The HARP Project

Organization
The HARP B project is lead by MRAO with contributions from HIA Victoria, the UK ATC and JAC. The SIS devices are developed and produced by DIMES in Delft. The areas of responsibilities are:

Specifications

• Average pixel performance meeting or exceeding the performance of RxB3 at the JCMT.
• Automatic tuning in less than 60 seconds.
• Calibration accurate to 10% with inter-pixel calibration accurate to 5%. Hence, single sideband operations as well as stable and uniform calibration loads.

Design
The design philosophy behind HARP-B is that each pixel should performance as well as a state of the art receiver. According to the HARP-B design specifications is should be as good as B3 or better when the performance is averaged over the pixels. The alternative having many more but less sensitive pixels was ruled out before a proposal was put to the JCMT board. A less sensitive array will have high mapping speed, for extended sources, if large enough. However, the backend would be prohibitive expensive. Further, a larger less sensitive array would not perform well on small sources. Sources less extended than the array are of great importance.

In order to ensure state of the art performance a lot of effort has gone into design the imaging array as well as the cold and warm optics. Another effect of the philosophy is the K-mirror, which ensures no degradation occurs due to gridding effects or that empty space not is observed unnecessary.

The imaging array is based on Stafford Withington's mixer design using a radial wave-guide probe and a horn/elliptical mirror as beam launcher. The LO is injected through a meander line scheme with low losses. The meander line LO injection scheme uses thin Mylar sheets to reflect in a small portion from a LO beam meandering across the array. The SSB filter dumps the unwanted sideband to a very low temperature and well-defined cold stops ensures that the imaging array not sees much stray radiation from warm telescope parts.

The array is automatically tuned with a tuning time not exceeding 60 seconds according to the specifications. To reduce the cabling between the electronics and the different subsystems the serial CAN bus is used. The CAN bus is used is auto industry for control and is robust against interference.