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October 2022

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

By | Radio astronomy projects

C. S. LEUNG(1), THOMAS K. T. FOK(2), KENNEITH H. K. HUI(2), K. W. NG(3), C. M. LEE(3), S. H. CHAN(3)

(1) Department of Applied Mathematics, Hong Kong Polytechnic University, Hong Kong SAR, P. R. China, Email: chun-sing-hkpu.leung@polyu.edu.hk
(2) Ho Koon Nature Education cum Astronomical Centre, Sik Sik Yuen, Hong Kong SAR, P. R. China
(3) Hong Kong Astronomical Society, Hong Kong SAR, P. R. China

Abstract. Compact SPIDER 300A radio telescope has been used to study the partial solar eclipse at 21 cm wavelength, with magnitude 0.89, in Hong Kong on 21st June, 2020. The radio telescope SPIDER 300A was designed and constructed by the PrimaLuceLab company, Italy. Radio flux density time curves (light curve) and a two-dimension mapping of the eclipse are presented in this paper. Standard radio data reduction methods were used to obtain the intensity time curve. We also adopted the semi-pipeline method for the reduction of data to obtain the same results as with the built-in software of the radio telescope SPIDER 300A. The total solar radio flux of the eclipse was found to reduce by maximum 55±5%, while the maximum eclipsed area of the same eclipse is 86.08%.

Key words: Solar observations.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

 

1. INTRODUCTION

The first radio solar physics paper was published in 1944 (Reber, 1944). After that, solar radio physics was born. Following this development, several interesting books introducing solar radio physics kept momentum in the field (Kundu, 1965; Kruger, 1979; Kundu and Gergely, 1980; McLean and Labrum, 1985). For solar radio astronomy studies, total or partial eclipses provide great op- portunities for solar radio observations. Solar radio observations primarily are time-dependent. Furthermore, several radio solar eclipse observations were carried out in China in 1958, 1968, 1980 and 1987, and the findings were briefly introduced (Liu and Fu, 1998). On November 3 1994, high spectral and time resolution radio observations of solar eclipse were carried out at Chapeco, Brazil for the first time (Sawant et al., 1997), asymmetrical limb brightening at 1.5 GHz was observed and recorded. Besides, thanks to the advance in technology, sensitivity for the backend system had improved a lot so that two-dimensional mapping for the radio Sun can easily be achieved. Recently, one of the interesting results was published by making use of a UK made telescope at University of Baghdad (Jallod and Abood, 2019). However, the excellent results for the cm radio wave bands can be attributed to the extensive study from Goldstone Apple Valley Radio Telescope Observations (Velusamy et al., 2020). From this study, the observations obtained made us believe that the radio emission originated in the chromosphere and corona; furthermore, they obtained the source brightness temperatures and angular sizes as a function of frequency. These results were made known in terms of gyroresonance mechanism across the active region of the Sun.

Solar eclipse is a rare and attractive event occurring 2-5 times per year. Optical astronomers usually study the solar features happening at the photosphere, chromosphere and corona. We combined radio observations for the 2019 and 2020 solar eclipses by making use of 3 small radio telescopes in Hong Kong. The results were cross checked as to identify any discrepancy. The dates of observations are 26th of December, 2019 and 21st of June, 2020, respectively.

This study has three goals. First, to obtain the time variation of radio two-dimensional spatial mapping of solar eclipses. Second, to obtain total radio flux density time curves (light curve analogy in radio). Lastly, to verify if 21 cm radio band can reveal any physical properties of solar eclipses through analyzing historical worldwide radio eclipses. This paper is also a collection of historic radio observations in Hong Kong, which will make further studies of comparison between radio quiet zone in Hong Kong and that in other countries feasible. Italy, in which our telescopes’ manufacturer PrimaLuceLab company bases, would be the first on our list, as the same SPIDER 300A radio telescope system is also installed at their National Radio Observatory in Bologna.

 

2. OBSERVATION

Partial Solar Eclipse on June 21, 2020 (magnitude 0.89, max eclipsed area 86.08%)

The radio observations of the eclipse were made on June 21, 2020 at the Ho Koon Astronomical Center (hereafter HKAC, Longitude: 114° 6′ 29.3076′′E, Latitude: 22° 23′ 1.644′′N, Altitude:149m), Stanley Ho Astronomical Observatory (hereafter SHAO, Longitude: 114° 13′ 24.0414′′E, Latitude: 22° 14′ 32.2362′′N, Altitude: 4.6 m), and Physics Building Dome of University of Hong Kong (hereafter HKU, Longitude: 114° 8′ 23.262′′E, Latitude: 22° 13′ 59.7′′N, Altitude: 120 m), respectively.

