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GBT Scan Types

A Scan, formerly referred to as an observing procedure, is a pattern of antenna motions that when used together yield a useful scientific dataset. This page describes the various scan types that are available for use within GBT Scheduling Blocks. Each scan consists of one or more subscans, which are the individual components of the antenna pattern. You may find the Observing API Building Blocks documentation to be a useful reference when reading this page. It contains descriptions of various support classes like Catalog, Location, and Offset.

Please note that the syntax for all Scan Types is case-sensitive.

AutoFocus

AutoPeak

AutoPeakFocus

Syntax:

The intent of this scan is to automatically peak and/or focus the antenna for the current location on the sky and with the current receiver, therefore it should not require any user input. However, by setting any of the optional arguments the user may partially or fully override the the calibration search and/or procedural steps as described below.

The sequence of events in full automatic mode, i.e, with no arguments are:

  1. Get recommended beam, antenna/subreflector motions, and duration for peak and/or focus scans.
  2. Get current receiver from the control system.
  3. Get current antenna beam location from the control system.
  4. Run a configure for Continuum with <receiver> using named configuration cases.
  5. Run a balance to obtain accurate system temperature readings from the DCR.
  6. Select a source using computed minimum flux, observing frequency, location, and search radius.
  7. If no qualified source is found within the radius, then provide the observer the option to use a more distant source (default), and if none found either aborting (second default) or continuing the scheduling block.
  8. Slew to source.
  9. Run a balance to set scan power levels.
  10. Run a scan using either Peak and/or Focus.

Note that these Auto scan types are the only standard scan types that autonomously select a source, configure the telescope, or access a pointing Catalog without additional calls in the block.

Setting optional arguments will cause the scan to skip unneeded steps:

Parameter Info:

The example, demonstrates the expected use:

Used with these GBT Standard Observing Modes: Focus Peak Corrections

BalanceOnOff

Syntax:

Parameter Info:

When there is a large difference in power received by the GBT between two positions on the sky, it is advantageous to balance the IF system power levels to be at the mid-point of the two power levels. Typically this is needed when the "source position" is a strong continuum source. This scan type has been created to handle this senario. BalanceOnOff() slews to the source position and then balances the IF system. It then determines the power levels that are observed in the IF Rack. Then the telescope is slewed to the off position and the power levels are determined again. The change in the power levels is then used to determine attenuator settings that put the balance near the mid-point of the observed power range. Note that the balance is determined only to within +/- 0.5 dB owing to the integer settings of the IF Rack attenuators.

The following example balances on 3C48 and remeasures 2 degrees off:

Daisy

Syntax: This command scans around a central point in the form of a rose curve.

Parameter Info:

The area of the sky covered will be circular, with a diameter equal to twice the specified map_radius. For map radii of a few arcminutes, a radial_osc_period of 60 sec or longer is recommended; a scanDuration of 20 radial_osc_period's will result in an approximately closed pattern. For beamsizes of 20 arcsec (fwhm) or so, the circular area mappped will be fully sampled if the map radius is less than 6'. It is not an especially useful observing mode for general-purpose single-beam mapping, since the largest "hole" in the map is approximately 0.3 x map_radius. However it is useful for focal-plane arrays.

This example produces a three leaf map about "3C123":

Used with these GBT Standard Observing Modes:

DecLatMap

Syntax: This command scans in the Dec or latitude-like direction, and steps in RA or longitude by hDelta.

Parameter Info:

A Declination/Latitude map, or DecLatMap, does a raster scan centered on a specific location on the sky. Scanning is done in the declination, latitude, or elevation coordinate depending on the desired coordinate mode. This scan type does not allow the user to periodically move to a reference location on the sky; please see DecLatMapWithReference for such a map. The starting point of the map is defined as (-hLength/2, -vLength/2).

This example produces a map with 21 columns each 30' long, using a spacing of 6' and scan rate of 180'/min using beam 1. The map is 2 degrees in RA by 0.5 degrees in Dec.

Note that the "Location" may be in a different coordinate system than the lengths.

Used with these GBT Standard Observing Modes: OTF Map; Continuum Map

DecLatMapWithReference

Syntax:

Parameter Info:

A Declination/Latitude map with reference source, or DecLatMapWithReference, does a raster scan centered on a specific location on the sky and periodically moves to a reference location on the sky. Scaning is done in the declination, latitude, or elevation coordinate depending on the desired coordinate mode. This procedure does allows the user to periodically move to a reference position on the sky, please see DecLatMap if no reference point is required. The starting point of the map is defined as (-hLength/2, -vLength/2).

This example produces a bidirectional map with 21 columns each 30' long, using a spacing of 6' and scan rate of 180'/min using beam 1. This map goes to a reference point 2 degrees north and 2 degrees east of the map center every 3 columns:

Used with these GBT Standard Observing Modes: OTF Map; Continuum Map

Focus

Syntax:

The only required parameter is location.

