Mullard Radio Astronomy Observatory
The Cambridge Low-Frequency Synthesis Telescope (CLFST)
Since the early days of Radio Astronomy in Cambridge we have been building instruments to survey the sky at low frequencies. The latest in this long line of telescopes was Cambridge Low-Frequency Synthesis Telescope (CLFST), an east-west aperture synthesis telescope operating at either 151 or 38 MHz.
The CLFST was built in 1980. Initially it was designed to operate at 151 MHz and comprised 60 tracking yagis on a 4.6-km baseline, giving 776 simultaneous baselines. These provided a resolution of 70×70 cosec(declination) arcsec2, with a sensitivity of about 30 to 50 mJy/beam in 12 hours, and a field of view of about 9°×9°. The photo on the right shows a typical group of 151-MHz aerials. The 151-MHz telescope far exceeded its original projected lifetime and eventually ceased operating in 2000, but a group of aerials similar to those pictured here has been preserved at Lord’s Bridge.
The 38-MHz capability was added in 1984 in order that operations at that frequency would coincide with the time of minimum sunspot activity in the eleven-year solar cycle. This was an important consideration, as the disturbance of the ionosphere by solar activity would severely affect observations at a frequency as low as 38-MHz. 58 × 8-metre yagis were mounted on poles along a parallel 4.6-km baseline to give a resolution of 4.5×4.5 cosec(declination) arcsec2, deliberately similar to that of the earlier 6C 151-MHz telescope to allow comparison between measurements of sources at the two frequencies. Being rather large and unwieldy, the 38-MHz aerials did not track, but had to be pointed manually in the desired orientation and secured with guy ropes. The observing method used had been established for the 6C survey – by making observations at a number of different hour angles at a chosen declination, a coarse approximation to a tracking observation was built up. The photo on the left shows some of the 38-MHz aerials; a group of 151-MHz aerials can be seen on a parallel baseline alongside. Because of their unwieldy size and susceptibility to high winds, the 38-MHz aerials have now been dismantled and removed.
The CLFST was used to make the 7C survey at 151MHz and the 8C survey at 38 MHz, the latter attaining approximately the same resolution as the earlier 6C survey at 151 MHz. The 6C telescope was the predecessor of the CLFST and was a non-tracking instrument which had used 40 of the same 151-MHz aerials, pointable manually on a 1.8-km baseline, giving a resolution of 4.2×4.2 cosec(declination) arcmin2). In recent years these three surveys have found some of the highest redshift (most distant) radio galaxies known.
For information on other MRAO telescopes, follow the links on the left.