Small Radio Telescope (SRT), developed by MIT Haystack Observatory, was used in HKAC. SRT is centered at 1,420 MHz with half-power beamwidth (HPBW) of 7 degrees. While SPIDER 300A radio telescope systems, developed by PrimaLuceLab company, were used in both SHAO and HKU. The telescopes were optimized at 1,420 MHz with HPBW of 4.03 degrees. The diameters of the telescopes that we used in SHAO, HKU and HKAC are 3 m, 3 m and 2.3 m respectively with their corresponding bandwidths at 50 MHz, 50 MHz and 50 kHz. The so-called “on-the-fly two dimensional mapping” used at SHAO is indeed a mode of moving the antenna horizontally from left to right. Once the antenna completed mapping the first row, it moves downward and starts the succeeding row. This mode of movement of scanning horizontally and one row downward will continue and repeat until the designated area of sky had been fully scanned.

And the “on-off method” is that the antenna will be operated to scan the designated sky so that the feed horn is made to align to the target “on mode” for a period of time, occasionally the feed horn is made to align to the ambient space “off mode” for another period of time as to get the ambient signals. This “on-off method” is adopted repeatedly during the observing time as to ensure to properly receive the signals as well as the ambient noises.

Always on-source tracking method was used at HKAC and HKU, while on- the-fly two-dimensional mapping method was applied at SHAO, which allowed us to obtain the time variation mapping of radio Sun during eclipse, which will then be presented as sequence diagrams in the later section. The Sun was mapped with a 7 × 7 grid covering 10 × 10 degrees2, and with step size of 1.6 degrees. Integration time for each grid was 1 second. All the data recordings were saved in the standard FITS format.

 

3. DATA REDUCTION

We note that the calibration system of SPIDER 300A is still under development at the moment of this manuscript being drafted, arbitrary unit and percentage changes were used during analysis. Since the telescope was scanning across the surface of the Sun, the offsets between the telescope axis and the Sun in azimuthal and altitude at each moment were continually varying. Those offset functions of time can be retrieved from fits headers. We can then scale the received power, which is diminishing along the off-axis distance, with respect to each channel according to the beam profile. The scaled powers were added together after flagging particular channels which were severely polluted by the local interference. We assume the solar power spectrum smooth and without strong emissions in any particular frequency, so that flagging some channels will not significantly affect the result if we only consider the percentage changes in power throughout the eclipses. For our SPIDER 300A system is an affordable radio telescope system, the small deviations from the electronic components were inevitable. During the observation, we made use of different gain values for the both left and right hand polarization as to minimise the deviation.
For the 2020 eclipse, the Sun was observed by on-the-fly two-dimensional mapping method mentioned in the previous section. Since the angular distance between the Sun and the measurement center varied from time to time throughout the scanning process, in order to obtain the true radio flux of the Sun at each moment, the power diminishing effect along off-axis distance must be compensated according to the beam profile. The offset distances and beam parameters could be found in FITS, through dividing measured flux by beam respond at corresponding offset, time series of the power change will then be obtained. Power of the uneclipsed Sun is estimated by averaging the data from −6 hr. to −2 hr. before the eclipse maximum.
For comparison, the change of eclipsed area is also included in the plots noted as optical. Equatorial positions of the Sun and the moon were calculated by Skyfield (Rhodes, 2019), and the area changed was calculated according to equations provided in Maplesoft webpage (Jason, 2019).

 

4. DISCUSSIONS

For this study, we accomplished the 3 major goals mentioned in the Introduction section. Hence, for this discussion, we will make reference on these 3 goals to proceed with detailed consideration.

