RoenneSpectrometer Simulator
Joachim Köppen Kiel Jan 2017

Some brief explanations

This is a simulator of the software with which the 1.4 GHz antenna at DL0SHF in the village of Rönne is controlled and operated to make radioastronomical measurements. It permits to make 'observations' of sky objects in the same way and to produce data in the same quality as the real instrument. The graphical user interface closely resembles the real thing, except for some features that make sense only in the real program. This is the version for spectroscopy in the 21 cm line of neutral hydrogen at the theoretical frequency of 1420.4057517667 (±...9) MHz.

The simulator produces spectra for a number of positions in the Galactic Plane which are based on real observations with this instrument.

The real instrument has these technical data:

  • Diameter of the parabolic dish: 9 m
  • Effective diameter of the illuminated dish: 7 m
  • Effective area of the illuminated dish: 40 m2
  • Antenna gain: 40 dBi
  • System temperature of the receiving system: 50 K
  • Sensitivity: 69 Jy/K

Access to the simulator is organized in three pages: Operate, Output: Power, and Output: Spectrum. Here is a description of the controls:

Operate: this is the principal page, from which all operations are effected, and where all information are displayed.

  • On the top, there are two rows of buttons which give controls to the telescope
  • On the right hand side a number of quantities are displayed
  • Below the buttons five panels give graphical displays of data:
  • map or image
  • the spectrum
  • waterfall of the spectrum
  • time plot of the received power
  • the large panel shows the current sky with the radio sources

  • Startup: click this to start the operations. If everything starts up properly, the clock will show the current time in UT, and the local sideral time LST. The plot below then shows the current situation on the sky. Also, now all controls can be operated.
    A second click will stop the operations -- however, this would make real sense only with the real instrument, of course.
  • Clear: clears the waterfall and spectrum plots. Spectrum averaging starts afresh.
  • Now: click this to show current situation.
  • -1 hr: click this to show the situation one hour earlier ...
  • +1 hr: ... or later
  • Record: click on this button starts recording all measurements as text on the Output pages. Another click will stop recording. Recording data is only possible when the program works in the current time (Now). Note that both Output pages start afresh when the next recording is started.
  • ...: [for future use]
  • BaselineSub: Clicking this enables the subtraction of a linear baseline from the spectra shown in the Waterfall plot, which makes any spectral features stand out more prominently. Note that one has to adjust the plot range fields of the waterfall accordingly.
  • Track: This button shows blue text when a source is being tracked. Clicking it can then switch off tracking, and the source will drift out of the antenna beam.
  • Map/Image: [not yet in operation]
  • DriftScan: When a source has beeen selected and is being tracked, a click on this button will move the antenna to a position where the source will be in 10 minutes. The antenna will stay on that position, waiting for the source to pass through the antenna beam.
  • Park: puts the antenna to its resting position.
  • The right hand display:
  • AzElCmd: In these two fields one can enter the desired sky position. Hitting the 'Enter' key will let the antenna move to this position. When a source is tracked, these fields show the predicted position of the source (hence, don't enter anything!)
  • offset: here one may enter any horizontal and vertical offsets from the above commanded position. Thus one may apply an offset to a source that is being tracked.
    Note that the horizontal offset is not equal to the difference in azimuth! This is because the azimuth circles bunch together and all go through the zenith. Thus one has Δhoriz = cos(El) * ΔAz ! For example, at EL=60° moving by ΔAz = 10° moves horizontally by only 5°.
  • AzEl: is the display of the current position. Whenever the antenna is moving, the background of these fields is blue.
  • Galactic: The current position in galactic coordinates ...
  • RaDec2000: ... and in Rectascension and Declination (both in degrees)
  • VLSR:give the correction of the true solar motion with respect to the Local Standard of Rest in our Milky Way galaxy.
  • Park ...: Here appears the name of the source that is being tracked.
  • UT: The current time
  • LST: The Local Sideral Time
  • Measure interval: [not yet operational]
  • 128 bins: The spectrum - about 2 MHz wide - is divided into the specified number of spectral bins, from 32...1024. For measurements in the Milky Way 128 bins are completely sufficient, since there are no spectral features so narrow that they require a higher resolution. Thus more bins mean more data and longer files!
  • centre frequency: here one can set the frequency at the centre of the spectrum [for further extension]
  • Freq, Vrad: When the mouse is moved over the spectrum and waterfall plots, the frequency and the radial speed - corrected to the Local Standard of Rest - are displayed here.
  • Az, El: When the mouse is in the sky plot, the azimuth and elevation of this point are shown.
  • Map/Image Display: [not yet operational]
  • Spectrum Display: The black curve is the current spectrum, the red curve is the average of all spectra taken since the present source had been chosen, or since the 'Clear' button had been clicked.
  • Waterfall Display: Each time a new spectrum is taken, it is added as a line to this plot. The intensity is coded as colours from a rainbow which is shown near the centre of the screen. Black/violet means low values, red is high values. The minimum and maximum values for this range are found in the two fields below the waterfall. Enter any values and hit the 'Enter' key (or click the Set button and all the following spectra will use the new range. Intensity values below the minimum value show up as grey, values higher than the maximum value are white.
  • Power Display: The power, averaged over the entire spectrum, is shown as numerical values below the waterfall plot, and these values are plotted against time in this plot window.
    Note that lower and upper bounds are the same as the minium and maximum values for the waterfall plot. If one uses 'BaselineSub' to emphasize spectral features in the waterfall, and thus one uses an appropriately narrow range below the values for the total power, the curve in the power plot goes out of range. [perhaps i'll change that later ...]
  • Sky Display: shows the situation in the sky above the antenna. The grey area near the horizon shows the ground, two antenna masts, and the small grove of trees in the south-west which serves as the flux calibrator.
    Sun and Moon are indicated as yellow and cyan disks, other celestial sources are shown as blue dots with their name. The grey curve across the sky is the mid-plane of the Milky Way, with positions in the Galactic Plane indicated by their galactic longitudes, such as G30. The red cross shows where the antenna is pointing at.
    The red frame indicates the permitted range in azimuth and elevation. Note that in this cartesian plot of the sky hemisphere, the zenith (el=90°) is represented as the entire top border. This distortion has an advantage: It shows thaat the azimuth motor takes the same amount of time to move over a certain azimuth angle at all elevations. Hence, a small correction in horizontal angle takes up much time near the zenith, and one should avoid observing a source close to the zenith.
    When the simulator has been started up, this plot is updated in real time.

