REAL TIME DATA COLLECTION SYSTEM AND TWO-DIMENSIONAL FOURIER TRANSFORM HARDWARE FOR MI-YUN METER-WAVE APERTURE SYNTHESIS RADIOTELESCOPE

In this paper two equipments for Mi-Yun meter-wave aperture synthesis radiote-lescope are described. These two equipments are used for data acquisition and handling with on-line computer, NOVA-3/D.The Mi-Yun aperture synthesis system consists of an E-W array ,giving 192 simultaneous signal pairs of the form R=W cos, I=W sin per 10 sconds. Where W is the fringe amplitude and=(2d/) cos sinHA, the phase. (d-the interferometer spacing,--the wave length,--th declination and HA--the hour angle of field center.)The first equipment is the data collection system which performs the follwing function: 1 In every 10 msec generates a value,0=A sinHA, where A is thecarefully chosen constant, HA is the hour angle of the field center. 2. In the same 10 msec generatesk, where k=k0(k=3, 4,,194), accurate to 2.3.Converts each of the 192 signal pairs into parameters Wk andk; perform the fringe stopping by substracting k from eachk, giving a new signal set of the form Wk cos(k-k) and Wk sin(k-k); accumulate for 10 seconds and send the output data to the on-line computer. The function0(t)=A sinHA is generated by the digital differential analyzer (DDA) developed by one of the authors (C.K.Kwong).AM-2901 has been used for the CPU of our system. Block diagram of the whole system is shown in Fig.4. Fig.5 is the flow chart of the microprogram. The system can work either with mean time or sidereal time. The other equipment is the two-dimensional direct fourier transform (DFT) hardware based on the previous work of R.H.Frater, adapted for the data handling of the Mi-Yun system. The observation will be carried out in 12 hours, filling up 2160 equally spaced hourangles with 192 spacings each, thus 2160192 grid values in u-v plane (as defined in Fig.7(a)). Fourier transform of this u-v distribution is to be performed by the DFT hardware to give a radio sky map on a 256256 grids (Fig.7(b)). The performance can be completed in less than 6 minutes. In the construction, the principle and scheme developed by Frater has been used with some imnor modifications, such as using the hardware multiplier (MPY-12 AJ) in place of Log. Table, increasing the accuracy with interpolations, etc.Fig.8 is the block diagram of the DFT hardware. As a laboratory test, a set of signals simulating a point source located at the field center was sent to the DFT hardware. The result of the fourier transform is shown in Fig.9 they agree well with the estimation. The two equipments have been completed now and have worked normally for several months.