Time variation of radio two-dimensional spatial mapping of solar eclipses

The two-dimensional mapping false colour diagram was created by making use of the 1.42 GHz uncalibrated SPIDER 300A data in arbitrary units from 2020 eclipse at SHAO. The location for the SHAO is in the valley of the Tai Tam reservoir, where a hill blocks the west side of the sky, the last hour of 2020 eclipse was not recorded. Fortunately, the sequence diagrams demonstrating the first contact, partial phases and the maximum eclipse of the partial solar eclipse were obtained. Please be reminded that this was 0.89 partial solar eclipse instead of total solar eclipse. The mapping result is shown in Figure 1. Figure 1 shows a mapping diagram before the solar eclipse. The UT that we made the figure was at 03:56. We adopted 2 s for the integration time and a step size is of 1.614 degree for each pixel. Figure 1 was obtained before the solar eclipse. It was included as to give a proper comparison to the later figure obtained during the eclipse. Figure 2 shows the real time dual circular polarization time plots generated from the built-in software RadioUniversePRO v.1.4.8. Figure 2 shows schematically a part of real time intensity changes during mapping as the telescope beam scanning across the Sun. The intensity was found to vary as shown in Figure 2.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Figure 1 – The standard interface for the RadioUniversePRO software shows the Sun in raw data format.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Figure 2 – The real time flux density curve versus time plots. The plots represent dual polarization for the Sun’s signal

 

We would admit that it took roughly 15 minutes for a scanning and the follow- up scanning which included the mechanical movement by the antenna, setting of the device parameters, the fine tuning during the observation, recording the data, and the resetting for the follow-up scanning. New mapping with the same setting was repeated immediately after the previous scan. The whole process continued until the Sun was blocked by the hills at west of SHAO. A sequence of 10 × 10 degrees2 images of the eclipsed Sun was then obtained. Figure 3 shows an example for the uneclipsed Sun that observed in some other day for comparison. Figure 4 shows a sequence of diagrams as to present the different stages of the eclipse.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Figure 3 – The usual daily mapping for the Sun by making use SPIDER 300A

 

Figure 4 clearly shows that the received power of the Sun decreases as the eclipsed area increases. Since the beam of the telescope is 4.03 degrees, which is much larger than that of the Sun (<1 degree); unlike what we expect in optical images, the radio morphology of the Sun remains circular throughout the process.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Figure 4 – Sequence diagrams of the 0.89 partial solar eclipse in Hong Kong on June 21, 2020: a. Eclipse begins, b. Partial phase I, c. Maximum eclipse, d. Partial phase II, and e. Near the end of eclipse (blocked by hill).

 

The latter half of the sequence diagrams of the eclipse demonstrates the effect from the blockage of the hills nearby. Although our sequence diagrams are not able to cover the whole eclipse from the start to the end, the moment of the maximum eclipse (frames from UT06:28 to UT09:19 in Figure 4) is recorded, enabling further analysis of the process from the beginning to the maximum of radio eclipse. For interpolation of Figure 4, we use “RectBivariateSpline” function from SciPy package to interpolate data over rectangular meshes by bivariate spline approximation. In our actual operation, 36 values are interpolated between meshes in each direction, with 3 degrees of the bivariate spline. More comprehensive radio solar eclipse observations from better sites with better occasions and better instrumental conditions in coming years are expected, our result will serve as one of those records in Asia for future studies. Based on the data processing described, the percentage change of received radio power during the eclipse is shown in Figure 5 for 2020 partial eclipse.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Figure 5 – Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

 

As the resolution of the radio observation is less than that of the optical, previous studies (Sherwood, 1978; Tan et al., 2009) showed that the radio Sun in 21 cm appeared larger than that in optical, wider and shallower dips of radio power change compared to that of optical are to be expected. Our results from 2020 eclipse agree with the expected demonstrating shallower dip when compare with the optical eclipse models. For SHAO 2020 data, although we covered the very beginning of the eclipse, no significant delay is found from the reduced data. We postulate the delay may be too small due to the almost perpendicular intersecting angle between the Sun and the moon such that our instrument was not sensitive enough to determine the beginning moment of the radio eclipse. The expected delay mentioned is reasonably attributed to different paths, and so different thickness of the atmosphere of the optical and radio waves taken passing through between the almost perpendicular angle and later more “inclined angle” for the Sun’s positions appeared in the sky. And no significant delay was found. If more data at around −2 hour before eclipse maximum were obtained, a better estimation might be achieved. From the light curve, we found the drop in radio power is 55±5% at maximum eclipse, which means that the radio radius of the Sun is larger than that in optical. We can estimate the ratio between radio solar radius and that of optical from their differences in power drop. The simulation result is shown in Figure 6. Assuming the radio behavior of the Sun is similar to that in optical, the morphology of radio pattern is basically symmetric circular disc. We can obtain the expected light curves by substituting different solar radii (ranging from 1 to 1.5 solar radius) in the equation (Jason, 2019). The most fitted model would indicate the measured solar radius of our observations.

