7. Observing Modes
7.1 ON-OFF Techniques
In order to reduce as much as possible the atmospheric contribution during an observation, it is possible to apply some techniques based on at least a couple of exposures, one on source and one on an adjacent area ("OFF source" reference position), sufficiently free from emission.
At high frequencies short-scale and strong atmospheric fluctuations affect the observation, hence the need of quick antenna shifts between the two positions (which have to be reasonably next to each other) or the usage of other techniques which do not involve the movement of the entire structure.
The Medicina antenna offers the following ON-OFF techniques :
The antenna shifts between two different positions. The time needed to cover some beams is nearly 5 seconds at all frequencies.
The shifting of the beam is obtained moving the secondary mirror only. This technique requires always a shorter time than the Position Switching.
In both cases the algorithm used is of the type ON-OFF-ON-OFF.
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