Position Switching - PS and DPS

To select normal position switching which uses a reference position provided by the observer (see section 3.2.2), the user must type in the command PSMODE. This is the most frequently used mode of the Observatory.

For position switching, (see Figure 4.1) the telescope moves to the REF position, takes data for TREF seconds, then moves to the MAIN position and takes data for TMAIN seconds. The process is repeated RPT times for each one of NSCAN scans. In CONT mode, RPT must be at least 2.

Figure 4.1: Sample single position switched observations

For Double Position Switching (see Figure 4.2): the telescope moves to position 1R and takes data for TREF /2 seconds, then moves to the MAIN position and takes data for TMAIN seconds. Then the telescope moves to position 2R and takes data for TREF /2 seconds. If RPT is greater than 1, then the telescope stays at position 2R, takes data for TREF /2 seconds, moves to MAIN and takes data there for TMAIN seconds. Then it moves to 1R and takes data for TREF /2 seconds, and so on. "Mirroring" of the REF position is an option in DPS . Typing REFSAME causes the same reference position to be used at all times and, in effect, DPS becomes the same as PS except for the symmetric TREF/2 integration in the REF position at the beginning and end of the SCAN. Typing REFOPP causes the reference position offsets to change sign every repeat.

Figure 4.2: Sample double position switched observation using REFOFF and REFSAME options

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Table 4.3

PS or DPS Keywords

REFOPP or REFSAME

In DPS mode either change sign or do not change sign of REF position offsets each RPT.

TMAIN (seconds)

The integration time for an on-source observation.

TREF (seconds)

The integration time for an off-source observation. In DPS mode, one-half of time TREF will be spent in each position set by REFOPP or REFSAME.

RPT (integer)

The number of off-source/on-source sequences to be done and stored as one scan.

NSCAN (integer)

The number of scans using the above parameters to be done consecutively.

TSAMPLE (seconds)

For CONT mode only. The integration time (in seconds) for each data point recorded by the computer. TSAMPLE must evenly divide into TMAIN and 0.5TREF for DPS mode.

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