 

Solar eclipse observations with SPIDER 300A radio telescope in Hong Kong in the 21cm radio frequency band

Figure 6 – The simulated light curve for the 2020 partial solar eclipse with different radio Sun’s radius.

 

During the solar eclipse, the eclipsed area in optical was smaller than that of the radio signals. Therefore, the power drop of the signals in optical would be more than that of the radio. And the result was shown in Figure 6. Solar radius is scaled with specific factors to see how the curves change. From the above result, we estimated that the radio solar radius is roughly 1.4±0.1 of that in optical. (Sherwood, 1978; Tan et al., 2009)

 

4. CONCLUSION

Based on the above observational results, we were glad to have accomplished the original planned goals for this study. Firstly, we managed to obtain the two-dimensional radio mapping animation for the 0.89 partial solar eclipse on 21 June 2020 in Hong Kong. The eclipse light curve in 2020 partial solar eclipse of radio was observed different not in proportion to the eclipsed area from of the optical as expected. There was about 55±5% of the maximum radio eclipse recorded compared to a 86.08% eclipsed area in optical. And the radio solar radius detected during the solar eclipse in 21 cm was related to chromosphere and corona, but not to the photosphere. The photosphere of the Sun defines the solar disc optically. During the solar eclipse, the optical light of the Sun is blocked by the moon. The radio signals in 21 cm at the chromosphere and the corona of the Sun, however, remain observable during the solar eclipse. Therefore, we may recognise the signals observed as the radio solar disc.

The 2020 observation for the 21 cm radio solar eclipse was unprecedented in Hong Kong. Probably, other South East Asia countries may have similar observations. Therefore, our observation data obtained can serve effectively to contribute to the radio data all over the world. There was a radio quiet zone in Hong Kong, such that we propose constructing a radio interferometer array over there. In fact, we wish to implement the plan soon since it is reasonably good for the development in radio astronomy in Hong Kong. And we developed a simple systematic pipeline data reduction approach for dealing with the data obtained from the SPIDER 300A telescope. And the calibration function for the SPIDER telescope is expected to be completed soon, and hence we can properly calibrate all the coming observational results. The results we have obtained are good enough for initiating further studies and collaborations with other countries and research groups, and for comparing our data with their results.

 

REFERENCES

Jallod U. E. and Abood K. M., 2019, Progress In Electromagnetics Research Letters 85, 17.
Jason S., https://www.maplesoft.com/applications/view.aspx?SID=4343&view=html
Kruger, A., 1979, Introduction to Solar Radio Astronomy and Radio Physics, D. Reidel Publishing Company.
Kundu, M. R., 1965, Solar Radio Astronomy, Interscience Publishers.
Kundu, M. R. and Gergely T. E., 1980, Radio Physics of the Sun (International Astronomical Union Symposia, 86), Springer Company.
Liu Y. Y. and Fu Q. J., 1998, Progress in Astronomy 16, No.3 Sep.
McLean, D. J. and Labrum, N. R, 1985 eds., Solar Radiophysics, Cambridge University Press.
Reber, G., 1944, Astrophys. J. 100, 279.
Rhodes B., 2019, Skyfield: High precision research-grade positions for planets and Earth satellites generator, (ascl:1907.024).
Sawant H. S., Srivastava N., Trigoso H. E., Sobral J. H. A., Fernandes F. C. R., Cecatto J. R., Subramanian K. R., 1997, Advances in Space Research 20, 2359.
Sherwood H., 1978, Radio Universe, Chinese edition.
Tan B. L., Yan Y. H., Zhang Y., Tan C. M., Huang J., Liu Y. Y., Fu Q. J., Chen Z. J., Liu F., Chen L. J., and Ji G. S., 2009, Science in China Series G: Physics, Mechanics and Astronomy 52, 1765. Velusamy T., Kuiper T. B. H., Levin S. M., Dorcey R., Kreuser-Jenkins N., and Leflang J., 2020,
Publications of the Astronomical Society of the Pacific 132, 1015.

