Two Beam Continuum Observations with PS or DPS (REFSAME)

When beam chopping in continuum mode it is possible to approximately double the S/N for a given PS or DPS (with REFSAME ) sequence by setting the REF position equal to that of the reference beam of the beam chopper. Before attempting a 'two beam' observation observers must first determine the reference chopper beam offset from the main beam and then carefully determine the optimum subreflector focus for each beam (see also Balonek Thesis, FCRAO report #190, pp. 83-106).

1. Enter CONT mode with VSYN. Put a strong continuum standard source (planet, etc.) in MAIN and type CSTART to turn on beam chopper.
2. Set the REF position to be at least 0.5$^\circ$ away (e.g. REF SAME AZOFF 0.5).
3. Find optimum subreflector focus for main chopper beam by using FIVE to point up at several focus settings (e.g. FOCUS -5, -4 . . . +2).
4. Place MAIN position in reference chopper beam by adding 0.1$^\circ$ to azimuth thumbwheel offset digiswitches determined in (3) at a given focus.
5. Repeat focus setting and FIVE points as in step (3) to determine the optimum focus for the reference chopper beam.
6. Plot the intensity vs focus results from steps (3) and (5). They should look something like the graph to the right.
7. Choose the intensity crossover point as the optimum 2-beam focus. Subtract the thumbwheel offsets for the reference chopper beam at this focus from the main beam thumbwheel offsets and put this difference in the REF position (e.g. if the main and ref beams differ in AZ by 0.109 and in EL by 0.001 then type REF SAME AZOFF .109 ELOFF .001).
8. The MAIN position is now in the main chopper beam and the REF is in the reference chopper beam. Do a final point up with FIVE using the multiple offs option (i.e. OFFS 1). Obviously, do not vary the focus.
9. The system is now peaked up for 2-beam continuum observing. Use the ratio of the true source temperature and the peak temperature from the step (8) pointing fit as the 2-beam calibration. Steps 1-7 should not have to be repeated again but step (8) should be repeated throughout the day to find the optimum focus. Remember, when mapping a continuum source using offsets from MAIN aways keep the AZOFF , ELOFF values from step (7) in the REF and add mapping offsets to both the MAIN and REF coordinates.