HartRAO Home > news > Cryogenic 22 GHz receiver on 26m Telescope 2014-05-05

During 1998 to 2005 a new main reflecting surface was installed on the HartRAO 26m telescope, with the objectives of improving performance at high frequencies and increasing the usable upper frequency limit from 12 GHz to the 22 GHz band. An uncooled test 22 GHz receiver was installed in 2007 to test performance in that band after the upgrade. This provided a system temperature of over 200 K, so it was not particularly sensitive. However it demonstrated that the upgraded antenna did provide useful performance at 22 GHz. Use of the telescope for VLBI at 22 GHz with the test receiver has grown, and requests for 22 GHz continue to increase. The orbiting Russian radio telescope Radioastron operates in this band, and since December 2013 HartRAO has been doing frequent VLBIs with Radioastron and other radio telescopes on the ground, including at 22 GHz. Another new request was for 22 GHz astrometric VLBI to help improve the International Celestial Reference Frame (ICRF), which uses thousands of distant quasars as fixed sources as its basis. A decision was therefore taken late in 2013 to develop a more sensitive cryogenic 22 GHz receiver, with the target of having it working on the telescope for the 24-hour 22 GHz astrometric VLBI scheduled for 4-5 May 2014.

The feed horn and waveguide components for the receiver are sufficiently small that they could be included inside the cryogenic vacuum package and cooled down with the low noise amplifiers. This resulted in a very low receiver noise temperature of 20 - 25 K. The contribution of the dry winter atmosphere at zenith roughly matches this. The sensitivity near zenith is about five times better than the uncooled test receiver. This degrades away from zenith as the noise contribution from the atmospheric water vapour increases and the paraboloid dish surface deforms under asymmetric gravitational loading.

The receiver was installed on the 26m telescope on 23 April 2014, and operated successfully in the astrometric VLBI on 4 - 5 May.

Astronomer Aletha de Witt is leading the astrometry research at HartRAO, and commented:

"K-band (22 GHz) radio observations have the potential to form the basis for the most accurate Celestial Reference Frame (CRF) ever constructed. K-band CRF observations currently exist only from the all-northern Very Long Baseline Array (VLBA) and therefore must be complemented by observations from southern arrays in order to realise this potential. A collaboration has been formed with the goal of completing sky coverage at K-band with specific emphasis on the Southern hemisphere where K-band CRF coverage is weak. Preliminary astrometric observations were carried out on 23 August 2013 between telescopes in Australia (Hobart 26m telescope), Korea (Tamna 21m) and South Africa (HartRAO 26m). More extensive astrometric observations were carried out on 21 December 2013 that also included the Tidbinbilla 70m NASA Deep Space Network (DSN) antenna in Australia."

"We identified the uncooled K-band receiver on the HartRAO 26m antenna as one of the major error sources in our K-band CRF work and thus made it high priority to develop a cooled receiver. Our first CRF observations with the new HartRAO receiver ran for 24 hours starting on 4 May 2014 between the Hobart 26m, Tidbinbilla 70m and the HartRAO 26m telescopes."

The pictures below show stages in the development of the receiver and VLBI fringe test results.