Parameter Info:

The Focus procedure sweeps the subreflector or prime focus (depending on the receiver in use) through the axis aligned with the beam. Its primary use is to determine focus positions for use in subsequent scans. Note that the FocusLength? and scanDuration should be overridden as a unit since together they determine the rate.

Important: Because of the addition of the location keyword, Focus will not be backwards compatible.

In the following example a focus subreflector is performed from -200 to +200mm at 400mm/min using beam 1:

Or using the defaults:

Used with these GBT Standard Observing Modes: Corrections

LSFS

Syntax:

Parameter Info:

This scan type performs a "Least Squares Frequency Switch," where a single scan is broken into 8 equal parts, each observed at a unique frequency (as described above). This routine was devised by Tim Robishaw and Carl Heiles to minimize the IF ripples in the bandpass.

This scan type is used for polarization studies, but could be used for other observing schemes. It only works with the Spectrometer backend.

The following example generates a subscan at 1258+6126:

Note that to accomplish this ability, the spectrometer and LO1's switching signal states are defined differently, i.e., the spectrometer (the data collector and switching signal generator) is set up for one scan with 8 equal integrations and 1 sig/ref period per integration. The LO1 (the frequency controller) is set up for an one scan, but with 8 sig/ref periods per scan. Then the delta-frequencies fed into the LO1 are doubles, i.e., f1, f1, f2, f2, f3, f3, ..., f8, f8, so each integration has two switching states which are equivalent.

Used with these GBT Standard Observing Modes: Z17; Spider

Nod

Syntax:

Parameter Info:

The Nod procedure does two subscans on the same sky location with different beams.

The following example does a Nod between beams 1 and 2 with a 60 second subscan duration:

Used with these GBT Standard Observing Modes: Position Switched (Dual Beam); Beam Switched

OffOn

Syntax:

Parameter Info:

The OffOn scan performs two subscans. The first subscan is offset from the location, used in the second scan by the referenceOffset and subscan is on source.

The following example does an OffOn scan with reference offsets of 1 degree in width and height and a 60 second subscan duration, using beam 1:

Used with these GBT Standard Observing Modes: Position Switched (Single Beam)

OffOnSameHA

Not implemented in beta version due to time-sensitive limitations of M&C system.

Used with these GBT Standard Observing Modes: Position Switched (Single Beam)

OnOff

Syntax:

Parameter Info:

The OnOff scan performs two subscans. The first subscan is on source, and the second subscan is at an offset to the source used in the first subscan.

The following example does an OnOff scan with reference offsets of 1 degree in width and height and a 60 second subscan duration, using beam 1:

Used with these GBT Standard Observing Modes: Position Switched (Single Beam)

OnOffSameHA

Syntax:

Parameter Info:

The OnOffSameHA scan performs two subscans. The first subscan is on source, and the second subscan follows the same HA track used in the first subscan (now off source).

The following example does an OnOffSameHA scan with a 60 second subscan duration, using beam 1:

WARNING: This procedure should be used with caution at high frequencies or in confused regions of the sky. Due to imprecisely repeatable scan-start overheads, the location of the OFF beam from one scan to the next is not guaranteed to be in a fixed place on the sky. Variations greater than an arcminute are not uncommon.

Used with these GBT Standard Observing Modes: Position Switched (Single Beam)

Peak

Syntax:

The only required parameter is location.

Parameter Info:

The Peak scan type sweeps through the specified sky location in the four cardinal directions. Its primary use is to determine pointing corrections for use in subsequent scans Note that the hLength/vLength and scanDuration should be overridden as a unit since together they determine the rate.

The following example does a peak in encoder coordinates with 90 minute lengths and a 30 second subscan duration using beam 1.

Peak("0137+3309", Offset("Encoder", "00:90:00", 0), Offset("Encoder", 0, "00:90:00"), 30, "1")

or with the defaults.

Peak("0137+3309")

Used with these GBT Standard Observing Modes: Pointing Corrections

PointMap

Syntax:

Parameter Info:

A PointMap constructs a map by scanning on fixed positions laid out on a grid. This scan type does not allow the user to periodically move to a reference location on the sky; please see PointMapWithReference for such a map. The starting point of the map is defined as (-hLength/2, -vLength/2).

The following example does a 4x4 point map using beam "C":

Used with these GBT Standard Observing Modes: Point Map

PointMapWithReference

Syntax:

Parameter Info:

A PointMapWithReference constructs a map by scanning on fixed positions laid out on a grid. This scan type allows the user to periodically move to a reference location on the sky; please see PointMap if no reference location is required. Reference interval is the number of points that should be completed before moving to the reference offset. The starting point of the map is defined as (-hLength/2, -vLength/2).

The following example does a 4x4 point map using beam "C" that moves to a reference source every 2 points:

Used with these GBT Standard Observing Modes: Point Map

RALongMap

Syntax:

Parameter Info:

A right ascension/longitude (RALong) map performs a raster scan centered on a sky location. Subscans are performed in the right ascension, longitude, or azimuth coordinate depending on the desired coordinate system. This scan type does not allow the user to periodically move to a reference location on the sky; please see RALongMapWithReference for such a map. The starting point of the map is defined as (-hLength/2, -vLength/2).