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

By | Radio astronomy projects

by

Sandri Mario (1,2), Antonacci Simone (2), Bellotti Tommaso (2), Bonadiman Simone (2), Cavallari Silvia (2), Goldoni Michele (2), Poletti Francesco (2), Soussane Marwa (2), Tavonatti Carlotta (2), Zanella Francesco (2), Zanella Giovanni (2)

(1) IARA, Astronomia Valli del Noce, SdR RadioAstronomia UAI, IMO
mario.sandri@gmail.com, www.astronomiavallidelnoce.it
(2) Liceo “Bertrand Russell” Cles (TN)

 

1. Introduction

Our galaxy is an SBbc medium size and mass spiral type. It is only partially visible, since we are inside it; the plane of the disk and the thousands of stars it contains appear to us as a milky white stripe on the celestial vault, called the Milky Way. The Galaxy is composed of a central core, a bulge, a disk and a halo. In particular, the disc contains the spiral arms. The spiral nature of the Milky Way was confirmed through the study of the distribution of the HII regions, consisting mainly of bright nebulae of ionized hydrogen (HII) that form right inside the spiral arms. The spiral arms are regions of active formation of new stars, dominated by young stars, dust and gas.

It is possible to derive the rotation curve of the Milky Way, map the spiral structure of the Galaxy itself, to determine the integral mass, i.e. the mass contained in concentric shells, in particular in correspondence with the Sun and describe the kinematics of the Milky Way by means of the Oort constants through the study of the concentration of galactic hydrogen, the most abundant element present in the universe and located on the arms of spiral galaxies.

The universe is composed minimally of planets and celestial bodies, most of the galaxy is composed of gases (molecules, atoms, ions) that form interstellar matter. Among the various elements present in space, the most common is atomic hydrogen HI. Atomic hydrogen is the simplest atom: it is formed only by a proton and an electron. The ground state of the hydrogen atom consists of two levels that correspond to the parallel and antiparallel spin configurations of the electron and proton. The energy difference between the two levels corresponds to the emission of a photon with 1420 MHz frequency, corresponding to 21cm wavelength. The greatest intensity of the signal is found on the galactic plane as the concentration of hydrogen is greater. The bodies that make up the Milky Way are not stationary: they revolve around the galactic center. All objects observed from the Earth are therefore in motion with respect to it, which in turn is moving. As a consequence of this, also the wave emitted by the hydrogen is shifted: the observed frequency is different from that actually emitted by the source. Radio astronomers use the Doppler effect to calculate the speed at which celestial objects move relative to the Earth.

 

2. The radio telescope and data acquisition

The instrument used for this study is the SPIDER 300A radio telescope designed by PrimaLuceLab and allows schools, universities, scientific museums and other educational institutes to carry out radio astronomy research. Designed with the same functionality as a large professional radio telescope, SPIDER 300A is a complete, reliable and easy to use instrument. It comes with a 3 meter diameter parabolic antenna, which ensures a high gain and a waterproof altazimuth mount with high load capacity that allows high movement precision. It comes with a receiver designed to receive 1420 MHz frequency with 50 MHz instantaneous bandwidth. Everything is remotely controlled by the RadioUniversePRO software that controls the radio telescope and records signals.

This is a time-lapse video shows the SPIDER 300A radio telescope during capture of the data used for this study about the galactic hydrogen distribution:

 

 

Data acquisition was done by PrimaLuceLab staff by using the SPIDER 300A radio telescope (installed at the “Marcello Ceccarelli” Visitor Center of Medicina radio telescopes – Italy – near the Croce del Nord professional radio telescopes and the large 32 meter diameter dish) with the addition of BKND-Pro backend (actually in development) that allows acquisition of high-resolution spectra. Galactic hydrogen spectra were recorded from areas located on the galactic plane with longitudes from 0° to 210° with intervals of 2° from one spectrum to another. The integration time of each set was 180 s. The data were recorded in text format, returning the signal strength in arbitrary units at each sampled frequency (from 1419.25 to 1421.75 MHz with 4000 channels).

 

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope: one of the emission spectra of galactic hydrogen captured with SPIDER 300A

 

3. Spectra analysis

For each data set the position and intensity parameters of the various peaks were obtained. The Doppler effect can be clearly seen, in fact the maximum peak of the graph, which indicates the concentration of hydrogen, does not have 0 as abscissa, but is slightly shifted to the right or to the left and this indicates a move away or an approach of the gas cloud. Leaving aside the mathematical procedure that allowed us to process the data, we highlight the obtained results. Proceeding in sequential order, the rotation curve of the Galaxy in the first quadrant was initially obtained.

