NRAO Logo

GBT Mapping Calculator

Last Modified: December 31, 2014
Ronald J Maddalena
Calculates the time needed to map,an area including overhead, based on the integration time per beam area (e.g., the results from the Sensitivity Calculator) and the area to be mapped.  Provides example ASTRID commands as well as other mapping parameters. Supports the typical mapping commands for on-the-fly (OTF) rectangular maps, OTF Daisy maps, and point rectangular maps.

                                

Input Values Instructions

Backend and Observing Modes         

Mode:
Frequency Switching (Line Only)
Total Power (Line or Continuum)

Noise Diode:
On
Off

Doppler Tracking (Line Only):
On (Hardware)
Off (Software)

Used by the calculator to provide values for the minimum time resolution, minimum switch period, and minimum integration time.  Minimum switch periods for VEGAS are derived from Tables 2, 3, and 4 in GBT Memo 288, plus the values you supply for sky frequency, and the TINT/SWPER ratio. 

The values used by the calculator for any backend/mode are summarized  .
  • For DCR, CCB, and Mustang observing the calculator will use Total Power with Doppler Tracking Off, regardless of what you specify. 
  • For Mustang observing the calculator will use Noise Diode Off, regardless of what you specify. 
  • For VEGAS continuum observing, the calculator makes no assumptions and you must specify Total Power and turn Doppler Tracking Off.
  • For VEGAS spectral line observing, we recomend using Hardware Doppler Tracking unless you are sampling very fast and must use Software Doppler Tracking.
  • Depending upon the results of the Sensitity Calculator, specify either Frequency Switching or Total Power.
  • Specify Noise Diode On for all observations excpet those with extremely fast sampling.  But, always specify Noise Diode Off when using the 68-92 GHz receiver.
Map TypeOTF Rectangle
OTF Daisy
Point Rectangule

The type of map.  Either:
OTF Rectangle: On-the-fly mapping of a rectangular area.  The OTF motion is either along rows of the map (i.e., RALongMap) or along columns (DecLatMap).  OTF Rectangle is the default map type.
OTF Daisy: On-the-fly map of a circular region of the sky where the telescope is in continuous motion making petal-like tracks
Point Rectangle:  Map of a rectangular region where the telescope moves to a map position, sits for a specified amount of time, moves to the next position, and so on.
Frequency  MHz

Sky Frequency in MHz.  If source is at high velocities, this might not be the rest frequency.  Regardless of what you enter, the calculator will use 86000 MHz for Mustang observing and 30000 MHz for CCB observing.
Integration Time Per Beam seconds

Integration time per beam area in seconds, typically the results produced by the Sensitivity Calculator.  The lower the value, the faster the map.  For multi-feed mapping, use the integration time per beam area divided by the number of feeds.
Map SizeEither Rectangle Size:
Horizontal Deg
Vertical Deg
Or Map Radius:
  Deg

Map dimensions in degrees.

Horizontal and vertical sizes are ignored if map type is OTF Daisy.

Radius is ignored if map is OTF Rectangle or Point Rectangle.
Row/Column/Petal Oversampling:

A value of 1 indicates Nyquist (critical sampling, which is the default value).  The lower the value, the faster the map.  Usage depends upon the type of map:
  • For OTF Rectangle maps, specifies the amount of oversampling between rows (for RALongMap) or between columns (for DecLatMap). 
  • For Point Rectangle maps, specifies the amount of oversampling between rows and columns. 
  • For OTF Daisy maps, specifies the amount of oversampling between the petals at the very edge of the map. 
If the value is:
  • > 1, the map is oversampled and will be as accurate as is possible.  Values > 1.2 are seldom used.
  • < 1, the map is undersampled, will be missing information, and have low fidelity.  Typical values for undersampled maps are:
    • 0.4 produces approximate FWHM spacing.
    • 0.8 produces approximate HWHM spacing.
On-The-Fly Oversampling

Ignored if map type is Point Rectangle as these do not OTF observe.  Otherwise, specifies the oversampling in the direction of the OTF slew.  Usually > 1 (where 1 = Nyquist sampling) since high values produce the less degradation from smearing along the slew direction.  The lower the value, the faster the map.

