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

• Raster scans
  • The 20-meter can do two types of mapping: raster (i.e., back and forth), or daisy patterns.
  • The raster scans come in 2 flavors: "RALongMap" or "DecLatMap".
  • Mapping is in either RA/DEC or Galactic Latitude/Longitude coordinates depending on the coordinate type selected in the Target screen.
  • RaLongMap: sweep along the Right Ascension (or Longitude) direction, and step sucessive sweeps in Declination (or Latitude).
  • DecLatMap: sweep along the Declination (or Latitude) direction, and step sucessive sweeps in Right Ascension (or Longitude).
• Daisy: scan in a flower-petal pattern with the object of interest at the center.

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)

• The 8-10 GHz receiver has a beam size of about 7 arcmin.
• The 1.4 GHz receiver has a beam size of about 44 arcmin.
• The 1.4 GHz receiver is presently on the telescope.

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

• For the 1.4 GHz receiver, the beam is about 44 arcminutes, so raster maps should be no smaller than 4x4 degrees; and daisy maps should have a radius of 2 degrees or more.

  
  Cautions:

• Blocking near the horizon: Use a minimum elevation somewhat higher than the default 5; there is blockage in certain directions near the horizon. The horizon is less blocked to the south; for far south sources such as the galactic center, the minimum elevation of 3 degrees will probably be ok.

• Also, due to the radiation from the atmosphere, mapping near the horizon shows the variations in system temperature due to the atmosphere, which may drown out the brightness from weaker sources. Best to use a minimum elevation of 25 degrees when doing maps.

• The 20-meter can pick up radiation from the Sun in its far sidelobes. To avoid this try to keep all parts of a map at least 15 degrees away from the Sun. In the Skynet interface, set the "Solar Separation" to 15, unless you really want to observe the Sun.

Quick Advice

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!