 

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope: Rotation curve of the Galaxy obtained from experimental data (black dots). The blue line represents the quadratic interpolation, while the red one a polynomial interpolation of ninth order.

 

The method used to determine the rotation curve of the Galaxy has limits of use for longitudes of less than about 10°, since the Doppler effect is not particularly noticeable. Furthermore, this method for speed estimation cannot be used for longitudes between 90° and 270°, since in these directions there is no longer a univocal correspondence between speed and distance. The data were also compared with the Clemens rotation curve, which was derived from the survey on the study of the CO line on the galactic plane. This curve is approximated by a seventh degree polynomial whose parameters obtained are visible in the table.

ParameterValue
A0-1149 ± 344
A12757 ± 747
A2-2579 ± 637
A31226 ± 280
A4-318 ± 69
A546 ± 9
A6-3,5 ± 0,7
A7

0,11 ± 0,02

 

After obtaining the rotation curve of the galaxy, the radio map of the Milky Way was drawn in the 21 cm hydrogen line.

 

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope: Galaxy structure obtained from experimental data.

 

From the graph of the distribution of hydrogen obtained from the experimental data, it can be seen that the latter is not uniformly distributed, but appears concentrated in filamentary structures. The latter are considered strong evidence of a spiral structure. This image, in particular, would seem to reveal a spiraling structure highlighting five spiral arms, two of which are well formed and not so marked. By superimposing the image obtained with a representative of the spiral structure of the Galaxy, one can guess how the three spiral arms highlighted are, from top right to bottom left, that of the Regulus-Swan, Perseus, Orion, Carina-Sagittarius and Scutum-Crux.

 

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope: Reconstruction of the spiral arms: Regulus-Swan (green), Perseus (yellow), Orion (red), Carina-Sagittarius (light blue) e Scutum-Crux (fuchsia).

 

In correspondence with the Sun position there are many experimental points due to the fact that it is surrounded by spiral arms and therefore the presence of hydrogen in this area is abundant. The intensity and size of the various points indicates the intensity of the signal which is an indication of the concentration of hydrogen. As was to be expected, the areas around the position of the Sun have a greater intensity comparable to that of the clouds that are located at almost the same distance from the galactic center. This last feature highlights how the intensity of the hydrogen concentration decreases uniformly in every direction away from the galactic center.

From the knowledge of the rotation curve it was possible to derive the mass of the Galaxy within radius R: this is commonly called integral mass.

 

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope

Study of the galactic hydrogen distribution with SPIDER 300A radio telescope: Integral mass as a function of the distance from the galactic center

 

From the obtained data, galaxy mass is equal to 81.22 x 109 M☉. It’s important to notice that these data do not consider the fact that a supermassive black hole is located at the center of the galaxy, and therefore the values are underestimated. It is impossible, without this hypothesis, which cannot be deduced from the data obtained, to accurately detect the mass of the galaxy. Finally, the Oort constants were calculated considering only the data that met the condition of identifying positions in the vicinity of the Sun.

The Oort constants A and B respectively measure the deviation from rigid rotation and local vorticity, from which the angular velocity corresponding to the Sun and the trend of the differential rotation are derived.

 

 

4. CONCLUSIONS

From the obtained results, compared with those expected, it emerges that they are in good agreement. In fact, as previously stated, the rotation curve of the Galaxy is comparable, within the limits already stated, with historical values. The map of the Galaxy also reflects the characteristics attributable to other observation campaigns, in particular the historical data acquired, and mainly highlights five structures attributable to five spiral arms. Compared to some researches with amateur instruments, it is evident that the greater sensitivity of the instrument allowed to highlight a much more complex structure than expected. Finally, the mass obtained within the orbit of the Sun is comparable to the one commonly accepted, with the discriminant of not taking into account the mass concentrated in the galactic center, which, as already expressed, is not identifiable with this technique. Finally, the Oort constants were found to be compatible, as already highlighted, with the expected data. The analysis carried out with the data obtained from the PrimaLuceLab SPIDER 300A radio telescope proved to be extremely interesting and allowed us to obtain excellent scientific results.

