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ON/OFF (Position Switching)

This is the standard procedure for basic spectral line observations at Arecibo. The procedure simply observes ON your source for x minutes and then takes a `blank sky' observation OFF your source for the same amount of time and tracking the same azimuth and zenith angle range. This is followed by recording your chosen noise diode configuration ON for 10s and OFF for 10s. This procedure will provide the simplest means to eliminate the standing waves in your baselines. These standing waves are due to the unique geometry of the Arecibo Gregorian system which allows for considerable reflection from sources inside and outside the beam.

Your choices with this procedure are:

• Scan duration (for the ON-source observation) - You can set this to any integral number of minutes, up to 20. Choosing a large integration time, though, is not necessarily beneficial. First, the greater the integration time the better the chance that there will be a slight change in system temperature, system gain, etc. between your ON and OFF-source observations. Second, as you need to track the same range of azimuth and zenith angles for your ON and OFF scans, your slew time between the ON and OFF observations increases as your observation time increases (see ``wait time" below). Typically, observing times are kept to within 5 minutes (with 5 minutes as the most efficient, and therefore default, choice). If you wish to observe for more than 5 minutes at a time, please carefully consider your project, slew rates, etc.

• Wait time - This is the length of time (in minutes) the data-taking system waits while the telescope slews (and then settles) between acquiring the ON and OFF-source data. Again, this must be an integer. If your scans are for 5 minutes or less, your wait time should be set to 1 minute (the default setting). For longer observations you need to increase your wait time (particularly near zenith). Your total observation time is:
  2$\times$(scan duration) + (slew time) + (calibration time)
  A 5 minute ON/OFF + 10s CAL takes $\sim$11.3 minutes. A 7 minute ON/OFF, (which has a 2 minute slew time plus a cal) takes 16.3 minutes.

• Number of Loops in a Sequence - This sets the total number of ON/OFF + CAL cycles to be taken on each object each time the ``Observe" button is pushed. The default setting is for one loop (one ON+OFF+CAL-ON+CAL-OFF cycle).

• Adjust power - This setting allows you to adjust the correlator power levels. The options are: never, before the first ON/OFF+CAL pair only, or before every ON/OFF+CAL pair. The default setting is for the power to be adjusted before every ON/OFF+CAL pair.

• Doppler update - This allows the observer to choose when the Doppler update is applied - either at the start of every ON/OFF+CAL pair, or only at the beginning of the entire observing sequence. The default setting is for the Doppler to be updated at the beginning of each ON/OFF+CAL pair.

Doppler correction options are described in Section 1.8, and the noise diode configuration options are described in Section 1.6.