Phone: +39 070 711801


Pranusanguni - San Basilio (Cagliari) Italy

Project

Front-End

One of the most advanced feature of SRT is the so-called 'frequency agility' i.e. the capability to switch among different receivers remotely and automatically by using several minor servo-systems. In order to test both these mechanisms and the antenna’s response from different antenna focal positions, the first light receivers have been designed to cover as many foci of the antenna as possible.

The focal position and the operational frequency band of each receiver are reported in Table I.

 

 Receiver

Freq. 

Beam-size

 T-sys [K]

 Gain[%]

   Primary-focus L-P band dual-frequency

 305 - 410 MHz

 56.2'

 (65)

NA

 (45)

NA

1.3 - 1.8 GHz

 12.6'

(21)

NA

(47)

NA

 

 BWG-focus C-band mono-feed

5.7 - 7.7 GHz

 2.8'

 (19)

33

 (66)

48

 

 Secondary-focus K-band multi-feed

 18 - 26.5 GHz

 50"

(45 - 75)

70 - 90

(57)

44 

For each receiver the frequency coverage, the beam-size in arcmin/arcsec, measurements and estimations (between brackets) of system temperature at 45 degree elevation and antenna gain are shown (NA: not available). The substantial degradation in the antenna gain is mainly due to the current problems to move the subreflector necessary to maximize the optical alignment. As far as the system temperature this is worse than expected because of unpredicted losses in the receiver cabin’s screen and probably for another source still to be identified. Concerning the next generation receiver for SRT, the front-end group has now started working on a secondary focus Q-band 19-pixel dual-polarized cryo-cooled system.

All front-ends are dual-polarized, cryogenically cooled and take advantage of the latest HEMT technology. The refrigeration system is based on a two stage Gifford- McMahon cooler (20 K and 77 K) in a Helium 5.5 closed cycle system. The K-band receiver is based on a 7-pixel multi-feed configuration to improve sky map capability (beam separation is 138"). Also the low frequency receiver shows an original design with a coaxial configuration to simultaneously observe both P- and L-band. This latter is a key-aspect in pulsar research enabling astronomers to evaluate and subtract the interstellar medium dispersion effects. However, the first receiver installed in SRT has been the C-band mono-feed receiver. Its beam size was the preferred one for testing the preliminary pointing models. This receiver was first installed in the Gregorian focus to minimize possible misalignments in the optical path. Then, it was moved to its final position in the BWG. Currently, all three microwave front-end receivers are installed in the radio telescope. The receivers commissioning now in progress includes measurement of the receiver stability and system temperature, the manual pointing, the pointing model and the gain curve versus elevation. The C- and Kband receivers have almost completed this procedure. Experimental measurements on antenna are not yet available for the dual-frequency receiver which was mounted in May, 2013. Preliminary technical results of the receivers together with estimated values (in italic and between brackets) are presented in the table below.