Solar Drift Scans

Joachim Köppen, Strasbourg, 2011/12

Observations

• Goto Calibrator and Start Observe & Record; measure here for about 5 min
• Sun now then Goto
• When telescope reached position, manually move it to get the maximum signal. The position may deviate by about 1° in elevation and as much as 3° in azimuth. The maximum reading is about +48 dBµV.

If you want to make a full drift scan (may need luck, takes 30 min):

• Determine the offsets in AZ and EL between this maximum and the predicted sun position. Apply these offsets to the predicted position which appears after one clicks Sun+15min
• Goto this position. Note the present reading which is the sky background. Then wait ...
• As the sun drifts through the antenna beam, the signal hopefully rises up the previously found maximum value. Keep observing until the signal has gone down and is constant again
• Measure the constant level for some minutes (the second measurement of the sky background), then
• Finish by Goto Calibrator; stay there 5 min, and then Stop & Finish the observation run

If you want to make a half drift scan (is safer, takes 15 min),

• From the movement of the sun, you can guess where it will go: always right, but up or down. Therefore, move manually by touching the keys of the rotrator controller just briefly! The signal should now be about 1 or 2 dB lower
• As the sun drifts through the antenna beam, the signal hopefully rises back to the maximum, and then goes down. Keep observing until the signal has become constant
• Measure the sky background and the Calibrator again
• Stop & Finish the observation run

Interpretation (more details):

• Import the text file with the data into Excel
• Convert all the dBµV values into linear units (P = 10^(dB/10))
• Determine the average power from the calibrator: Pcal
• Determine the average power for the empty sky: Psky
• Convert all powers into antenna temperatures: Tant = 290K * (P-Psky)/(Pcal-Psky)
• Plot the antenna temperature as function of time (scatter plot)
• Make sure that the measurements of the sky average about zero temperature, while the calibrator measurements are around 290 K. If not, adjust Psky and Pcal
• Determine the times at which the bell-shaped curve of solar measurements reaches the half its value at maximum.
• Convert this time span into the angle that the sun had moved. This is 15°/min*cos δ with the present declination δ of the sun ( e.g. from here).
• This angle is the Half Power Beam Width of the antenna.

From this we can compute the solar flux and compare it to other measurements:

• The antenna beams solid angle is Ω = π/4 * (HPBW*π/180°)²
• This gives its effective area: Aeff = λ²/Ω
• The flux F = 2kTant/Aeff can be compared with the data for that day and frequency published by NOAA (note that 1 SolarFluxUnit = 10000 Jy).

The final step is to derive the temperature on the solar surface:

• From here) one gets the angular diameter of the sun: D.
• The filling factor with which the sun fills the antenna beam is given by: (D/HPBW)²
which allows the compute the surface temperature from the maximum antenna temperature
Tsun = Tant_max * (D/HPBW)²

last update: Sept. 2011 J.Köppen