Last Update: 27^th Jun 2020

How to Plan an Observation using the Nobeyama 45-m telescope

Target
Observing Plan for Line Observations

Target

In making your observing plan, please note that targets fulfil following conditions:

1. Elevation and Azimuth: 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).
The observable azimuth range is from -87 to 447 degrees (software limits).
2. Avoid the Sun: On sunny days, when you observe at less than 30 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 in 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 (click to enlarge)
4. 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 causes lower observing efficiency (also see, Observing Plan for Line Observations).

Fig. 2: Elevation Angle vs. Hour Angle (click to enlarge)

Observing Plan for Line Observations

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 that 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

line sensitivity

, where T_sys 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 (T_MB: it can almost be regarded as brightness temperature, T_B), but antenna temperature (T_A*). The conversion from antenna temperature to main beam temperature is
Ta* to Tmb

,
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 intervals. 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 T_B. You can convert flux density S into brightness temperature T_B 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 T_B is only a lower limit (actual T_B 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 (T_sys = 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 (T_A* > 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 -87 degrees to 447 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, Observation 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 dump time is 0.04 sec, the scan speed of the telescope should be slower than 125"/sec at 115 GHz. Empirically, the sky stability at Nobeyama is less than 30 seconds (typically 20 sec). Thus, the maximum length along the scans is ∼60 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 the 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.