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

Happy mapping!

Some Example Setups

Quick, pretty good raster Map.
Larger, better quality Map.
Daisy: quick look
Daisy: larger and denser


Lots of gory details and calculations follow, if you are interested --


Link to: Daisys - Sweeps

Daisy Scans

The daisy scan is specified in Skynet by four parameters:
4-petal daisy scan 12-petal daisy scan

How Large a Radius?

The radius depends on the size of the region of interest, and, as mentioned above, a minimum radius of 2-3 beamwidths is desirable. Thus, a good choice would be the size of the object to be observed plus 2 beamwidths.

How Many Petals?

For a quick look, the 4-petal pattern is sufficient. This gives one a measurements of the peak brightness of the object as the telescope scans through the center. This is good for "point sources", i.e., ones with angular diameter less than about half of the beam width.
For sources whose angular sizes are several beam widths across, one can use more petals in the pattern to make an image of the object. But note that the daisy pattern is more dense near the center than at the edges, so one gets a more detailed image nearer the central part of the pattern.
For a fully sampled map one needs the data points to be spaced closer than a half beamwidth. One can estimate the size of the fully sampled part of a daisy scan by the approximate formula :
  • Rfs = radius of fully-sampled area = 0.5BeamWidth/tan(360degrees/2Np)

    In general if you are interested in a detailed image of an extended object, the raster scan (RALongMap or DECLatMap) is a better choice.

    This table shows the approximate radius of the fully-sampled area for various choices of Np and observing frequency.
    Frequency Beam Width N petals Radius of fully-sampled area
    1.4 GHz 44 arcmin 4 22 arcmin
    1.4 GHz 44 8 54
    1.4 GHz 44 12 84
    1.4 GHz 44 20 142
    ..........
    8.5 GHz 7.5 arcmin 4 4 arcmin
    8.5 GHz 7.5 8 9
    8.5 GHz 7.5 12 14
    8.5 GHz 7.5 20 23

    How much time?

    As the telescope sweeps across the source, the quality of the data depends on the number of samples per beam width. One needs at least 3 samples per beam; we will adopt "good", "better", or "best" the choice of 3, 5, or 9 samples per beam (Nsb).
    The time spent on each sample is the specified integration time, Tint. Typically the minimum Tint is 0.1 seconds. Adopting some values for "good", "better", or "best" quality:
  • good: Tint = 0.1 sec, Nsb=3 samples/beam
  • better: Tint = 0.2 sec, Nsb=5 samples/beam
  • best: Tint = 0.3 sec, Nsb=9 samples/beam
    Examples for 1.4GHz observing (44' beam)
    Radius Npetals Quality Tint Nsb Time per petal Tdur
    120' 4 good 0.1 3 22 sec 90 sec
    120' 4 better 0.2 5 22 sec 90 sec
    120' 4 best 0.3 9 45 sec 180 sec
    .....
    300' 4 good 0.1 3 45 sec 180 sec
    300' 4 better 0.2 5 45 sec 180 sec
    300' 4 best 0.3 9 90 sec 360 sec
    Due to the limit in maximum telescope move rate of 40'/sec, there is no difference between the "good" and "better" cases.
    A complication arises due to the cosine elevation factor in the maximum slew rate.

    If you want more than 4 petals, just multiply the Tdur by Npetals/4.



    Raster (or sweeping) Scans

    The raster map is specified in Skynet by several parameters: Near the bottom of the screen is "Duration Information", which gives the number of sweeps, duration, and total map duration, depending on what numbers have been put in. Here is a plot of a typical back-and-forth sweep pattern.

    Map Size

    Map Size may be given in either "beam widths" or "degrees". The best choice is to use beam widths. If you want the map to be equal angular size in both dimensions, specify equal number of beams in both RA and DEC.

    Sampling Density

  • Gap between sweeps: in units of the beam width. Usually use 1/4 or 1/5. One may use larger fractions such as 1/3 and 1/2 for quick maps of extended sources such as galactic plane areas.
  • Gap along sweep: normally check the box so that this is the same as the gap between sweeps.

    Map Depth

    Set the integration time to 0.2 seconds, then check the Slew Speed. If the slew speed is greater than 0.6 degrees per second, increase the integration time in steps of 0.1, i.e., try 0.3, 0.4, etc, until the Slew Speed is 0.6 or less.
  • If you select "Slew Speed", then you can set the slew speed in the box, and it will set the corresponding integration time.

  • The relation between Slew speed, Integration time, and Gap is as follows:
    Given Gs = gap along sweep; Beam Width = BW (in degrees);
    tint = integration time in seconds; Sslew = slew speed in degrees/second.
    
    Then :  Sslew x tint = Gs x BW 
    
    Given any two of the quantities Gs, Sslew, tint, the third one can be calculated.
    
    The time to do one sweep is Tsweep = Length (in degrees) / Sslew.

  • 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]