Miyun metre-wave aperture synthesis radio telescope

Chin.Astron.Astrophys.10 (1986) 3-11 B (1985) 245-.^54

Pergamon Press. Printed in Great Britain 0275-1062/86$10.00+.00

Act.Astrophys..Sin.

MIYUN

METRE-WAVE

APERTURE

SYNTHESIS

RADIO

TELESCOPE

Beijing Observatory Metre-Wave Radio Astronomy Group*

Beijing

Observatory,

Academia

Sinica

ABSTRACT The Miyun metre-wave aperture synthesis radiotelescope, working at frequency of 232 MHz, consists of an E-W array of 28 elements, each of 9m aperture. 192 baselines are effected with a full coverage of the U-V plane (Fig. 2). The longest baseline is 1,164m. This instrument is designed for source survey and detection of peculiar sources in northern declinations. A set of observations completed in 2 x12 hours gives a thermal noise limited sensititity of 0.05Jy and a resolution of 3'8x3'8~~~6. The field of view is 8'~ 8". This should enable us to complete an overall survey of the region 6, + 30' within two years, and to carry out monitoring of selected areas. Figures 1 and 2 show the main properties and general design of the instrument and Figures 3 and 4 give some preliminary results of sky mapping.

1.

DESIGN PHILOSOPHY AND STRATEGY

Synthesis aperture radio telescopes are high sensitivity, high resolution imaging devices in radio astronomy. Their implementationin the mid-sixties brought in a major breakthrough in radio astronomical techniques t11. Their originator was awarded the Nobel Prize in 1974 for this work [2]. To image in the metre-wave range with high sensitivity and high resolution astronomical objects has become one of the most urgent problems in radio astronomy, especially since high quality observations in the centimetre range over the past 10 years or so have led to many important discoveries. To understand the structures of bodies in the metre range is now a pressing demand. In this respect, the synthesis aperture method is undoubtedly the most effective means. At present, the number of recorded and published radio objects over the whole sky is about 30000per steradian. This number is minute compared with the number of photographicallyrecorded bodies, reckoned in hundred millions. In view of the role in the development of astronomy played by the discovery of radio objects, a further radio survey over a large area of the sky to find more hitherto unknown objects constitutes an

important basic work in the further development of astronomy. Radio surveys are most effective with metre-wave techniques, but we require a high sensitivity, a high resolution and we would wish to complete the survey observations within a reasonable time. These requirements can be met by a metre-wave aperture synthesis system. The design plan for the Miyun aperture synthesis radio telescope was first put forward in 1973 [3], and was re-adjusted in 1979 following the adoption of digitization techniques. The design strategy was based on answering three questions, how advanced ? how practicable ? any scope for futre development ? 1. Target Figures Noting the constraints of our practical conditions, our design targets are: a) a resolution of 3.8' x3.8' csc 6. b) A thermal noise limited sensitivity of 0.05Jy. We estimate we shall be abIe to record between 20000and 30000 sources per steradian, 5 times higher than present available surveys. This target at present is matched only by the Cambridge 151MHz survey system (the U.S. Texas Radio Source Catalog, now published in instalments, records some 7000sources per steradian. c) A field of about 8'~ 8" or larger, so that a general survey over one-quarter of the whole sky

* The following persons took part in the designing and making of the telescope: WANG Hong, WANG Xin-min, WANG Shou-guan, LIU Fu-you, PU Ting-yi, CHEN Hong-shen, QIU Yu-hai, YANG Yi-pei, PANG Lei, ZHANG Chun-lu, ZHANG Guo-quan, ZHANG Xi-zhen, JIN Tie-lin, ZHENG Yi-jia, ZHAO Hui-ping, NAN Ren-dong, KANG Lian-sheng, BAO Hong-qi, WEI Ming-zhi.

