Multi-Beam Optics

STEAM-R

STEAM-R (Stratosphere-Troposphere Exchange And climate Monitor Radiometer) mission is currently being developed as part of the next ESA Earth Explorer Mission, PREMIER, as the first submillimeter multi-beam limb-sounder for atmospheric research.

The prime contractor for STEAM-R is Omnisys Instruments and the focal plane array optics has been designed by Axel Murk, Mark Whale, Matthias Renker and their colleagues at the Institute of Applied Physics (IAP) at the University of Bern under a research and development sub-contract as part of the overall project activities.  It consists of two multi-faceted mirrors: one with 7 parabolic facets and one with 7 hyperbolic facets. The first of these mirrors has 7 cylindrical apertures to accommodate Ultra-Gaussian feed horns. The monolithic design with multi-faceted mirrors was chosen to minimize alignment errors between the beams.  In order to achieve the desired beam spacing on the sky, the beams must be closely packed in the array.  This leads to a high level of truncation at the facet boundaries (up to -17dB, as compared with  typical levels of -35 dB), and therefore the need for the machining of very sharp interfaces between the facets.

Thomas Keating, under contract from Almatech has recently manufactured a breadboard model of the array optics. This included these two multifaceted mirrors, feed horns, a polarizing grid and the support structure as shown in the photograph below.

The optics was checked with extensive CMM (co-ordinate measuring machine) measurements, for example the feed horn apertures checked in the report shown below:

For the mirror shown above, up to 1200 points were measured on each facet.  The surface accuracy is given in the table below, where a common offset has been fitted to all 7 facets. This illustrates the success of the monolithic construction in aligning the individual facets.

In the following photograph, the alignment of the whole system was being checked on the CMM:

Beam Patterns

The performance of this breadboard model has now been measured at the IAP, Bern, and gives an impressive match between the GRASP-predicted results and the measurement.

The 2D near field beam pattern, at 340 GHz, is given in the plots below.

Comparison with Simulation

The performance of the STEAM-R focal plane optics has been simulated with GRASP using the Method of Moments. The beam pattern shown below is the result of this simulation at 340 GHz, and the following contour plots overlay the measured and predicted beam patterns.

 323 GHz

340 GHz

357 GHz

Complete Radiometer Calibration Load System

A complete radiometer calibration load system is now available, consisting of copies of the calibration loads we have manufactured for ALMA with a copy of the ALMA load controller electronics.

The calibration loads have a convenient compact envelope as a result of having been designed to fit into a constrained volume in the ALMA telescopes.  The need to fit a high performance conical load design into a limited length lead to the development of the current “folded cone” structure.  This features a central absorber cone, surrounded by a conical reflector. The space between this reflector and a secondary absorber forms a second absorbing cavity.

Complete system.

The ambient calibration load is designed to cover all ALMA bands from 31 to 950GHz.  It averages -55dB of backscatter (S11) for the lowest band, and is mostly below -60dB above 95GHz, as shown in the graph below.  The hot calibration load is designed for use from ALMA band 3 (95GHz) and above where it too is mostly below -60dB in S11. The loads have been extensively tested — both electromagnetically and thermally — more details are given in the manual and the references therein.

S11 performance of a production hot calibration load (HCL) and ambient calibration load (ACL). Spikes at 120 and 140 GHz are test artefacts due to reduced VNA sensitivity at these frequencies.

The Calibration Loads

The ambient calibration load has a long main absorber cone with a wide aperture in order to accommodate as much of the beam as possible, as was required to for the lowest frequency bands.  It is equipped with 3 PRT100 temperature sensors.

Ambient calibration load.

The hot calibration load features a main reflector which folds back over the aperture of the main absorber cone to provide additional thermal isolation.  It is equipped with 5 PRT100 read-out temperature sensors and a further 3 PRT100 sensors used for thermal control.

Hot calibration load.

The Controller

The load controller includes thermal control and temperature read-out electronics, and features a CAN interface.  A USB-to-CAN adaptor is provided with the system to allow it to be conveniently connected to a PC running the console application.

The controller operates three independent heater circuits, which can be commanded to temperatures up to 90°C, and a further constantly-on heater circuit warms the shroud around the aperture of the hot calibration load.

It reports the temperature from the separate read-out sensors in the hot load, and from sensors in the ambient load.  There are an additional four temperature channels which would be used at the ALMA telescopes to monitor the solar filter and the ambient temperature.

Calibration loads controller.

A LabVIEW-based console application is provided to allow commands to be sent to the firmware running in the load controller.  It displays temperature and status information, and also includes a function for logging the data to a file.

Console application.

Downloads

version 4.1.0	Installation and Operating Manual for the current software version.
version 4.0.0	Console application.

Acknowledgement

The product contains ALMA load technology, developed by ESO. Thomas Keating Ltd is a icensee of the ESO ALMA load technology developed and transferred by ESO (http://www.eso.org/).  Technology partially protected by patent application, ESO, 11 007 146.1.

The terms of Thomas Keating Ltd’s license from ESO precludes use in any military application. We therefore expect customers to comply with this limitation and not to sell, transfer or otherwise commercialize the product to any entity under United Nations or European Union embargo.

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Quasi-Optical Network for MetOP-SG  MWS: Designed, built and tested by TK

Thomas Keating and QMC Instruments Ltd have combined their skills to provide Dr Kawahata of NIFS with two
three channel,  multi-polarization multiplexers for use in a diagnostic instrument on the Large Helical Device
in Nagoya. These combine co-polarized waves over three waveguide bands 10-26.5 GHz; 26-40 GHz and
40-60 GHz, though the use of  in-waveguide low pass filters. Such filters are more associated with free-wave
operation and this is a novel application of this technology.

The solid construction multi-layer filters are shown below

and these are inserted into the precision machined waveguide holders designed and made by TK.

The results are really very good -  given the constrained waveguide and high incidence angle of the filter's operation, recalling that the best a neutral density based system could do would be to give  -3dB (high band), -6 dB (middle band) and -6 dB (lower band) of loss. This would be worse if you used conventional -3dB waveguide couplers, because part of the guide would be operating way out of band. The performance is shown below:

Our results give losses of  -1 dB, around -2 to -3 dB and -1 dB  in the Eplane (E field in plane of bend) slightly
worse in the H plane, in those respective filter bands. The benefit to NIFS in this lower loss is doubled, given
that both transmit and receive multiplexers are required