NRAO Skynet 20-meter telescope

Links UNC Skynet 20m skynet interface Log of 20m results. NRAO Skynet main Observing Advice

Details for various kinds of observing projects
Main Observing Advice Mapping/Imaging Advice Spectrum Observing Advice Pulsar observing



Making Images

Preamble -- For those not familiar with radio astronomy, note that the 20-meter telescope is like a camera with one pixel, which we call a "beam". To make an image, one must scan the telescope back and forth across the object and build up an image, pixel by pixel, a process called mapping.

Beamwidth -- The beamwidth of a telescope is given by Bw = 1.2λ/D (in radians).
in which λ is the observing wavelength, and D is the telescope diameter.
If you use the frequency in GHz (Fghz) instead of wavelength, then the
beamwidth in arcminutes is: Bw = 62/Fghz (for a 20-meter telescope)

General Advice -- Maps should be at least 5 beamwidths across, otherwise you cannot distinguish the object from the background.


Quick Advice

For the 20cm receiver (installed as of Aug 2013)
Here are some rules of thumb that will get you pretty good maps.
    Set the minimum elevation to 25 degrees  
    Set the solar separation to 15 degrees  

Daisy Pattern:
-----------------
  1. use a radius of at least 90 arcminutes.
  2. number of petals: 4 for a quick look,
      8 or 12 petals for an reasonable image of the central part of the field.

  3.  For Radius:
       up to 180 arcmin -- use 45 sec per petal
        200-300         -- use 75 sec per petal
        300-400         -- use 100 sec per petal
        (above that use radius divided by 10)

     Multiply these times per petal by the number of petals
     to get the total duration.

  4. use integration time of 0.2 seconds in all cases.


Map pattern:
--------------
  1. "Map Size" : Use a size of 6x6 beam widths or larger.

  2. "Sampling Density" : 
	Set the gap between sweeps to 1/4 for average maps
	Set the gap to 1/5 for higher quality.

  3.  Under "Map Depth"
	Select "integration time"
	Look at the Slew Speed: this should be 0.6 or less.
	Put integration time = 0.3 seconds.
	Then check the Slew Speed, if greater than 0.6, 
		increase the integration time in steps of 0.1
		until slew speed is less than 0.6

Usually you can just set the map size and the gap between sweeps.  The interface will
fill in reasonable values for the rest.

Happy mapping!

  • At the bottom of the page, note the number of sweeps, the sweep duration, and the total duration of the map. Generally you are discouraged from making maps that take more than 30 minutes or so to complete. If you want to map a very big area, it is more efficient to make several smaller maps.



    [Frank D. Ghigo, NRAO-Green Bank, Nov 2013, rev Jan 2014, rev March 2015, rev June 2020]