4

Beijing

Radio

(declination>30°N) can be completed within 2 years. If these targets are met, then this equimment can be regarded as an effective instrument for fulfilling the above pressing need in metre-wave radio astronomy. Practicability Under the conditiones of 2. the above targets having been fulfilled, we adopted the following technical guidelines with respect to the three major structural components : a) for the system of aerials which usually contains the lion’s share of the cost, we use the aerial array of our original Miyun interferometer, and to make the necessary additions and modifications to achieve the required total receiving area and resolution in ways that are simplest, most reliable and most economical. For transmission lines, we use the cheapest, core cables and bury them China made, solid at a depth of 1.5m be~owtbegro~d so as to reduce the variation in the electrical length caused by temperature fluctuation. Under the premise of the amplifiers reaching the sensitivity of the overall system, emphasis is placed on their resistance to interferance by limiting the bandwidth to l.SMHz, and by a careful choice of the b) For the numerous pre-filters etc. complex structured receiver systems (including a large quantity of correlators, time-delay systems, “fringe tracking”, data acquisition, pre-treatment etc.] we adopt the plan of introducing from abroad digitization techniques. We received assistance from our colleagues in Sydney University, Australia, and introduced this part of technique used in the Fleurs radio The results obtained showed telescope. c) For the accurate and reliable working. post-treatment of the large amount of data and the various auxiliary operations (such as real-time control, auto-calibration, etc.) we developed various pieces of software and initiated bilateral exchanges with major aperture synthesis centres abroad, for the introduction of software. Scope for Future Development Because 3. the target objects for radio astronomical observation are usually somewhat limited in number, the usual working life of a radio telescope can be very short, so we must keep on developing and renovating techniques so as to avoid the mistakes of some early instruments ceasing to be effective in a Right from the first draft short time. design for the Miyun metre-wave aerial array, we saw to it that there was always going to be scope for future expansion, and that the development at any stage can be made on the basis of the configuration so far achieved by adding more aerial units and using newer Further development on the techniques. present array includes a) adding a second working frequency (408MHz) on the existing

Astronomy

Group

system, to facilitate the estimation of the spectrum of the newly discovered objects and the search for variable sources in metrewaves, and b) the addition of more aerials (specifically, we have already added two more dishes at a distance of lkm)to double the resolving power. Next development will be a metre-wave VLBI system.

2.

ASTRONOMICALTARGETS [4]

The working targets at the present stage for the Miyun aperture synthesis system are a general metre-wave survey of the sky of northern declinations and a search for accurate and detailed peculiar sources; measurement in metre-waves of large, extended sources will also be made at the same time. Hp to now, the number of recorded radio sources over the whole sky is only a few tens of thousands, and of these, only a small fraction have been subjected to detailed study. Nevertheless, the great significance of the results obtained has created a large demand for more sample objects, from which we may pick out or discover more, especially significant sources and this has directed attention to a general survey in metre-waves. It is hoped 1) that, by raising the the survey will be made technological level, to a great depth and in a highly uniform manner and 2) that development of methods will lead to a better classification and discrimination of the objects found. Another demand in metre-wave observation is precise measurement of target sources. Here, the most difficult part concerns the morphoIogy and fine details. Because of limitations in sensitivity and resolution of measurements at metre-waves, the data so far gathered in this wavelength range falls far below those in cm-waves and the optical But the metre-waves are two decades range. away from the cm-waves, and possess a rich, intrinsic, physical content, so the everdeepening study of various kinds of radio astronomical bodies has meant an ever pressing demand for raising the observing capabilites in metre-waves so as to make combined studies possible. We now discuss the capability of the Miyun 232MHz aperture synthesis system in regard to these specific jobs. General 1. Sources

Survey

and Search

for

Peculiar

For the northern hemisphere, the most important surveys are the Cambridge catalogues of radio sources. The 4C catalogue, working at 178MHz,records 4843 sources in the region -0,7’s5r80°, with fluxes greater than 2Jy. Results of Phase I of the 6C Survey were published in 1976 [SJ. The working frequency was 151.5MHz; aperture synthesis technique was used; resolution

Miyun Radio

3.7’ x 3.7’ csc 6, and field, 17” x 17’. The first map is of the region of the North Pole and is the average result of 10 separate runs of 12 observing hours each. The limiting flux is 0.05Jyand it is estimated that a density of 35 000 measured sources per steradian will be reached. The 6C Survey for the whole Northern sky is in progress. A series of results have also been published from the U.S. Texas Survey Plan These are the most accurate position [61. catalogues so far produced of sources in metre-waves. The Plan covers the range 6 1-350, the working frequency is 330 - 380 MHz, flux limit, about 0.2Jyand their first target is to give positions accurate to 1” for 5000 sources. Figure 1 shows the working frequency and limiting flux of some major programs in Of these, northern hemisphere survey work. will carry out or and “Texas” 6C, “Miyun”, complete the survey within the next few years.

