Multi frequency feed system for high magnification Cassegrain radiotelescopes at millimeter wavelengths Samuel López Ruiz, Félix Tercero Martínez. José A. López Fernández, Seog-Tae Han Centro Astronómico de Yebes Instituto Geográfico Nacional 19141-Yebes (SPAIN) samuel.lopez@oan.es, f.tercero@oan.es, ja.lopez@oan.es, sthan@kasi.re.kr Pablo-Luis López-Espí, Rocío Sánchez-Montero, Francisco J. Beltrán Martínez, Signal Theory and Communications Dpt. University of Alcala Alcalá de Henares (SPAIN) pablo.lopez@uah.es, rocio.sanchez@uah.es, 10.fjbeltran@gmail.com Abstract— This paper presents the design of a feed system at K and Q frequency bands which permits simultaneous observation at a radio telescope. The system will be used for radio astronomical VLBI applications. With this design we get simultaneous operation with two different cryogenical receivers placed at the focal zone of the radio telescope with a relative pointing error lower than 3arcsec and an aperture efficiency higher than 75%. Quasi-optical techniques have been used to implement a frequency independent system. Phase corrected feeds have also been developed in order to reduce the size of the feeds and to allow cooling down and receiver noise temperature improvement. The design has been validated with laboratory test and on site tests after positioning and alignment at the focal zone of the 40 meter Yebes Observatory radio telescope. Keywords— multi-band feed system, radio telescopes, quasioptics, corrugated feeds, phase reference, dichroic mirrors I. INTRODUCTION Very long baseline interferometry (VLBI) allows high resolution radio astronomical observations, up to 100 μas in Q band, with Earth based radio telescopes. One of the major error sources in this technique is the phase fluctuation suffered by the different stations mainly due to troposphere water vapour. It fluctuates highly between stations and introduces a random and time dependent phase variation. Multi-frequency phase reference technique reduces these effects as it obtains linear phase fluctuation with frequency and it allows its calibration [1, 2]. There are several approaches to combine radio astronomical observations simultaneously: using a broad band receiver with bandwidths as wide as 7:1 [3], a multi-frequency receiver using coaxial feeds [4] or a multi-frequency receiver using different feeds but a common multi-mirror solution [5]. This last approach offers the advantage of minimizing receiver noise temperature and optical losses then enhancing the radio telescope efficiency. The Yebes 40 meter dish antenna [6] is a multipurpose radio telescope which operates from 2 to 116 GHz. Recently, multi- frequency phase referencing observations at K and Q bands have started under an agreement between IGN (Instituto Geográfico Nacional from Spain) and KASI (Korean Astronomy and Space Science Institute). The radio telescope was equipped with receivers at both interest bands, covering 21.75-24.45GHz and 41.00-49.00GHz. Simultaneous observations were not possible and a new system has been designed and implemented to get this capacity. The new system is based on an optical bench with two different feeds and several common mirrors. This paper is organized in the following way. First we addressed the optical elements design, that is, the different mirrors needed to focus the beam from the radio telescope to the feeds. Secondly we introduce the feed size reduction and optimization to get the required beam patterns taking into account the size limitations imposed by the cryogenical receivers. Finally, we detail the system test after installation in the radio telescope and draw the conclusions. II. OPTICS DESIGN A. General description The 40 meter radio telescope at Yebes Observatory is a classical Nasmyth-Cassegrain system with a main parabolical dish of 15m focal distance and a hyperbolical 3.28m secondary mirror with 26.6m focal distance. Its high inter-foci distance allows the installation of several receivers at its huge receiver cabin [7]. As the system magnification is 7.909, the required feeds to illuminate with a -12 dB taper is as narrow as 3.6º. The high magnification at the focal zone fulfils the paraxial condition allowing quasi-optical approach to be used for the calculation of system beam waist and its position [8]. A -12 dB taper and a perfect phase matching at the secondary mirror edge is used to get the maximum efficiency. We use the common quiasioptic parameters to characterize the beam, w (Gaussian beam waist at -8.7dB) and R (curvature radius of the phase front). At the sub reflector edge we need ws=1395mm and Rs=25396mm. When both conditions are fulfilled, the maximum aperture efficiency is achieved, 0.79 (including the sub reflector blockage) [8]. B. Optical system design A dual band system has been developed working at K band (21.75GHz-24.45GHz) and Q band (41.00GHz-49.00GHz) (Fig.1). A dichroic mirror allows the simultaneous operation in both bands [9]. The K band beam passes through the dichroic mirror with 0.2 dB estimated losses and the Q band is reflected (Fig. 2) [10]. As a secondary goal, the optical system must be also efficient outside both bands to a foreseen extension to higher frequencies. Extra focusing elements are needed to reduce the high magnification of the radio telescope optical