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OTF Observations with the 45-m Telescope

On-The-Fly (OTF) is a technique to perform mapping observations: the antenna is driven continuously in a mapping region and the data are taken in a short interval (t_dump: in case of 45-m system, it can be set to be 0.1 or 0.04 times N [s]). OTF is effective in particular when a wide area is surveyed in relatively short integrations. The OTF observations have advantages over usual position-switch observations as follows.

the dead time of the telescope reduces.
the system variation (e.g., atmospheric conditions, pointing of the telescope) does not affect the map since the entire map can be scanned in a short period.
since the data are acquired more frequently than the Nyquist sampling rate, the spatial information is not lost.

For details, please refer to Sawada et al. 2008, PASJ, 60, 445 . Please read the page Status Report on the details of the receivers and spectrometers that can be used for OTF observations.

Scan Pattern and Observation Parameters

Scan Pattern

Schematic illustration of an OTF scan pattern is shown in Fig. 1-1. The behavior of the antenna is similar to those in raster-scan mapping of continuum observations. The "on-source" part of the figure stands for the main-run of a scan (hereafter simply "scan"), during which the antenna is driven at a constant speed and the data are taken at every t_dump seconds. The "approach-run" is inserted before every scan to let the antenna move stably during the scan. If there is no OFF (integration on emission-free reference position) between two scans (Fig. 1-1 middle), a "transit-run" (from the end point of the first scan to the start point of the second approach run) is inserted.

Like usual position switch ("step-and-integrate") observations, the "chopper-wheel" calibration (R-SKY) data should be obtained during the observation in an appropriate interval. OFF points are taken before every scan or several scans.

Fig. 1-1: scan pattern

Fig. 1-1: Schematic illustration of an OTF scan pattern. (top) in case of a sequence pattern of "1*" (OFF-ON); (middle) in case of a sequence pattern of "1**" (OFF-ON-ON). (bottom) A sample scan for the FOREST receiver. About the beam rotation angle, please see this page.

As a result of the observations, the mapped region is filled with (spectrum) data points with spacings smaller than the Nyquist sampling rate. Since the data points do not align on any regular grid, the data should be regridded onto a regular grid using a convolution function. Convolution functions are listed on this page.

Observation Parameters

You need to specify the following parameters for the OTF observations.

The dimension of the mapping area l₁ ["] × l₂ ["] (l₁ along the scan, l₂ across the scan).
Note that, if you use the FOREST receiver array, the region covered with all the 4 beams roughly becomes (l₁ - 50) ["] × (l₂ - 50) ["].
Time to be taken in a scan row (on-source) t_scan [s]
Scan speed on the celestial sphere then becomes v_scan ["/s] = l₁/t_scan. Please specify the scan speed so that the moving distance of the antenna (t_dumpv_scan ["]) does not exceed 1/3 or 1/4 of HPBW, where t_dump is 0.1 or 0.04 [s].
The number of on-source scans per one OFF: N_scan^SEQ, the separation between the scan rows: Δl ["], and the grid spacing of a map to be made: d ["] × d ["].
Please set Δl to be about 1/3 of HPBW, and d to be 1/3 - 1/2 of HPBW. The duration between the OFF-point observations (~N_scan^SEQ × t_scan) is recommended to be 10 - 30 sec.

The resultant map has so-called scanning effects if scans along only one direction are coadded. The scanning effect will be significantly reduced by combining orthogonal scans. The so-called PLAIT algorithm described by Emerson & Gräve (1988) is implemented in the data reduction software. Please refer to the Data Reduction section.

Determining Parameters and Sensitivities

The observation time and the sensitivity can be estimated by

Time estimator (Web ver.)

An Example of Parameters
Using FOREST, and assuming parameters l₁=l₂=1200 ["], maximum elevation of 60 [deg], t_scan=20 [s], Δl=5 ["], d=6 ["], d_OFF=40 ['], T_sys=300 [K], B=100 [kHz], and the convolution function of "Bessel×Gauss"

	* Parameters :
	1.Receiver = FOREST
	2.Pol = Double
	3.Frequency = 115 GHz
	4.Max Elevation [deg] = 60
	5.Length along the scans [arcsec] = 1200
	6.Length perp. to the scans [arcsec] = 1200
	7.Time for scan [sec] = 20
	8.Num. of ONs per OFF = 1
	9.Separation between scans [arcsec] = 5
	10.Map grid [arcsec] = 6
	12.OFF-point separation [arcmin] = 40
	12.Tsys_inp [K] = 300.0
	13.Resolution
	  - Frequency resolution (kHz) = 100
	  - Velocity resolution  (km/s) =
	14.Convolution function = Bessel*Gauss

	* Results :
	1.theta [deg] = 0.0
	2.Beam overlapped area = 1150.0 ["] * 1150.0 ["]
	3.vscan is 60.0 ["/sec] (6.0 per 0.1s sample)
	4.Nrow = 231
	5.tapp [sec] = 7, ttran [sec] = 8, ttranoff [sec] = 12
	6.tOFF =  4.7 [sec] ->  5 [sec]
	7.tOH = 31.0 [sec]
	8.ttot(ON) [min] = 77.0
	9.ttot(OBS) [min]/[hour] = 230.0 / 3.83
	10.eta(ON/OBS) = 0.33
	11.tcell(ON) [sec] = 4.13
	12.tcell(OFF) [sec] = 48.00
	13.Tsys_inp [K] = 300.0
	14.Tsys_obs_sa[K] = 306.0
	15.dTa*_sa [K] = 0.5641
	16.Tsys_obs_sb[K] = 422.8
	17.dTa*_sb [K] = 0.7794
	18.Version = 2.0170721

The optimal OFF-integration is 5 seconds. This table will take about 80 minutes to run and ΔT_A^*=0.56 [K] is obtained. About 1150" × 1150" region is covered with all the 4 beams. Effective spatial resolution becomes about 17" (if the telescope beam is 15"). About the effective resolution, please see Miscellaneous Information: Convolution in "Make Map".

Making Observing Scripts

Please use "nobs" to make observing scripts. The usage of nobs is described on this page. Please select OTF mode in "Scan Tab". The scan pattern made by nobs can be checked with obspoint. About the FOREST beam rotation angle, please refer to this page.

Data Reduction

Data Download

For data download, please refer to this page, which can be accessed only inside NAOJ and via NRO VPN.

Data Reduction

Please use " NOSTAR" to reduce the OTF data. The usage of NOSTAR is described in this page.

Miscellaneous Information

Derivation of sensitivity, effective angular resolution, notice for projection and others are summarised in Miscellaneous Information page.

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Last Update: 1^st August 2018