c

MIYUN II

U-Y)

s

2

1515159 178

I 0.W 23?#

TEXAS BOLOGNA r

I

11 02 335

0.2 408

Ml+

Fig. 1 Metre (and long decimetre) wave source surveys in the northern hemisphere

For survey work, there are 4 desiderata: 1) Ascertain the existence of a source and distinguish whether it is a point source or an extended source (of course, in respect of the resolving power of the instrument used). 2) Measure the position of the source to an accuracy around lo”, to facilitate optical identification. 3) Endeavour to reach an absolute flux density of lO%, thus to give a sufficiently accurate spectral index, and to reach such degree of repeatability in relative values that variable sources can be well detected. 4) Have a sufficiently large field so that the survey can be completed within a reasonable time. The corresponding technical demands are as follows : 1) For sources of 0.05Jyand a system resolution of about 4’, the “resolution limit” becomes all important and any lower resolution will cause source confusion. 2) For a resolution of about the position of the 4’9 we can estimate centre of the main lobe in from the image profile of a point source with an error better than about 10”. The absolute position of the source can be determined from the known positions of reference

Telescope

5

sources. 3) In determining the flux of the source using reference sources of known fluxes, we should aim at a relative precision of 2%. 4) The field should be large and grating lobe confusion should be suppressed as much as possible. In regard to items 1 and 4, the design of the Miyun system satisfies the requirements; for 2, in the 8” x 8” Miyun field, there should be, on the average, more than 100 Texas sources with positions accurate to l”, amply sufficient as reference sources; lastly, in regard to 3, the relative precision is determined by the signal-to-noise ratio, and for deciding whether an observed source is variable or it is best to have two completely not, independent observations for corroboration. At present it is still difficult to distinguish various intrinsic properties of the sources from just radio observations. However, certain peculiar behaviours in radio can be linked to certain peculiar properties, the most notable being This has the phenomenan of variable flux. been substantiated by a considerable amount of observations in the cm range, and some theoretical work, while examples of variable sources in the metre-wave range are still rather few and still await exploration. Another kind of peculiarities refer to abnormal spectra, those with abnormally large gradient, or abrupt changes. Lastly, for sources that have been identified, a classification according to the radio spectrum should lead to interesting statistics. With these in view, we made adequate provision in the Miyun system for our next stage of simultaneous work at two frequencies, 232 and 408MHz. (The reason for choosing 408MHz as our second frequency is that we can thus compare our work with the extensive international, well-advanced work at this frequency so that our system can be linked to the cm and dm systems that have already made a great deal of observations).

2.

Fine

Structures

of

Sources

in Metre-Waves

a source with a flux of 0.05Jyat 3OOMHz has a spectral index of (I= -1, then its flux at 10GHzwill be 1mJy; and if the spectral index is -0.8 or -0.5, then its flux at 10GHz will be 2.2mJyor 7mJy. Therefore, among the sources presently studied by large telescopes in the cm range, a considerable portion will show up in metre-wave observations down to O.OSJy. Certain diffuse sources, such as the Milky Way band and the very large sources clusters of galaxies, :;;;p;ya; &?z;;,“;;z; also require high resolution observatjon, and such resolving power (of the order of arc minutes) as possessed by the present Miyun system can already yield useful results. For searching If

6

Beijing

Aerial

Multiplie:

r-

Ai

(Dual

Radio

Astronomy

Frequency

reamplifier

Receiving)

Group

Array

Bi

THE

FRONT

Mixer e modulation

Oscillator

& Digitization 2%ch. Delay

Digital System

Pre-porcessing

Fig. 2

Direct FT tiardware

Niyun Aperture Synthesis Radio Telescone Block Diagram Peripherals

Miyun Radio Telescope

Fig. 3

Map of north celestial polar region (8*x 8")

for the central star of supernova remnants and the detection of exploding stars and comets, an arcminute resolution may be adequate in some cases.

