7. Observing Modes

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7.2 Mapping Techniques

If the radio emission is extended over an area larger than the antenna beam, several pointings might be necessary in order to cover the entire area of interest.

The Nyquist theorem states that the correct source sampling along a direction requires an angular distance between the pointings of :

The Nyquist sampling  is commonly expressed as beam's fraction :

The Medicina antenna mainly offers  two mapping techniques :

bullet Raster Scan

The map is obtained through discrete adjacent pointings ("point and shoot" mode). At every step the antenna stops and acquires data for the exposure time required.

The time necessary to cover an area A, considering the on-source time only, with a monofeed system, can be roughly estimated with the following :

Np = number of pointings

tesp = single exposure time (depending on the sensitivity required).

The Nyquist sampling is approximated with a beam shift in both directions (vertical and horizontal).

Usually this mapping technique is associated with an ON-OFF technique, therefore the total time necessary to complete a survey is given by :

tsh = antenna shifting time (Position Swiching) or secondary mirror shifting time (Wobbling)

The scan can be conducted in several user-defined ways, the most common is along two perpendicular directions ("cross scan").

bullet On-The-Fly

 

In the "On-The-Fly" mode the antenna is moved along one direction, usually with  a "rows-and-columns" path, at constant speed. The data are continously acquired and downloaded by the backend ("OTF dumps") and integrated according to the desired sampling interval - minimum value is 1 ms. This sampling corresponding to angular excursions of a few arcseconds (depending on the antenna speed).

To reach the required sensitivity it is necessary to scan the same area several times, preferably along different directions.

The ON-source time is :

td = acquisition time

Nd = number of dumps (depending on the required sensitivity).

The Nyquist sampling is obtained if the acquisition time, for each dump, corresponds to an angular antenna shift equal or shorter than the ideal Nyquist distance.

Also the distance between rows and columns must be coherent with the Nyquist sampling.

The On-The-Fly technique is characterized by very short scanning times, so it is the best one in order to reduce the atmospheric contribution (in any case it is necessary to use an ON-OFF technique). For a squared spectroscopic map the total observing time is estimable with the following :

The Medicina antenna offers the  On-The-Fly Mapping on a user defined RA/Dec, Az-El or GLon/Glat map, with a maximum scan speed of 15/m.

NEW: for continuum observations (in modes: ON-OFF, OTF cross-scans, OTF maps, raster maps) an Exposure Time Calculator is now available.


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