Last Update: 27th July 2020

Intensity Calibration

Since the system of the Nobeyama 45-m telescope may vary its intensity scale due to daily instability, the observatory strongly recommends the users to perform standard source observation at least once per a day. The aim of this page is to show the method of the standard source observation and to illustrate an alternative way for 2SB receivers to calibrate daily variation of the system when the standard source observation is hard to perform.

Standard Source Observation

Standard Source Observation is an observation toward a strong source that is already known its absolute intensity. For this reason, the standard source observation can not be done except for strong lines such as CO and its isotopic species.

Basic Standard Source Observation

This suits all receiver except for FOREST.

First of all, you need to check which standard source can be used for your observation. Check items are:

  1. Distance from Your Main Target

    It can almost be judged from the coordinate of the target. If there is time your target reaches the upper elevation limit (El > 80 deg.), it is good to perform the standard source observation during such time. Otherwise, you would be better to observe it at the begining or the end of your observing time.

    This can be checked with Parameters for Observation page.

  2. Strength of the Emission

    Even if one standard source is suitable for its coordinates, it can not be used if the intensity is weak (because it may not be wise to take time for the standard source observation).

    You can examine this via Spectra of Standard Sources (older but more molecules) and Spectra of Standard Sources, 2013 ~ (newer but only for 12CO and 13CO).

If you successfully find a good standard source, make observing scripts.

Please do not forget to make observing scripts for pointing for the standard source (The pointing source is written in Parameters for Observation).

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Standard Source Observation for On-On Scan with the FOREST Receiver

If your main observation is On-On scan mode, you should make a standard observation in order to correct both beams of the FOREST receiver (Beam 1 and Beam 3). Since standard sources are more compact than the distance between Beam 1 and Beam 3 (~70 arcsec), this can be achieved with On-On scan. Therefore, the difference to the basic standard source observation is that the scan table is for the On-On mode. Other setup is the same as the basic standard source observation.

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OTF Observation toward Standard Source for FOREST

Since FOREST has 4 beams, 2 polarisations and 2 sidebands, it needs to calibrate each beam. The observatory provides a sample scan file for the standard source observation with FOREST named f_cal.nscan which is OTF scan. The scan pattern is made to cover central ~3' x ~3' with all four beam. Thus, even if there is some pointing offset, all beam of FOREST can cover the peak of the emission (i.e., no need of pointing for this standard source observation).

The sample scan table can be used from the "Samples" button in Scan Tab. It takes about 20 minutes to complete the observation. Please note that since f_cal.scan is designed for one of the standard sources, IRC+10216, the Coordinates of Off Point must be modified for the standard source the users intend to use. The off point for each standard source can be found in Parameters for Observation in Spectra of Standard Sources.

Paramters of f_cal.scan
Parameters for Standard Source Observation

Scan Pattern of f_cal.scan
Central ~3' x ~3' region is covered with four beams.
Template Scan Pattern for Stardard Source Observation

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Alternative Intensity Calibration Method: IRR Correction

If you can not perform intensity calibration with a standard source observation, there is an alternative method to intensity calibration. This method is only vaild for 2SB receivers, in other words, T70 and FOREST. These receivers separate their side band into two: upper side band (USB) and lower side band (LSB). Ideally, a side band separation is done perfectly (100 %), while the separation stays at some level in real. This may make a contamination of emission from another side band. The degree of the contamination is measured as Image Rejection Ratio (IRR). Since the observatory provides the IRR measurement system for 2SB receivers, the users should measure the IRR every day. Details of the IRR measurement are described in Observations page.

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