3. AERIAL

ARRAY

AND

THE FORE SYSTEM

A Block diagram of the Miyun Aperture Synthesis Radio Telescope is shown in Fig. 2. The array of aerial is along the East-West line, and is divided into Array A and Array B (Fig. Za). The elements of Array A made

use of the 16 dishes of the original Miyun Interferometer,with each reflecting surface enlarged to 9 metres. These elements are placed 72 metres or 12do, apart, do=6m being the "basic interval" of our system. We took do= (2/3fD, D being the aperture of a single element, in order to have a completely covered baseline plane (the Wplane) with reduced grating sidelobe confusion, to facilitate the making of large field maps (and source cataloguesf: to this end, we put Array B whose elements are to interfere severally with the elements of

8

Beijing Radio Astronomy Group

Array A, on the two sides of Array A, so that, the successive intervals between the elements of Array B need not to be da, rather, it can be 2d0 = (4/3)0. Array B consists of 12 dishes, each with the same structure as the dishes of Array A. We use all the combinations AiB,(i= l-16, j= 1-12) to form the entire inter3erence unit, consisting of 192 interferometerswith

baselines incrementing by Ido from 3dO to 194da,inclusive; see Fig. 2(a). Thus, in the "Earth's rotation synthesis" [3], each cycle of 12 hours' observation, when weighted in a natural manner (that is, without any artificial weighting of the interference signals received from the various channels), gives an overall resolution of 3.8' x3.8' csc 6 for the 232MHz

62'2L

62'08

61'48

27*28~

LLl-Li 27"28'

Fig. 4

25*48'

29-48'

2"20m48*

24mOi3

22128'

2h20-4;

Dirty and Clean maps of the extended source 4C 61.5

Miyun Radio

Telescope

65’42’

Fig.

5

Iso-intensity

map of

system. We take the 9-metre dish working at 232 MHz to have an effective area of 22.3 m2, the noise of the system (sky background plus intsrumental noise) to be 400’K, the bandwidth to be a fixed 1.5 MHz, and the transmission efficiency between the aerial and the pre-amplifier to be 0.75, and we find a “thermal noise limited” sensitivity of O.OSJyover one cycle of 12 hours’ observation covering a field of 8’~ 8’, when we take a signal-to-noise of 5 to be the limit of detectability. The above targets have been met according to the requirements on the design. For an aperture synthesis system, an important limitation on the sensitivity comes from the sidelobes and grating responces of “strong” sources in the field or its vicinity, which drown the signals from weak sources. The intensity ratio between a strong source in the field and a nearby weak source is called the “dynamic synthesis observation. range” of the aperture The usual method of removing the sidelobes and grating lobes of strong sources is the trial-and-error program called CLEAN [7]. Since during our Phase One work on the Miyun system, we used a Fourier transform hardware [8] to carry out the mapping treatment, we developed a CLEAN program suitable for hardware treatment, [9]. The effectiveness of CLEAN is determined by how precisely we can control the relative phase errors between the various channels of the array and the non-uniformity in their gain. At present, by means of a simple CLEAN program,

the

extended

source

DA 240

we can achieve a dynamic range of lOO:l, and we expect this figure will be greatly improved upon in our next stage after we have implemented self calibration. Our aperture synthesis coverage lacks the intervals 1 do and 2 do, causing weak negative lobes over some 4 ’ in the synthetic directional pattern. This effect is not serious in the usual applications. We use cable SYV 75-15 to transmit the 202 MHz signal of the local oscillation of the Miyun system, and cable SYV 75-9 for the intermediate frequency of 3OMHz. The equally-spaced branches of the local frequency each have a path length of 650m, and the intermediate frequency cables each have a length of 1OOOm. The cables are buried in the ground at a depth of l.Sm, where the diurnal temperature variation is less than 1’C. The pre-amplifier has a noise of about 3 db, the total gain is 70 db and the total bandwidth, 5 MHz. Technical details relating to these items are given in Ref. [9].

4.

THE RECEIVER AND DATA REDUCTION

A block diagram of the rear portion is shown in Fig. 2(b). The relevant technical details will be published in Ref. [9]. After transmitting the intermediate frequency 3OMHz signals of the 16 channels of Array A and 12 channels of Array B to the control room, Array B was divided into two groups (B1 - 86 and B1’ - Be’ of Fig. 2a) and worked on alternately. Thus, 22 channels of

