Heterodyne ITC

Please see the ACSIS spectrometer information for more information, including the resolution constraints when observing multiple simultaneous frequencies.

“Chained” modes:
  • 250 MHz × 8192
  • 1000 MHz × 2048
Combine two correlators to increase the resolution (number of channels).

Basic modes:
  • 250 MHz × 4096
  • 1000 MHz × 1024
HARP: used in 400 MHz, 1600 MHz “hybrid” modes, or with two frequencies.
Other receivers: only used in wider hybrids or with multiple frequencies.

Please select “Other” to enter a specific resolution in km/s or MHz. If a resolution is entered in terms of frequency, it corresponds to the sky frequency.

Some receivers have two mixers observing the same position in orthogonal polarizations. Normally the data are combined to reduce the integration time required, but you can use this option to see what the effect would be of processing each polarization individually.

Heterodyne receiver information

Continuum mode can be selected when you are interested in the continuum level (e.g. for planets) at the expense of reduced observing efficiency.

Heterodyne observing modes

Opacity conditions at JCMT are divided into 5 bands. You may select a band to use a representative value for that band or select “Other” to enter a specific 225 GHz opacity value.

Weather band information

Grid: the telescope moves to one or more individual points.
Jiggle: the secondary mirror of the telescope moves in a pre-defined pattern.
Raster: data are recorded while the telescope scans an area.

Heterodyne observing modes

Beam: the secondary mirror of the telescope “chops” a short distance (≤ 180 arc-seconds) to either side.
Position: the whole telescope moves between the source and offset position.
Frequency: the observing frequency alternates during the observation.

Heterodyne observing modes

In raster mode with an array receiver, you can choose how far the telescope moves between scan rows. This does not affect the pixel size since this is set by the array geometry.

Small spacing values allow each point in the map to be passed over by multiple different receptors, giving smoother noise performance.

HARP raster examples

In raster mode with an array receiver, the map can be enlarged to ensure complete coverage of the target area.

Width: the distance along the first dimension is increased.
Height: this calculator will include additional scan rows, corresponding to an increase in the second dimension.

The default option “Width only” should approximate the distance automatically added by the telescope control system.

In a basket-weaved observation the telescope scans the map area in two orthogonal directions (along the height and width) to minimize the effects of sky and system uncertainties.