GBT Initial Beam-Steering System

Technical Description

DRAFT 9/15/1999

The beam-steering system consists of reflectors inserted in the optical path between the Gregorian feedhorns and the subreflector, and associated positioners and servo control systems. The purpose of this system is to:

• Provide a "jitter" beam-steering mode whereby the GBT beam may be rapidly steered to an arbitrary point within a 1 arcminute radius of the nominal beam position on the sky.



• Provide "chop" modes whereby the GBT beam is stepped between several programmable points at a rate up to 10 Hz.



• Provide calibration positions, terminating the feed inputs with various calibration loads.

The initial system will be designed primarily for operation at 40-52 GHz and will be installed with the GBT 4-Beam Q-Band Receiver. However, it is desirable that the beam-steering system design be usable from 30 to 116 GHz, realizing that scaling of reflector sizes might be required.

Beam-Steering Operation Modes

Figure 1 provides a graphical description of the operational modes. We will support multiple-feed clusters on the GBT, and in the descriptions that follow, it should be noted that "beam position" refers to the position on the sky of a beam from a feedhorn located at the center of a feed cluster (even though there may not be a physical feed at the center). The beam-steering optics path must support at least a four-feed cluster at Q-band (feeds located at the cardinal points of a 7.54 cm diameter circle) without serious degradation of performance. Beam steering accomplished by the subsystem simultaneously operates on all beams from a feed cluster. Beam positioning repeatibility should be within ±0.7 arcseconds, on the sky.

Figure 1(a) illustrates the Jitter Mode. The beam nominal position is at the system boresight position BP0. The instantaneous beam position may be steered to any point within a 1 arcmin radius of BP0. The purpose of jitter mode is for rapid pointing corrections and/or fast mapping observations. The jitter mode servo system should operate with a 5 Hz minimum bandwidth (10 Hz goal). The form of the jitter input command signal is to be determined.

Figure 1(b) illustrates the Jitter plus Two-Point Chop Mode. In this mode, the nominal beam position alternates between two positions BP0 (SIGNAL) and BP1 (REFERENCE), spaced up to 2 arcminutes apart, in response to an external binary control signal. The minimum phase time (PT) of the control signal is 50 milliseconds, and the beam position transition time (TT) should be 20 milliseconds maximum (10 milliseconds goal). The GBT Sig/Ref chopping control signal can be "advanced" to account for inertia in the beam-steering system, so the transition time should be measured from the time the beam position gets 1.5 arcseconds away from the first position, until it settles to within 1.5 arcseconds of the second position. The user should be able to specify the chop axis direction during observation setup, and the chop axis should track parallactic angle as the GBT tracks sources. Jitter control is also active in this mode, so the instantaneous beam position is offset from the BP0 or BP1 positions in response to a jitter command signal input.

Figure 1(c) illustrates the Jitter plus Three-Point Chop Mode. This mode is similar to the Two-Point Chop mode, except the REFERENCE positions alternate up to 2 arcminutes on either side of the SIGNAL position BP0. Requirements for transition time and positioning of the chop axis are identical.

Calibration Modes

The beam-steering system should be capable of terminating all feed cluster beams with either an ambient load, a cooled load, or a wide-angle (approximately 30) view of the sky. Transition time to one of these positions from the normal subreflector view position should be less than one second.