A Survey of Selected Radio Telescope Receiver Types Dana Whitlow Microwave Receiver Specialist, Arecibo Observatory Denis Urbain

In this talk we will consider several types of receivers: Single feed Focal plane arrays > Traditional (Arecibo ALFA, Parkes MB20) > Phased array (AO-40 (upcoming at Arecibo) > Incoherent detector array (USRA SOFIA)

Definitions COHERENT RECEIVER: one in which phase information is preserved in the signal chain, regardless of whether or not it is actually used. If your receiver has an RF amplifier, mixer, etc, it’s a coherent receiver. Coherent receivers are subject to a fundamental quantum sensitivity limit: Tnoise >= ~hn/k INCOHERENT RECEIVER: one in which the first element in the chain is a direct (power) detector, in which case phase information is destroyed. If the first active element is a heat detector warmed by signal radiation, you have an incoherent receiver. The quantum sensitivity limit is said not to apply.

Single-beam versus Multi-beam Single beam (single pixel) operation seems like a waste of a perfectly good (well, almost) optical system. Multiple beams permit considerably faster survey work, but having them is definitely an extra-cost option.

G. Cortes-Medellin, K.F. Warnick, B. D. Jeffs, G. Rajagopalan, P. Perillat, M. Elmer, D. Carter, V. Asthana, T. Webb, A. Vishwas. “Field of View Characterization of Arecibo Radio Telescope with a Phased Array Feed”. IEEE Antennas and Prop Symposium, Spokane, WA, Jul 2011

TRADITIONAL FOCAL PLANE ARRAY Receivers are independent, with no phase connection. Therefore each feed must take individual responsibility for matching its footprint to the main reflector, setting a minimum size requirement. Feeds of this size cannot adequately sample the spatial electromagnetic field configuration at the focus to correct for off-axis aberrations.

Example of Off-axis Aberration

ALFA 7-ELEMENT CLOSE-PACKED FEED HORN ARRAY

FOCAL PLANE PHASED ARRAY Here the array comprises a grid of antenna elements spaced by slightly less than l/2, thereby meeting the Nyquist criterion for full spatial sampling of the electric field configuration over the focal plane. The elements are often implemented as shortened “half-wave” dipoles. The outputs of the elements are vector summed with complex element- and beam-dependent weighting to produce the desired beam(s) on the sky. Assuming that sufficient processing capability is available, simultaneous production of many beams is possible. Beams can be well corrected for off-axis aberrations and small focus errors. Within limits, pattern notches can be formed to mitigate RFI. But there’s a catch: electrical interactions and noise coupling between the closely-packed elements complicate the design and may degrade noise performance.

BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY

BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY

INCOHERENT DETECTOR ARRAYS Incredibly, heat detectors (such as bolometers and arrays thereof) can be made sensitive enough to be very useful for astronomy. Greatest usefulness is in the mm-wave, sub-mm-wave, and mid-to-far IR regimes where fundamental quantum behavior places severe limits on the performance of coherent receivers. A variety of useful detection mechanisms are known and used; all require cooling to sub-one-degree-Kelvin temperatures to work.

SOME ADVANTAGES OF INCOHERENT DETECTION Extends upper frequency limits of high-sensitivity radio astronomy beyond the theoretical and practical limits of “conventional” (coherent) radio telescope receivers. Uncouples the strict connection between beamwidth and effective aperture area that is characteristic of coherent receivers. This can sometimes be exploited to obtain a sensitivity advantage if diffraction-limited angular resolution is not required. Very wide pre-detection bandwidths are available, which helps sensitivity.

SOME ISSUES WITH INCOHERENT DETECTION No phase information is available from the detectors; thus neither off-axis aberration correction nor participation in interferometry is possible. Spectroscopy is considered impractical because nothing can be done post-detection, and versatile or tight pre-detection filtering is too hard to implement. But one could imagine some quasi-optical techniques with diffraction gratings, I suppose. (Good luck with the cooling!) Extraordinary care is required in the design and implementation of the sensor (array) to keep out stray radiation everywhere in the electromagnetic spectrum, since the inherent bandwidth of a thermal sensor is essentially infinite. Accomplishing this adequately can be much more challenging than it looks at first glance. Cryogenic cooling is a challenge, especially in large arrays.