Default value is 2, as recommended by Mangum, Emerson, and Greisen (reference below).  Take note, if:
  • >= 2, oversamples and produces an accurate, high-fidelity map of a region with minimal defects from smearing due to the OTF motion.  Values > 3 are seldom used unless the calculator warns that you will have excessive line broadening.
  • > 1, oversamples and produces an accurate map but with possible defects from smearing due to the OTF motion.
  • < 1, undersamples and produces a map with missing information, with low fidelity, and suffering from the affects of smearing from the OTF motion,  Typical values for undersampled maps are:
    • 0.4 produces samples with widths equal to about  FWHM.
    • 0.8 produces samples with widths equal to about  HWHM.
Number of Switch Periods per
Integration (TINT/SWPER)

Specifies the integer ratio of the switching cycle to the integration time for the backend.  The higher the value, the higher the map fidelity but also the higher the overhead from blanking.
  • Calculatior will always use a value of 1 if Total Power, Doppler Tracking is Off, and Noise Diode is Off or if Mustang is the backend.
  • For CCB observing, the calculator always uses an estimate that keeps the switch period to ~1 msec.
  • For all other backens, we recommend using the default value of 4 as a  compromise between extra overhead and map fidelity.
  • You may opt for a lower value when the maps requires integration times that are close to the minimum allowed by the backend.
  • For frequency-switched, Doppler tracked, spectral-line maps, higher values may be used if the calculator warns that your line will be broadened too much for your selected VEGAS mode. 
  • For Total Power, Doppler tracked maps, higher values will lower the line broadening.  But, when Doppler Tracking is off (i.e., Software tracking), higher values will not lower the line broadening.


                               

ALGORITHM DETAILS:

The algorithms used by the calculator follow those described in Mangum, Emerson, and Greisen (2007, A&A 474, 679) but with the incorporation of the restrictions imposed by the the GBT's maximum slew rates and the backend's minimum sampling time and time resolution.  The calculated time per sample returned by the calculator always will be an integer multiple of the backend's time resolution and will have a value greater than the backend's minimum sampling time for the selected mode.  The calculated slew rate is kept below the telescope's maximum.  The time per row or column will always be an integer multiple of the sample time.  And, the number of rows or petals in the map will be integer.  Since the calculator always rounds up in its calculations, you will always get at least the desired time per beam area. 
   
Because of these restrictions, the actual time per beam area will be larger than that specified.  But, the calculator ensures that the returned value will always be as close as possible to the user's input value.  The user should be aware that there are a large number of situations when the hardware restrictions will produce values for the actual time per beam that are much larger than the desired values and, thereby, the map will take significantly longer than one would initially expect.  If such observations are run as the calculator specifies, the map will have higher sensitivity than was originally specified.  Optionally, if the actual time per beam or map time is much higher than you desire or can tolerate, and your observing isn't approaching the telescope's maximum slew rate, then using a faster-sampling mode, if one exists, might help.

The calculator warns the user whenever they will be exceeding the telescope's maximum number of accelerations of 2 per minute but the calculator does not enforce this restriction.  If you receive this warning, you may need to increase values for either the time per beam or the map size, which will increase the time needed to make the map.
    
Note: For map type of OTF Rectangle the calculator provides separate answers for RALongMap and the DecLatMap as the overhead, row spacing, etc will be different for the two styles of map. If you are using the Astrid procedures of PointMapWithReference, RALongMapWithReference, or DecLatMapWithReference, the calculator does not include the overhead or observing time for performing the reference observations.

Created and maintained by:
Ron Maddalena and the Green Bank Support Staff