9.1.1 Observing Modes

Four observing modes are supported by GREAT.

  • Position switching (PSW): In this mode the telescope nods between a target and nearby emission-free reference position. This mode should be used for observing one or more positions on an extended source. The reference position should ideally be less than 30' away from the source positions.
  • Beam switching (BMSW): In this mode, the secondary mirror chops between the source and a reference position less than 10' away at a rate of about 1 ‒ 2 Hz. The telescope nods between these positions at a slower rate. This mode results in better sky cancellation than position switching, but can be used only for compact targets, where the chop/nod throw would move the beam off-source.
  • Raster mapping: In this mode, the telescope points to an off position, followed by several on-position observations. The user must specify the map positions, the off-position for a PSW observation or the chopper set-up for a BMSW observation, and the totl integration time per map position. This mode is ideally used for small maps or strips, where relatively long exposure times (> 30 s) per map point are required.
  • On-the-fly mapping (OTF): In this mode the telescope scans along a line of constant latitude (a row) with the backends continuously integrating the incoming signal and recording an average after the telescope has moved a fraction (typically 1/3) the beam size. Once the end of the row is reached, the telescope moves off-source to a reference position. Then, the telescope steps half a beam width in longitude and scans the next row. This process continues until a map of the desired size has been observed. The whole map is repeated until the required sensitivity is reached.
Download the PDF Version

Share This Page