevation Angle vs. Hour Angle
For line observations, estimation of observing time is determined as follows:
- 1. Noise Level Estimation of the Source
- Two observation modes are available with the Nobeyama 45-m telescope.
One is a position switching observation which suits for few observing points.
The other is an On-The-Fly (OTF) observation suitable for mapping observations.
The expected one sigma noise level dT (K) for the position switching observations can be calculated as
, where Tsys is the system temperature
(including the atmosphere. The system temperature of each receiver is summarised in Status Report page),
df (Hz) is the frequency resolution, and
t (sec) is the on-source integration time.
Please note that derived dT is not in the unit of main beam temperature (TMB),
which can almost be regarded as brightness temperature (TB), but antenna temperature (TA*).
The conversion from antenna temperature to main beam temperature is
where η is the main beam efficiency.
For estimation, you should adopt the latest main beam efficiency
(you can consult the latest main beam efficiency at Antenna Efficiency).
If you are going to observe 2 polarisations simultaneously (a possibility of simultaneous observations depends on the receiver which you are going to use and your backend setup),
on-source integration time will be reduced by a factor of sqrt(2).
- The OTF mode is for mapping observations: the antenna moves continuously in a mapping region, and the data are taken in short interval.
How to estimate the expected noise level for OTF mode is described here.
In both observing modes, applicants are required to estimate the observing time with the time estimator and to append the result at the end of Science and Technical Justification.
The time estimators for the position switching mode and the OTF mode and the manual are found in Time Estimator (PSW) and Time Estimator (OTF), respectively.
- The Conversion from Flux Density S to Brightness Temperature TB
You can convert flux density S into brightness temperature TB with the following equation:
, where λ, θ are observing wavelength and angular resolution.
Alternatively, it can be represented using frequency ν instead of wavelength λ as,
Note that if a target radio source is smaller than the beam,
this TB is only a lower limit (actual TB is higher).
(the beam size can be roughly estimated as 1.2*λ[m]/(a diameter of a telescope)[m] in the unit of a radian.)
- 2. Time for Receiver Tuning
- The time needed for receiver setting varies greatly depending on the setup used by the previous observer and the status of the receivers.
From our experience, an average of 20 minutes per receiver is required for T70 receiver
(Other receivers do not require long receiver tuning).
- 3. Pointing Observations
- It is recommended to do pointing observations every 1-1.5 hours to check and correct the pointing accuracy of the telescope.
Pointing observations are mainly done using SiO maser sources (a compact and strong source) and otherwise continuum observations.
Since strong continuum sources (quasars) are few, most pointing observations are carried out by SiO masers.
The time required for the pointing check will, of course,
vary depending on the brightness of the SiO maser source used and the weather conditions,
but is usually of the order of 15-20 min.
If an SiO maser source with a line strength of 5 K in antenna temperature and
a velocity width of 3 km/s is observed as a pointing calibrator
with our standard pointing receiver H40 (Tsys = 250 K),
one can get an S/N ratio of 20 by spending an ON source time of 10 seconds,
which should result in a measurement accuracy of 1 arcsec rms
(see a plot of Pointing Error vs. S/N ratio).
SiO masers, which have been used at the Nobeyama 45-m telescope are compiled as a CSV file or are seen online in the list of SiO maser sources.
- 4. Observations of Standard Sources
- The intensity calibration is done by the chopper wheel method.
It is recommended to observe a standard source at least once every day
to establish the absolute line intensity scale and also to check the observing frequencies.
When the standard source is quite separated from the target objects,
a pointing check will be required prior to its observation.
For 3 mm observations, it is highly recommended to observe a standard source at least once during your observing run,
which is currently the most reliable system to measure absolute intensity at the 3-mm region.
It takes about one hour to observe a line with reasonable intensity (TA* > 1 K) toward a standard source.
Note that good weather condition is necessary to obtain reliable intensity (stable sky, preferably wind speed less than 3 m/s).
We also note that pointing observation for the standard source is also needed.
Pointing sources of the standard observations are described in the Standard Source List.
