Scan Types and other functions - Overview

Scan Types and other functions - Overview#

A scan is a pattern of antenna motions that when used together yield a useful scientific dataset. This section describes the various scan types that are available for use within GBT SBs. Each scan type consists of one or more scan, which are the individual components of the antenna’s motion on the sky. The scan types listed below are the functions within your SB where data will be obtained with the GBT.

Please note that the syntax for all scan types is case-sensitive.

Utility Scan Types#

Utility Scans generally describe procedures that are used to calibrate some aspect of the system such as pointing, focus, or power levels. Nearly every observing session will require the use of one or more of the utility scans.

Note

AutoPeakFocus, AutoPeak, AutoFocus, and AutoOOF automatically execute their own default continuum configurations unless configure=False has been supplied as an optional argument (only recommended for expert users). After running one of these Auto* procedures, you will need to reconfigure for your science observations.

Table 4 GBT Utility Scans#
Scan Type	Observing Type	Description
`AutoPeakFocus()`	Continuum	Selects and observes a nearby calibration source and updates the pointing and focus corrections.
`AutoPeak()`	Continuum	Selects and observes a nearby calibration source and updates the pointing corrections.
`AutoFocus()`	Continuum	Selects and observes a nearby calibration source and updates the focus corrections.
`AutoOOF()`	Continuum	Selects and observes a nearby calibration source with different focus settings to create an out-of-focus holography map to update the surface corrections.
`Focus()`	Continuum	Performs a focus observation.
`Peak()`	Continuum, Line	Performs a pointing observation.
`Tip()`	Continuum, Line	Performs an observation to derive \(T_{sys}\) vs. elevation.
`Slew()`	Continuum, Line, Pulsar	Slews the telescope to the specified source or location.
`Balance()`	Continuum, Line, Pulsar	Balances the IF system so that each device is operating in its linear response regime.
`BalanceOnOff()`	Continuum, Line, Pulsar	Move from a source to a reference position and balance the IF system at the mid-point of the two power levels.

Observing Scan Types#

Observing scan types will aquire scientific datasets by performing one or more scans at specific locations on the sky.

Table 5 GBT Observing Scans#
Scan Type	Observing Type	Description
`Track()`	Continuum, Line, Pulsar	Takes data at a single position or while moving with constant velocity
`OnOff()`	Continuum, Line	Observe a source and then a reference position
`OffOn()`	Continuum, Line	Observe a reference position and then a source
`OnOffSameHA()`	Continuum, Line	Observe a source and then a reference position using the same hour angle as the sourcec in the observation.
`Nod()`	Continnum, Line	Observe a source with one beam and then with another beam
`SubBeamNod()`	Continuum, Line	Moves the subreflector alternately between two beams

Mapping Scan Types#

Mapping scan types will record data over specified areas of the sky.

Most GBT mapping procedures have versions that allow for periodic reference observations. These may be used to correct for the instrumental bandpass shape in total power observations during data reduction. For OTF mapping, some observers may prefer to use the edge pixels of the map as a reference position if they are suitably “off-source”.

Note

The GBT mapping calculator is a useful tool for planning mapping observations It may be used to provide AstrID commands and parameters for many of the mapping scan types.

Important

An important restriction on mapping with the GBT is that the raster scan legs or petal lengths should be at least 30 s so that the telescope only turns around a maximum of twice per minute. This helps to minimize stresses on the telescope.

Table 6 GBT mapping scans#
Scan Type	Observing Type	Description
`RALongMap()`	Continuum, Line	Make an OTF raster map by moving along the major axis of the coordinate system.
`RALongMapWithReference()`	Continuum, Line	Make an OTF raster map by moving along the major axis of the coordinate system and making periodic reference observations.
`DecLatMap()`	Continuum, Line	Make an OTF raster map by moving along the minor axis of the coordinate system.
`DecLatMapWithReference()`	Continuum, Line	Make an OTF raster map by moving along the minor axis of the coordinate system and making periodic reference observations.
`PointMap()`	Continuum, Line, Pulsar	Make a map using individual pointings.
`PointMapWithReference()`	Continuum, Line, Pulsar	Make a map using individual pointings with periodic reference observations.
`Daisy()`	Continuum, Line	Make an OTF map in the form of daisy petals.

Utility functions#

Utility functions are used in SBs to control various aspects of the GBT other than data-taking scans. This includes such things as changing power levels, pausing the SB or waiting for a source to rise.

Table 7 GBT utility functions#
Utility Function	Description
`Annotation()`
`Break()`
`Comment()`
`GetLST()`
`GetUTC()`
`Now()`
`WaitFor()`
`ChangeAttenuation()`

Scheduling Block Objects#

Scheduling Block Objects are python objects that are used to contain multiple pieces of information within a single variable. These are used with positions, times, and for defining a horizon for the minimum elevation below which you would not want to observe.

Table 8 GBT Scheduling Block Objects#
SB object	Description
`Location Object`
`Offset Object`
`Horizon Object`
`Time Object`