Explanation of 20-meter telescope data files

Links: Latest Log 20meter info 20-m Skynet

There are, at the moment, 2 data displays; one the "standard" which is what you see initially with the quick-look web display; the "alternate" which you can see by clicking the butterfly.

In the "quick-look" web display one may click the links "Calibrated", "Raw Ascii", etc, to download tables of data.

Here are explanations of the information in these text files.

Data Headers

The header information consists of a list of keywords and their values, one keyword=value pair per line.
The last line of the header (starting with "#UTC_Time") is the column headers for the data.
Header Keywords

Continuum Data

For continuum raw data files referenced as Raw data or Raw ascii, the brightness readings are in "counts". For continuum calibrated data files, the calibration has been applied and the brightness readings are in Kelvin.

The first 5 columns are always the same:

  • UTC_Time(s) : UT time in seconds since UT midnight.
  • Ra : if HIRES data, RA is in degrees, otherwise in hours.
  • DEC : in degrees
  • Az : in degrees
  • El : in degrees

    For LOWRES data, there are 2 columns of brightness data labelled XL1 and YL1 or XX1 and YY1. For HIRES, there are 4 channels for the 2 frequencies and 2 polarization channels: XL1, XL2, YR1, YR2, ( or XX1,XX2,YY1,YY2).

    For LOWRES calibrated, standard processing, the minimum values have been subtracted. These minimums are given by the Keyword "MINS=". Otherwise, the brightness values are given in Kelvin.

    For each time step, the spectrum has been averaged over the frequency range STRTCHAN to STOPCHAN.

  • The "N" column, which may or may not be present, is the number of TBIN samples that have been averaged together.

    The "CAL" column:

  • If the noise cal is on, the value is "1"
  • If the noise cal is off, the value is "0"
  • If the noise cal is on for only part of the averaging period, the value is "-1"

    The "Sweeps" columns:

  • The first sweeps column counts the sweep number for mapping scans, starting at zero. It also counts on/off phases.
  • The 2nd sweeps column is the sweep valid flag. All good scan data has the value "1"; if "0" is it between sweeps, or in the calibration.

    the "On/Off" column:

  • for ON/OFF scans, this indicates whether on-source or off-source. Only valid when the "sweep-valid" flag is "1".

    Spectral Line Data

  • For LOWRES spectra, there are 3 columns, Frequency (MHz), and brightness for the 2 channels.
  • For HIRES spectra, there are 6 columns, the first 3 for frequency #1, and the next 3 for frequency #2.
  • The spectra have been averaged over all time steps where the sweep-valid flag is "1".

    There is special processing for the ON/OFF scan case when the alternate processing (butterfly) is selected. In this case the on-source spectrum is the average over spectra with the "ON" flag and sweep-valid is "1"; the off-source spectrum is the average having the "OFF" flag, and sweep-valid "1". The "on-off/off" spectrum is displayed and the corresponding text file may be downloaded.

    Header keywords

    Keyword Comments Typical value
    FILENAME Name of this text file "Skynet_57566_CAS-A_21903_22008.txt"
    BASENAME MJD, object name, ID number, scan number concatenated. "Skynet_57566_CAS-A_21903_22008"
    SRC_NAME The name of the source, as specified by the user. "CAS-A"
    OBSERVER The user's name and user ID number "fghigo_2839"
    DATE_OBS UT date and time of the start of the observation. "2016-06-27 15:00:23.00"
    OBSID Skynet assigns a unique ID number to each observation. "21903"
    SCANNUM Cyborg scan number (can be more than one scan number per observation) "22008"
    MJD Modified Julian date (days since Nov 17, 1858) "57566"
    UTC UT seconds since last UT midnight "54022"
    SCANTYPE Type of scan pattern, e.g. "Daisy", "Map", "Track", "OnOff" "Track"
    DATAMODE Type of observation: LOWRES, HIRES, or PSR_SEARCH "HIRES"
    RECEIVER Type of receiver: Rx1_2(1.3-1.8 GHz) or Rx8_10 (8-10 GHz) "Rx1_2"
    OBSFREQ Center frequency for the selected filter in MHz "1395"
    Actual_FREQ1 In HIRES mode, actual band center for frequency 1, in MHz "1668"
    Actual_FREQ2 In HIRES mode, actual band center for frequency 2, in MHz "1418"
    LO1freq Frequency setting of the LO synthesizer in MHz "7000"
    RFFILTER Selected filter "1355_1435"
    OBSBW Spectrometer bandwidth (minus means lower sideband mix) "-500"
    EFFBANDW Effective bandwidth of the selected filter, in MHz "80.00"
    NCHAN number of spectral channels "1024"
    STRTCHAN ** Starting channel number "747"
    STOPCHAN ** Ending channel number "911"
    TAVE User-specified integration time, in seconds "0.300"
    TINT Integration time, in seconds.
    Actual integration time may differ slightly from TAVE because
    it is an integral number of TBINs
    TBIN Spectrometer dump interval, seconds. "0.00419430"
    FOCUS Focus setting, mm "37.071"
    RA,DEC Right Ascension, Declination of the start of the scan. "23:38:54.48, 58:50:11.75"
    RA(deg) Right Ascension of start of scan (degrees) [***] "350.91371"
    DEC(deg) Declination of start of scan (degrees) [***] "58.83721"
    WIDTH(deg) Extent of mapping pattern (degxdeg) [***] "4.024x4.0058"
    AZIM(deg) Starting azimuth [***] "318.7034"
    ELEV(deg) Starting elevation [***] "41.6813"
    COORDREF User-selected coordinate mode [note zz] "RA_DEC"
    HREF User-specified "horizontal" coordinate [note zz] "350.8785"
    VREF User-specified "vertical" coordinate [note zz] "58.8370"
    IFattens Attenuator settings
    the system adjusts the attenuators prior to each observation
    to set nominal power levels to the spectrometer inputs.
    "7.00, 10.00"
    ACCLEN Accumulation length used by the spectrometer.
    Depends on the TAVE specified by the user.
    WARNING Warning message if there are hardware errors. "None"
    TSYS System temperature "YY1:32.50, XX1:32.00"
    TCAL Temperature of noise calibration at the observing frequency "YY1:8.47, XX1:7.28"
    TRX[L|R] Receiver noise temperature at the observing frequency "12.4285"
    PRECAL Length of noise calibration at the start of the scan, seconds "3"
    POSTCAL Length of noise calibration at the end of the scan, seconds "3"
  • ** Continuum data points are the sum over the spectrum from the start channel to the end channel for each time step (TAVE).
  • *** Alternate processing (butterfly)
  • zz The user can specify the type of coordinates (RA/DEC, AZ/EL, or Galactic longitude and latitude) HREF and VREF are the user-specified center of the pattern in whichever coordinates are specified. Note the data tables list RA/DEC and AZ/EL regardless of what coordinate mode was specified.
    (Frank Ghigo, NRAO-Green Bank, June 2016)