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Beijing

Radio

intermediate-frequency signal went to be treated in the control room. After amplification and correction for frequency response, the signal is mixed with a second local 30MHzoscillator to result in a zero intermediate frequency. The zero frequency signals from the various channels were then passed through a low-pass filter and an automotive gain control, and were then quantified into Z-bit digitized form. The digitized signals from the 22 channels were sampled at a sampling rate of 120 ns by a “sampling time-lag unit”, and were then given real-time time-lag compensation. After the time delay, each A channel was divided into 6 channels, and each B channel, into 16 channels, and the signals were passed into 96 AiB. numerical correlators for correlation ana I ysis, each correlator returning a sine output and a cosine output. The correlation was calculated once every 120ns, and the result of the calculation was accumulated by means of an 8-bit adder and an d-bit store. The averaged accumulated result from each was taken once every 10ms and sent to the data collector for pre-treatment. Every lOms, the data collector [8] generates a set of 192 “fringe phase tracking” signals (96 are used at a time) in the form of sinusoidal output. These signals give the real-time phases that should be obtained by the correlation measurement of the various channels of interferometers relative to the centre of the field. The data collector subtracts this “fringe phase ” from the input signals from the various correlators to give 96 channels of post - “fringe tracking” treatment (hence with much slower phase variations), sinusoidal signals. These, after each accumulated over 10 seconds, are then sent to a NOVA- 3D Computer. The “fringe phase tracking” signals generated by the data collector were also used to provide the real-time time-delays of the 22 aerial channels mentioned above. The correlation data of the 96 channels were sampled once every 10s by the NOVA-3D computer. Over 2 days of 1 cycle of “Earth’s rotation synthesis” observation (total observing time, Zxl2h), the size of the entire data is 3.4Mbyts. After applying gain weighting and various corrections for phase and gain, the data was then stored on magnetic disks. Correction for baseline parameters, various astronomical and ionospheric corrections, and corrections for the relative phase and error in gain of the

Astronomy

Group

different aerial channels were all made by the usual methods, [lo]. our data treatment is At present, carried out by means of the direct Fourier transform hardware designed by R.H. Frater [81> and the CLEAN program over a limited range. In future, this part of work will transferred to a VAX 11/780 computer for execution.

5.

be

TEST OBSERVATIONS

Figure 3 gives the result of one cycle (192 channels, 12 hours) of observation on the region of the north celestial pole. The dynamic range is about lOO:l, and the rms background fluctuation is about + 0.04Jy. Figure 4 shows the map of the extended source 4C 61.5, both the “dirty” map and the map after CLEAN. Figure 5 is a map of the source DA 240. The contour lines are at intervals of 0.21Jy/beamarea, starting at O.lSJy/beamarea.

ACKNOWLEDGEMENT We wish to express our sincere thanks to Professor W. N. Christiansen of Australia for giving us consistent support Our thanks are throughout the present work. also due to our colleagues in the Electrical Engineering Department, Sydney University, especially‘ Professor R;H.. Frater and Mr. C.K. Kuang for helping us to implement digitization techniques, REN Fang-bin and LIU Yi-song participated in the designing, during the early stage, WU Huai-wei and the late WU Lie-yu participated in the discussion of the overall plan, WU Huai-wei and GAO De-jie were responsible for installing the NOVA 3-D Computer, ZHOU Shu-fan, TANG Si-cheng, LIN Zai-yong and YU Jian-min were responsible for the positional measurement of the baseline and the design of the aerials, ZHU Jian-he and others were responsible for the modification of the HU Huan-bin was responsible for the aerials, designing of the aerial control system during the early stage, and XU Xiang, LI Shan-duo, XIN Jin-xia, FENG Shi-qin and HUANGCheng-yun took part in electronics work during the early stage. The entire engineering was completed through close and consistent meshing between various administrative sections, the factory and the Miyun Observing Station of Beijing Observatory. Contributions were made by each and every of these units.

Miyun Radio Telescope

REFERENCES [l] QIAN Shan-jie, Ziran Kexueshi Yanjiu ("Studies in History of Natural Sciences") 3 (1984) 194. [2] Ryle M., Science 188 (1975) 1071. [3] WANG Shou-guan, "The Miyun Metre-Wave Aperture Synthesis Radio Telescope: The Overall Plan" (1973) (An internal document in Chinese). [4] WANG Shou-guan, Publ. Beijing Ohs. Suppl. 9 (1984) 1. [S] Baldwin J.E., IAU Symposium No. 74, (1976) Ed. D.L. Jauncey. [6] Douglas J.N. and Bash F.N., ibid. [7] HllgbomJ., Astron. Astrophys. Suppl. 15 (1974) 417. [S] KUANG Zhen-kun et al., Dianzixue (1981) 201.

Tonyxun

("ElectronicsLetters") 3

Obs., Special Issue on the Miyun Metre-Wave Aperture Synthesis Radio Telescope.

[9] Publ. Beijing

[lo] Brouw, W.N., Meth. Comput. Phys. 14 (1975) 131. Academic Press.

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