Pointing

Point often for the first 2–3 hours after sunset because the pointing will shift as the temperature of the telescope changes (thermal pointing shifts). Repoint no later than an hour after the initial pointing. Repoint every 2.5–3 hours thereafter.

Beam switching with the wobbling Secondary mirror may give more consistent results, especially if the weather is marginal. Use a short subscan integration time because there are four subscans at each point. Position switching is also possible.

Before pointing,

• Check the correct pointing model is displayed on the antenna status monitor
• Load recent pointing offsets from the pointing logbook
  UIP> fazo az_offset
UIP> fzao za_offset

The pointing examples use ¹²CO(2-1) and beam switching.

There are many more line sources – typically evolved stars – than bright continuum sources. But it may be necessary to tune the receiver to a bright line. The example uses ¹²CO(2-1) and beam switching. Position switching is also possible.

• Tune the receiver, if necessary, and setup the spectrometer.
  UIP> LO 12CO2-1 /IF 5.28 /SIDE UP
UIP> SPEC 2 /FFTS1W 5.28 /FFTS2 /RES
• Choose a pointing source with the Orrery. Then track the source with the telescope.
  UIP> OBSERVE IRC10216
• Start the secondary wobbler.
  UIP> SEC 90 0.534
• Set the baseline windows. These are line free regions used to calculate the spectral baseline.
  UIP> SPEC /BASE 2500 3500 4700 5700
• Do a calibration.
  UIP> CAL
• Measure the pointing. The integration range must include the line.
  UIP> FIVE 20 /FFTS1 /RANGE 4000 4200
  • Choose a step size somewhat less than the beam size. Here the step size is 20 arc sec because the beam size is about 30 arcsec at the observing frequency.
• Update the pointing corrections. If the results of the pointing fit seem dubious, use the FAZO and FZAO commands to update the corrections.
• Repeat pointing measurements until convergence.
• Record the pointing results in the pointing log book to share with other observers.
• After pointing, remove the baseline windows.
  UIP> SPECTROMETER /NOBASELINE

If the line is not centered in the spectrometer or if there are lines in the baseline windows, take a spectrum prior to pointing to determine the line position and the line free baseline windows. This may be difficult if the pointing is far off.

For standard sources, it is good practice to take a reference spectrum just after pointing as a calibration comparison.

The bright planets, Mars, Jupiter, and Saturn, are good sources. Uranus and Neptune may be (very) difficult.

• Tune the receiver, if necessary, and setup the spectrometer.
  UIP> LO 12CO2-1 /IF 5.28 /SIDE UP
UIP> SPEC 2 /FFTS1W 5.28 /FFTS2 /RES
• Choose a pointing source with the orrery. Then track it with the telescope.
  UIP> PLANET MARS
• Start the secondary wobbler.
  UIP> SEC 90 0.534
• Do a calibration.
  UIP> CAL
• Measure the pointing. For a continuum source, integrate over the entire spectrometer bandpass.
  UIP> FIVE 20 /FFTS1
  • Choose a step size somewhat less than the beam size. Here the step size is 20 arc sec because the beam size is about 30 arcsec at the observing frequency.
• Update the pointing corrections. If the results of the pointing fit seem dubious, use the FAZO and FZAO commands to update the corrections.
• Repeat pointing measurements until convergence.
• Record the pointing results in the pointing log book to share with other observers.

It is good practice to take a reference spectrum of the planet just after pointing to check the beam efficiency.