Acknowledgements. Thanks to Filippo Bradaschia and all PrimaLuceLab staff for providing us with the instrument data that for the first time were used to carry out this type of research.

Radio astronomy in Hong Kong in 21cm radio band

Radio astronomy in Hong Kong in 21cm radio band

By | Radio astronomy projects

C. S. LEUNG (1), C. M. LEE (2), K. W. NG (2)

(1) Department of Applied Mathematics, Hong Kong Polytechnic University, Hong Kong SAR, P. R. China, Email: chun-sing-hkpu.leung@polyu.edu.hk
(2) Hong Kong Astronomical Society, Hong Kong SAR, P. R. China

Abstract. In this conference paper, we present the development of radio astronomy in Hong Kong in 21 cm wavelength since 2006. We discuss the subtropical region with its usual cloudy condition in Hong Kong for launching radio astronomy. The MIT small radio telescope and Italy SPIDER radio telescope series were good starting points to create radio astronomy for a dense human populated region like Hong Kong. We present some interesting results with respect to these two types of radio telescopes. We also introduce the future possibility for the developing radio interferometry array in Hong Kong for research and teaching.

Key words: Education in astronomy, Instruments.

 

1. INTRODUCTION

Hong Kong is a business-oriented city, highly compacted within about 1,100 km2. However, this densely populated metropolitan among those highest ones in the world is not good for developing optical astronomical observation. The radio pollution is serious and adversely affects the radio astronomy developed in Hong Kong. In October 2006, the University of Hong Kong (hereafter HKU) brought in a small radio telescope which was developed from US MIT subsidiary company operating in 21 cm wavelength for research and teaching in the Physics Building Dome of HKU (Longitude: 114o 8′ 23.262′′E, Latitude: 22o13′59.7′′N, Altitude: 120m) and a private observatory titled Ho Koon Astronomical Center (hereafter HKAC, Longi- tude: 114o6′29.3076′′E, Latitude: 22o23′1.644′′N, Altitude: 149m). In total, we constructed four 2.3 m radio telescopes in Hong Kong. The telescopes worked in full function mode until 2014 as we lacked experienced people for calibration start- ing from zero. Based on these small radio telescopes, we managed to reproduce the work from MIT small radio results on the Galactic rotational curve.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 1 – Galactic rotational curve obtained from the Hong Kong sky by using the small radio telescope in 21 cm frequency band (Credited from HKU SRT team).

 

We were grateful to have received the donation for the funding from Dr. Stanley Ho association, so we were able to buy a more powerful SPIDER 300A radio telescope system from the PrimaLuceLab company from Italy. We installed this power radio telescope in Stanley Ho Astronomical Observatory (hereafter SHAO, Longitude: 114o13′24.0414′′E, Latitude: 22o14′32.2362′′N, Altitude: 4.6m). This observatory was under the management of Queens College Old Boy Association. It was the first pre-ordered claim from the PrimaLuceLab company so we managed to have acquired their long-term technical support. It was very effective for their super sensitive design for the SPIDER series radio telescope in 21 cm frequency band such that we detected more interesting sources in the sky with better quality for the radio 2D mapping photos. Most of them were the historical detections from our Hong Kong sky. In fact, we did not think we still have a radio quiet zone in our Hong Kong sky at the first light of the SPIDER 300A system as we have major radio pollution.

 

2. OBSERVATION

As we mentioned that we used the MIT small radio telescopes (hereafter SRT) to do observation before, we started to do solar observation by using SRT as our startup project. For the SRT, it is a 2.3 m dish in diameter with 7 degrees beamwidth. Furthermore, the bandwidth for this telescope is only 50 kHz. The design for this telescope and receiver with its major emphasis on transient targets. But, it also has a function so called 25 points mapping with unadjustable integrating time for getting large 2D mapping. So, bound by this limited capability, we tried to test our sky by observing the standard radio source. Figure 2 shows the results based on the 25 points mapping with the related spectrum. The first radio signal outside the solar system was detected by Jansky in 1931 (Jansky, 1932) which the first radio solar physics paper was published in 1944 (Reber, 1944). After that, solar radio physics was born. As the starting work in Hong Kong, we tried to reproduce all these radio solar works such that we can be familiar with the system performance of our radio telescopes.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 2 – Results based on the 25 points mapping with the related spectrum. The “bull eye” shows that we point to the sun correctly (Credited from HKAC).