This example produces a map with 6 rows each 120' long, using a spacing of 6' and scan rate of 720'/min:

Used with these GBT Standard Observing Modes: OTF Map; Continuum Map

RALongMapWithReference

Syntax:

Parameter Info:

A right ascension/longitude (RALong) map performs a raster scan centered on a sky location. Subscans are performed in the right ascension, longitude, or azimuth coordinate depending on the desired coordinate system. This scan type does allow the user to periodically move to a reference location on the sky; please see RALongMap for a map that does not use a reference. The starting point of the map is defined as (-hLength/2, -vLength/2).

This example produces a map with 6 rows each 120' long, using a spacing of 6' and scan rate of 720'/min that moves to a reference position every 7 rows:

Used with these GBT Standard Observing Modes: OTF Map; Continuum Map

Slew

Syntax:

Parameter Info:

Slew moves the telescope beam to point to a specified location on the sky.

  1. If only a location is given the antenna slews to the indicated position.
  2. If a location and offset are given, the antenna slews to the indicated position plus the specified offset.
  3. If only an offset is given, the antenna slews to the current location plus the specified offset.

The following example slews to 3C48 using the center beam:

Note that Slew() with no arguments is basically a NOP, i.e., it slews to the current location (but in J2000):

Though one could change beams at that position:

Spider

Syntax:

Parameter Info:

Spider executes the specified number of slices of length scanDuration through the specified location. Each slice is of length 2*startOffset. The argument startOffset also specifies the angle of the initial slice. The user may specify unidirectional or bidirectional subscans of length calDuration and when to run calibration subscans relative to each slice, i.e., at "begin", "end", or "both".

This scan type is used for polarization studies.

The following example generates subscans through 1258+6126 starting the first leg 40' from the source's "right."

Used with these GBT Standard Observing Modes: LSFS; Z17

SubBeamNod

Syntax:

Parameter Info:

For two-beam receivers SubBeamNod causes the subreflector to tilt about the x axis between the two feeds at the given periodicity. The primary mirror is centered on the midpoint between the two beams. The beam selections are extracted from the scan's beamName, i.e., MR12 or MR34. The "first" beam ("1" or "3") performs the first integration. The periodicity is specified in seconds (float) per nod (half-cycle). A nod is limited to a minimum of 4.4 seconds. An example:
Track("3C48", None, 60.0, beamName="MR12", submotion=SubNod(4.4826624))
Alternatively, one can specify the nod time in units of the primary backend's integration times (integer) by setting the periodicity units to integrations instead of the default seconds, e.g.,
Track("3C48", None, 60.0, beamName="MR12", submotion=SubNod(nodLength=3, nodUnit="integrations"))
If the backend's actual integration time is obtainable then a warning is issued if the alignment between the integration times and the nod times shift over the duration of the scan by more than 10% of the nod time. A warning is issued in any case if the backend's actual integration time is not obtainable. Attempting to use integrations as the unit when the integration time cannot be obtained from the selected backend will cause a failure.

The following example does a subreflector nod between beams 1 and 2 for 60 seconds, each nod or half-cycle lasts for three integrations where Rcvr26_40 was selected in the configuration:

Used with these GBT Standard Observing Modes: Position Switched (Dual Beam); Beam Switched

Tip

Syntax:

Parameter Info:

The Tip scan moves the beam on the sky from one location to another location. The coordinate modes of the locations must both be AzEl. Scan timing must be specified by either a scanDuration, a stopTime, a startTime plus stopTime, or a startTime plus scanDuration.

The following example tips the GBT from 6.0 degrees in elevation to 80.0 degrees in elevation over a period of three minutes using the center beam:

Used with these GBT Standard Observing Modes: Tipping Scan

Track

Syntax:

Parameter Info:

The Track scan follows a sky location while taking data. Scan timing must be specified by either a scanDuration, a stopTime, a startTime plus stopTime, or a startTime plus scanDuration.

The following example tracks 3C48 for 60 seconds using the center beam:

Used with these GBT Standard Observing Modes: Total Power Track; Frequency Switched Track; VLBI; Pulsar; Radar

Z17

Syntax:

Parameter Info:

Z17 executes two circles of point subscans around location at 45 degree intervals. The first circle with a radius of startOffset and the second circle at a radius of sqrt(2)*startOffset. The initial subscan is at the angle specified by the startOffset. After circling twice, the procedure executes a subscan on location. The entire set of 17 subscans each of length scanDuration, is sandwiched between two cal subscans of lengths calDuration which consist of equal parts calibration noise signal on and off.

This scan type is used for polarization studies.

The following example generates subscan points around 1258+6126 starting the first circle at the source's "right."

Used with these GBT Standard Observing Modes: LSFS; Spider

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