NRAO Skynet 20-meter telescope

[Frank D. Ghigo, NRAO-Green Bank, Jan 2014]

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



Spectral Line Observing

There are two spectral line modes: high-resolution (hires), and low-resolution (lowres). The user can select the mode in the Skynet 20-meter observing web page.

the mode is selected in one of the "Add Radio Observation" dialogs:;


Quick advice for good 21cm Hydrogen spectra:

Select High Resolution Mode 

Set the two frequencies to 1420 and 1420 MHz 
Set Number of channels to 1024 or 4096

Galactic Hydrogen on the galactic plane:
   pattern: Track; duration: 60 seconds;  integration time: 1 sec

Extragalactic objects:
   pattern: On/Off;  duration: 200 seconds;  integration time: 1 sec; Offset: 3 degrees
   And repeat the on/off 4 or more times.

   Calculate the frequency of the HI line for the velocity of the object, 
   and set one of the frequecies (or both) to that frequency, in MHz.

Quick advice for OH spectra:

Select High Resolution Mode 

Set the two frequencies to 1664 and 1664 MHz 
Set Number of channels to 1024 or 4096

Select a source that is likely to have OH emission such as Orion A or W3
     Pattern: On/Off; duration 120 seconds; integration time 1 sec; Offset: 4 deg.

 For both HI and OH:
    Set the two frequencies to 1418 and 1668 

NOTE: in high resolution mode, it is not recommended to do maps or daisies, as there is no software available to process such data. If you do mapping in high resolution mode, use integration time recommendations from the mapping advice page.

Low Resolution Mode

Low resolution mode provides 1024 spectral channels in a 500 MHz band. Thus the channel spacing is 0.488 MHz. The portion of the 500 MHz band that can be observed depends on which feed/filter combination is installed.
If the 21cm or 18cm feed is installed, then you have no further choices.
(although you can type a number into the "Center Frequency" box, it is used only if the 8-10 GHz receiver is installed.)

Receiver Feed Filter Center Frequency Available Bandwidth
Rx1_2 21cm 1355-1435 MHz 1395 MHz 80 MHz
Rx1_2 18cm 1600-1730 MHz 1680 MHz 100 MHz
Rx8_10 3.3cm 8000-10000 MHz settable by user 500 MHz


High Resolution Mode

In this mode, there are two bands of 15.625 MHz each that can be placed within the total 500 MHz bandpass of the spectrometer. The user can specify center frequencies and number of channels. The center frequencies are not totally arbitrary, but have to be integral multiples of 15.625, as follows:
Receiver Feed band centers
Rx1_2 21cm 1000 + n*15.625 MHz
Rx1_2 18cm 2000 - n*15.625 MHz
Rx8_10 3.3cm Fc +250 - n*15.625 MHz
Where n is a positive integer; Fc is the user-settable center frequency.

You may enter any desired two frequencies into the Skynet web form, and the actual frequencies used will be the closest possible ones. For example, if you specify 1420 MHz, the band will be centered at 1421.875 MHz.

The same filters as listed above are used in both modes; thus if the 21cm feed is in use, the frequencies should be between 1355 and 1435 MHz.

Number of Channels
The number of channels may be selected from the following table, which shows the channel spacing in each case.
Number of Channels channel spacing in frequency velocity spacing (21cm)
1024 15.26 kHz 3.22 km/sec
4096 3.81 kHz 0.81
8192 1.91 kHz 0.40
16384 0.95 kHz 0.20


Web displays and downloadable files for High Resolution Mode

The quick-look web page is found at the following web url:
  • http://www.gb.nrao.edu/20m/peak/latest/

    For high resolution data the display looks different from what you are used to for low resolution. Here is an example for a daisy scan on Cass-A.
    Example quick-look web display for a high resolution observation

  • The left-hand plot shows the continuum; this is the average of the central 80% of each spectrum plotted vs time. The 4 traces are two polarizations for the two frequency bands.
  • The right-hand plot shows the average spectrum, averaged over all integrations. The upper and lower plots are for the two frequency bands, and the 2 traces in each plot are for the two polarizations.

  • The "continuum data" button accesses the continuum data as a text file. Note that the last column indicates the state of the calibration: if =1, the cal is on, if =0 the cal is off, and if = -1, the cal is in an intermediate state. So for calibration purposes you should ignore the lines with the cal flag = -1.
  • Likewise, the "spectrum data" button shows the data that went into the spectrum plots.


    Observing High Resolution Spectra

    The sky patterns used for spectra are "Track" or "On/Off". One can do daisies and maps, as in the example above, but if one is primarily interested in spectra, then the "Track" and "On/Off" patterns are preferable.

  • Integration time: for "Track" and "On/Off" procedures, integration time should be 1 or 2 seconds. Using shorter times just unnecessarily increases the size of the data files.

    Track: In the Skynet web form, you need to specify the duration of the track, and the integration time.

    On/Off: The duration is the time to spend on the source and the time off source, so the total time for the observation is twice the duration.

  • the Offset gives the number of degrees the "off" scan should be from the "on source" scan.
  • Normally the integration time should be 1 second.

    The "On/Off" pattern is used for fainter sources, in which the "off" data needs to be subtracted from the "On" data in order to emphasize the spectrum of the desired object.

    Some handy quick advice:
    For observing the 21 cm line of Hydrogen
    Set the two frequencies to 1421 and 1405 MHz 
    Set Number of channels to 1024 or 4096
    
    Galactic Hydrogen on the galactic plane:
       pattern: Track; duration: 60 seconds;  integration time: 1 sec
       (use longer duration for better signal-to-noise)
    
    Extragalactic objects:
       pattern: On/Off;  duration: 200 seconds;  integration time: 1 sec; Offset: 3 degrees
       And repeat the on/off 4 or more times.
    

  • Example of galactic hydrogen at 60 degrees longitude