    The plot has also an important function: Click on a source will make the antenna move to that position and then track the source. The 'Track' button text will become blue to indicate this state.

Output: Power: when data are recorded, the total power summed up over the spectrum is added to this page. From here they may be copied and pasted into a text file for further interpretation. Each datum is composed of time [UTC], azimuth, elevation, power [dB]. When a large number of data are recorded - for a couple of hours, say - the browser may have problems showing all the data easily. Therefore, it might be advisible not to make recordings too long.

Output: Spectrum: when data are recorded, the spectra are added to this page. From here they may be copied and pasted into a text file for further interpretation. The format is identical to that used by the real software: each datum line is composed of time [UTC], azimuth, elevation, number of spectral bins, frequency [MHz] of the first bin, frequency step between two bins [MHz], followed by the powers of all the bins of the spectrum.

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How to ...

  • ... take spectra of a source: for instance a position in the Galactic Plane
    • click on the source
    • thereupon the antenna will move to this position, and will start tracking it
    • as soon as the antenna reaches the position, it is a good idea to again click on the source, because this will record the source's name as a comment line in the data, as to mark the start of the good data.

  • ... make a drift scan of the Sun: This is to point the antenna at a position where the Sun will be some time later. One waits for the Sun to pass through the centre of the antenna beam, thereby allowing to measure the peak power (i.e. the power from the solar radio emission) but also the antenna pattern, i.e. the shape of the antenna beam and measures the angular resolution of the antenna (or HPBW = Half Power Beam Width) which is an important value for the interpretation of the measured solar data.
    • click on the "Sun"
    • record the data
    • after the antenna reaches this position, click on "DriftScan" button
    • the antenna will move there, in order to wait for the passage of the Sun
    • wait for the Sun to pass by ...

  • ... measure the system temperature of the instrument: Because the noise produced by the receiving electronics itself is one important (if not dominant) part of the measured power, it determines the sensitivity of the apparatus and its knowledge is necessary when one wants to interpret the data quantitatively.
    • go to the Calibrator (Az=220, El=1) to measure the radio noise produced by the trees or the ground Tcal = 290 K), which adds to the receiver noise, represented by Tsys. The (linear) power is pcal = a (Tcal + Tsys) with some instrumental factor a.
    • move the antenna to a position high in the sky, say elevation 60°, and measure
      psky = a (Tsky(e) + TCMB + Tsys)
    • As Tcal = 290 K from the calibrator, TCMB=2.7 K from the cosmic microwave background, and Tsky(e) = Tzen/sin(e) are known or can be measured, the two above equations can be resolved to yield the system temperature. Then one may also determine the instrumental factor a.

  • ... measure the sky's thermal radiation: Although the sky does not absorb much radiation at 1420 MHz, it does contribute to the noise measured by the antenna, because it emits thermal radiation. This foreground noise is present in EVERY observation of a celestial object, and hence we need to know it well, if we want to interpret our data.
    Fortunately, this atmospheric emission can well be modelled by a simple expression: The sky temperature depends on the elevation e in this manner
    Tsky(e) = Tzen/sin(e)
    The zenith temperature is determined in this way.
    • go to the Calibrator (Az=220, El=1) to measure the radio noise produced by the trees or the ground Tcal = 290 K), which adds to the receiver noise, represented by Tsys. The (linear) power is
      pcal = a (Tcal + Tsys)
      with some instrumental factor a.
    • move the antenna to a number of positions in the empty sky at various elevations, and measure:
      psky(e) = a (Tsky(e) + TCMB + Tsys)
    • Plot the (linear) powers psky(e) as a function of the so-called 'Airmass' = 1/sin(e), and fit a straight line through the data points, such as p = b + m/sin(e).
    • From this fit and with the other equations one gets the instrumental factor a, the system temperature, and the zenith temperature Tzen = m/a. The fit also allows us to extrapolate it to Airmass=0, i.e. if there was no atmosphere!

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Recorded data (UT, Az, El, pwr):
Recorded spectrum data (UT, Az, El, vlsr, n, fstart, fstep, pwr[0..n]):

The HI spectra of the Galactic Plane

This is an overview of the spectra from the Milky Way's plane. At the time of our first sky survey, the DL0SHF 1 GHz antenna still had uncorrected pointing errors of about 2°. Therefore we do not use our data from those observations, but we prefer data from the LAB survey, smoothed to the 2° antenna beam. Comparison with our survey data shows a very good agreement.
The data are shown as a false colour map of the flux as a function of longitude and radial velocity, and for a specified longitude, the spectra are also plotted in terms of antenna temperature. Since the emission is fairly extended, antenna temperature is equal to brightness temperature.

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