Optics Design and Verification for the APEX Swedish Heterodyne Facility Instrument (SHeFI)

Paper in proceeding, 2008

We present the design and verification of the receiver optics for the Single-Pixel Heterodyne Facility Instrument (SHFI) of the APEX telescope [1]. The SHFI is designed to cover the frequency band 211 – 1390 GHz in 6 receiver channels. Four of the receiver channels have been designed, installed and characterized: 211-275 GHz (Band 1); 275-370 GHz (Band 2); 385- 500 GHz (Band 3); 1250-1390 GHz (Band T2). The first three bands employ 2SB SIS mixer technology and Band T2 employs HEB mixers in a waveguide balanced mixer configuration. The entire optics design was driven by the receiver position in the telescope Nasmyth cabin “A” (Fig.1) and the aperture limit of Ø150 mm, introduced by the elevation encoder inside the Nasmyth tube A. This layout and the telescope geometry (~ 6 m distance from the focal plane to the Cabin A) lead us to choose a single-pixel configuration and required using intermediate optics with long focal distances. The common optics path, coupling the receivers to the Cassegrain sub-reflector, consists of the three offset ellipsoidal mirrors, M3, M6, M8s, and three flat mirrors, F4, F5, and F7s. The combination M3 and M6 via flat F4, F5, creates a Gaussian telescope, providing frequency-independent re-imaging of the antenna focal plane from the Cassegrain cabin into the Nasmyth cabin A. Switching between channels is achieved by the precision rotating of the active mirror M8s. The mirror M8s in combination with each channel active mirror M10 provides re-imaging of the secondary onto the feed horn aperture of the selected channel. Such a configuration provides frequency independent illumination of the secondary with the edge taper -12dB. The angular position of the flat mirrors F9 is adjustable and gives additional possibility of fine-tuning of the beam alignment from the common optics to every receiver channels. Verification of the optical design through measurements is essential in order to align the beams from the cryostat windows to the common optics to minimize loss in the quasioptical guiding system. In order to verify the design of the cold optics (corrugated horn + M10) in terms of Gaussian beam parameters, a new wideband vector field measurement system was developed [2]. Vector field measurements were performed for band 1, 2, and 3, and scalar measurements were employed for the THz band.