This article describes an experiment carried out with the radio telescope SPIDER230: the registration of the radio source Taurus A (M1) transit. The results underline the performance of the instrument and lend themselves to some interesting observations on observational radio astronomy techniques.
SPIDER230 radio telescope used to record Taurus A (M1) transit.
The experiment was conducted by Dr. Filippo Bradaschia, CEO of PrimaluceLab, RadioAstroLab partner in the construction of the SPIDER230 radio telescope that we thank for his cooperation. The receiving station, installed at Polo Tecnologico of Pordenone, includes the 11.2 GHz RAL10PL Total-Power receiver specially made by RadioAstroLab for SPIDER230 and the antenna system (circular 2.3 meters diameter parabolic reflector) with equatorial mount, motorization and dome of protection made by PrimaluceLab. The instrument is completely controlled via an Ethernet line by RadioUniverse software.
Specified design criteria adopted for the RAL10PL receiver, for the antenna and for the pointing system, the RadioUniverse software for data acquisition and data processing, guarantee to SPIDER230 an high sensitivity and stability required for radio astronomy observations. We have repeatedly underlined how the sky observation in the frequency band close to 10 GHz offers several advantages:
- Reduced sensibility to external radio interferences;
- Possibility to use antennas of acceptable dimensions;
- Higher resolution capabilities related to higher radio frequencies.
These benefits allow you to install a radio telescope also in the "home garden", however in an urban environment without too many penalties related to interferences. In any case, it is always advisable to check the suitability of the site for installation. The main disadvantages are related to the reduced number of radio sources observed with amateur or semi-professional radio telescopes.
Objects observed by SPIDER230 (at 11.2 Ghz frequency) are:
- SUN: flux of about 3 milion Jansky [1 Jy = 10-26 W/(m2 ∙ Hz)]
- MOON: flux of about 30000 Jansky
- CASSIOPEA A: flux of about 423 Jansky
- M17: flux of about 550 Jansky
- TAURUS A (M1): flux of about 506 Jansky
- ORION A (M42): flux of about 480 Jansky
Band radio specters of the main radio sources accessible by SPIDER230.
It's easy to note that the Moon is 100 times fainter than the Sun and Cassiopea A is 50 times weaker than the Moon. Due to the intensity of the Sun, the observation of our star with SPIDER230 requires the setting of the minimum amplification factor for the receiver and the insertion of a 22 dB attenuator along the coaxial line. The transits of the Moon are easily recorded by setting medium-low amplification factors (without attenuator), while the other items are more difficult to record. The need to verify the instrument limit performance, encouraged by the excellent sensitivity and stability measurements found for the Moon, has brought Dr. Filippo Bradaschia to schedule the data recording using the transit technique with RadioUniverse control software.
During RAL10PL receiver's design, some simulations were carried out to verify, at least theoretically, the suitability of the system as a radio telescope. We remember that the instrument is a Total- Power radiometer running at a 11.2 Ghz frequency with high sensitivity and stability. The last feature is particularly important for a radio astronomy receiver: environment temperature changes cause small variations in the gain of the receiving chain that make it very difficult to measure, causing drifts and fluctuations in data recording. The problem is most evident when the radio source to be observed is weak and the setted amplification factor is greater. The issue is resolved by adopting appropriate design criteria and thermo-stabilizing the receiver electronic circuits. Remain the daily temperature that affect the RAL10_LNB external unit, installed on the antenna: in this case it is quite easy to characterize the behavior from the thermal point of view and adopt appropriate procedures for compensation.
The Taurus A emission profile looks very "diluted" by the significant difference between the amplitude of the beam receiving antenna and the angular extent of the source (top chart). The chart below shows the signal power estimated increase, seen by RAL10PL receiver, due to the transit of radio source.
The image above show the results of simulations planned to investigate the possibility of receiving Taurus A with SPIDER230. The simulations are theoretical and consider an ideal behavior of the receiving system, perfectly thermally stabilized. The response of the radio telescope was calculated by setting the parameters of the receiver that will be actually used in the experiment. The antenna reception diagram and the radio source emissive one have been approximated as a uniformly illuminated circular apertures, simplifying the assessment of the spacial “filtering” effects that the antenna does on the true radio source profile. These simulations, while being very simplified, have the advantage to highlight the performance of the radio telescope. The Taurus A flux at a 11.2 Ghz frequency is about 506 Jy.
Calculations show that the variation in the antenna noise temperature due to the radio source transit is about 0.86 K and that the variation in receiver measured signal power is about 0.11 dB.
RAL10PL receiver input-output characteristic (determined in laboratory) and SPIDER230 theoretical response of Taurus A transit.
The Taurus A transit simulation is shown in figure above: is also represented in the drift of the base line radiometric that this will cause the extent "on the field".
We verify the correspondence of the theoretical simulation with experimental data.
The transit technique used for measurement consists of identify the object for which you want to record the radio emission, point the telescope in the sky area in which the object will move in the near future (eg 30 minutes later) and stop the telescope in that position. Because of the apparent sky rotation (caused by the rotation of Earth), the object will move towards the area of sky pointed by the antenna, will be intercepted by the receive beam and will pass through.
On February 24th 2014, PrimaLuce Lab technicians have pointed the SPIDER230 antenna to Taurus A (the M1 nebula in Taurus constellation that emits synchrotron radiation caused by electrons in fast spiral motion around magnetic field lines generated by the pulsar inside) recording a first 15 degrees transit. Plotting the intensity data obtained on the sky map they have verified that this increase occurred precisely at the theoretical position of Taurus A.
Verifying the first Taurus A transit with the RadioUniverse software.
Taurus A transits validation performed with RadioUniverse software.
To validate this registration, on March 9th 2014, 5 consecutive transits of the same area of the sky have been performed, this time 4 degrees each. SPIDER230 allows you to automatically record consecutive transits. RadioUniverse records, for each transit, a CSV file with 4 columns: each row is a record and contains date, Right Ascension, Declination and radio signal of the recorded point. So the different results obtained can be processed, for example by averaging the values to reduce random noise, increasing the visibility of the radio source. In the picture 6 we see the result of the processing of the 5 transits: the average curve is highlighted by red thick.
Taurus A (M1) radio source transits.
The "average" curve clearly shows the Taurus A transit which was also confirmed by the analysis of the Institute of radio astronomy of Bologna (IRA). The experimental results verify, taking into account the approximations related to the simulation, the theoretical recording, see the figure above.