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

Nearby Extragalactic galaxies:
   pattern: On/Off;  duration: 200 seconds;  integration time: 1 sec; Offset: 4 degrees
   And repeat the on/off 2 or more times.

   For more distant objects, try longer durations and more repeats.

    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 filter is selected.

Receiver Filter Center Frequency Available Bandwidth Num Channels
Rx1_2 HI 1355-1435 MHz 1395 MHz 80 MHz 164
Rx1_2 OH1 1630-1730 MHz 1680 MHz 100 MHz 205
Rx1_2 OH2 1650-1750 MHz 1700 MHz 100 MHz 205
Rx1_2 all 1350-1750 MHz 1550 MHz 400 MHz 820
Rx8_10 8000-10000 MHz settable by user 500 MHz 1024
There is a lot of RFI in L-band in the range 1550 to 1610 MHz. The HI filter cuts out most of the RFI, so is best to use for continuum observations. When observing the OH lines, the "OH2" is better since it includes less RFI than the "OH1".

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 spaced apart by integral multiples of 15.625.

You may enter any desired two frequencies into the Skynet web form, and the system will adjust them to be multiples of 15.625 MHz apart. The two frequencies may not be separated by more than 480 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