 

Apart from the sun, the SRT also worked on some other strong radio sources and lunar eclipse, namely CasA, Taurus A (M1), galactic rotational curve and so on. The galactic rotational curve obtained is similar to the MIT Haystack Observatory results, that means we can operate it in a similar way as that in US. Due to the location difference for all SRT in both HKU and HKAC, we missed the important observation for the annular eclipse on 21st May, 2012.
After we bought the new SPIDER 300A system from the PrimaLuceLab company as it has more powerful functions, we carried out some interesting observations in our sky. The SPIDER 300A has a 3 m diameter with 4.3-degree beamwidth. The bandwidth for SPIDER 300A is 50MHz, roughly of about 3 order of magnitude better than that of SRT. Furthermore, it has a built-in function so called Based-Band Converter Tool (BBC Tool) which is a digital filter for filtering the noise such that collecting better radio signals. Figure 3 shows the control software and the BBC Tool window. Figure 3 shows the RadioUniversePro software and the corresponding BBC Tool window. We can see the group of digital filters (16+16) fully tunable on the 2 Intermediate Frequencies such that we can avoid the series FRI signals.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 3 – RadioUniversePro software and the corresponding BBC Tool window.

 

By making use SPIDER 300A system, we detected more interesting objects in SHAO from Hong Kong Tai Tam. The followings are some interesting sources that we detected in SHAO.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 4 – 2D radio mapping of the Sagittarius A* before processing.

 

Figure 4, Figure 5 and Figure 6 show interesting results that we detected our Milky Way galactic center (SgrA*) on 1st January, 2021 and 9th December, 2021 respectively. These are the historical images from our Hong Kong sky. Figure 7 shows the daily observation of the 2D mapping for the radio sun. Furthermore, the interesting strong source so called Cygnus A (Cyg A) showing in Figure 8, is an extragalactic source about 600 million light years away from our earth.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 5 – 2D radio mapping of the Sagittarius A* after processing.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 6 – 2D radio mapping of the Sagittarius A* after processing with the Python scripts.

 

So, from all those observations in Hong Kong, we can setup the radio source database for our sky. In addition to accumulating observing data for our database, we also used the SPIDER 300A for doing the observation for the radio solar eclipses. We also got very meaningful results. For more technical and extensive details, please refer to the other paper for the radio solar eclipse in Hong Kong (Leung et al., 2021).

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 7 – 2D radio mapping of the Sun after processing with the Python scripts.

 

Radio astronomy in Hong Kong in 21 CM radio band

Fig. 8 – 2D radio mapping of the Cygnus A after processing with the Python scripts.

 

3. DISCUSSIONS AND CONCLUSIONS

For this presentation, we addressed why Hong Kong developed radio astronomy owing to the subtropical region and cloudy condition in the territory which is not good for developing optical astronomy. We still obtained some interesting two-dimensional radio mapping. This was the historical observation for the 21cm radio solar eclipse in Hong Kong. Based on these experiences, we trained some young students for developing their interest and career works in these discipline. For the SPIDER 300A system, it was easier for us to develop remote observation and data reduction pipe since their design can compromise with the professional telescope in other radio observatory in the world. The next stage for the radio astronomy in Hong Kong will be the radio interferometer such that we will combine all those radio telescopes in Hong Kong by making use software correlation. We will also work with other radio telescopes in other neighbouring countries to test the VLBI observation.

Acknowledgements. We would like to express our deep gratitude to the late Dr. Stanley Ho and his family for their generous donation and huge support to the Stanley Ho Astronomical Observatory at Tai Tam, Hong Kong. The funding provided the radio telescope and all accessories required for the radio observation and data acquisition. Special thanks are given to Prof. Yuen Kwok Yung as the facilitator and Ms Daisy Ho as donation coordinator and donor. We would like to give thanks for all radio observatories in the territory, namely HKU, HKAC, and SHAO. We are indebted to the management and technical teams from all these three observatories.

 

REFERENCES

Jansky K. G.: 1932, Directional Studies of Atmospherics at High Frequencies, Proc. Inst. Rad. Eng. 20, 1920.

Reber G.: 1944, Cosmic Static, Astrophys. J. 100, 279.

Leung C. S. et al.: 2022, Solar Eclipse Observations with Small Radio Telescope in Hong Kong in 21cm Radio Frequency Band, RoAJ, 32.

Received on 10 December 2021