- 5. Telescope movement:
- The maximum velocity of the Nobeyama 45-m telescope is 18 degree/min,
thus taking 20 min to rotate 360 degrees in azimuth.
The telescope can move in azimuthal angle from -90 degrees to 450 degrees.
Since in order to avoid to reach this limitation, our observing script checks the position of the target at the beginning of the script,
the telescope may rotate at the beginning of each observation about 360 degrees.
This happens especially when you change observing source from your target to pointing sources and vice versa.
To avoid a loss of your observing time due to this azimuthal rotation limit,
you may choose pointing sources in the same rotation as the target in the celestial sphere
(i.e., if your target is northbound source which is declination higher than 35d 56m 40.9s, northbound pointing source could avoid large azimuthal rotation when pointing observations).
- 6. The maximum mapping unit size
- Generally saying, the maximum mapping unit size is determined by the scan speed of the telescope, the dump time, sky stability, and pointing accuracy (also see, Hints on Choosing Parameters in the instruction of OTF page).
In order to avoid smearing and to obtain the data with Nyquist sampling, the product of the scan speed of the telescope and the dump time should be less than 1/3--1/4 of the HPBW of the beam (of course, it depends on the frequency you intend to obtain).
Since the minimum (and default) dump time is 0.1 sec, the scan speed of the telescope should be slower than 40"/sec.
Empirically, the sky stability at Nobeyama is roughly 20 sec.
Thus, the maximum length along the scans is ∼10 arcminutes.
Other matter also limits the maximum mapping unit size: Pointing accuracy should be checked every 1--1.5 hours.
This requirement demands the maximum length perpendicular to the scans about 5--7 arcminutes in case of the length along the scans of 10 arcminutes.
The users who want to map larger than these maximum map unit sizes observe to cover the desired area with the unit maps.
- 7. Total observing time
- Apart from the on-source observations, the total observing time must include the off-source observations and the movement of the antenna between the on- and off-positions.
For position switching mode, the integration time of the off-position is set at same as that of the on-positions.
The fraction of time spent for tuning of the instrumentation, observations for pointing and intensity calibration, telescope movement and software overheads is roughly 30% of the estimated observing time.
Thus, your total observing time can be derived by multiplying a factor of 1.3 to the results of the Time Estimator.
In order to scale an absolute flux and daily stability of arrays, it is highly recommended to observe the standard sources every day.
Since FOREST has four beams, OTF mapping observation of the standard source is recommended.
It takes about 30 minutes which is much longer than pointing observation of the standard source (∼5 minutes).
Therefore, applicants adopt the fraction of overheads of 40 % when they intend to observe with FOREST.
For continuum observations, applicants are required to calculate the observing time by themselves.
- Observing mode available
- On-The-Fly (OTF) mapping mode is used for mapping observations.
- The cross-scan mode is used for pointing and observations of a point source.
- On-Off scan mode also can be used for a point source
(This means position switch observations with a combination of beam switching.
This mode should be used for a point source to completely remove the effect of the atmosphere).
- Estimation of Observing Time
- For On-Off scan mode, the total observing time, including off-source time and antenna movement, is about 2.6 times longer than the observing time "t".
t (sec) can be estimated using the following equation:
where S is the sensitivity of receiver (mJy) in table 1 in Capability of the 45-m Telescope for this season and Frms is rms noise level required (mJy).
The total observation time including the corrections due to observing period and elevation can be derived by the following equation:
where f, ELmax and t are the correction factor of the observing period, the maximum elevation of the targets in degree and time necessary for observations (including ton-source and toff-source with the beam switch) in sec, respectively.
f is summarised in the table below:
Table 1: The Correction Factor of Semester B (from March to May)
*) In the observing season of Semester A (from December to February), f is 1.0 for all frequency range.
|Target Frequency (GHz)
- For OTF mode, a grid spacing of the final map is the same as the spacing of scan paths.
The total observing time tobs to get a map with Nyquist sampling,
including the dead time for acceleration and return of the antenna, is roughly calculated by next equation,
where S is the sensitivity listed in table 1 in Capability of the 45-m Telescope for this season,
Frms rms noise level required,
N grid number in the map.