Field Testing of the Durham/AGI Transmissometer

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Measuring visibility, or atmospheric transmission as it is known, is important for astronomers. In order to perform this type of measurement at their telescope site in Namibia, the Durham University Very High Energy Gamma Ray Astronomy group built an instrument known as a transmissometer. This uses LED and CCD-based technology to perform the measurements, which has advantages over the incandescent light bulbs and light detectors currently employed in instruments like this. In particular, our equipment is easier to align, requires less maintenance and uses less power than older instruments, so that it can be solar-powered, which is particularly useful for remote airstrips. So, we have patented the design. Having upgraded the instrument so that it will work during the day (astronomers need only night-time measurements!), found a cheaper CCD camera to incorporate in the instrument and changed to software so that it can be controlled using Windows, we needed to improve it further so that it can be used in airports and military airfields instead of more traditional instruments. This involves making the instrument more robust and performing extensive field trials side-by-side with one of the transmissometers currently built by our partner company, Aeronautical & General Instruments Ltd. (AGI) in. The work we needed to do was funded by a 2 year PIPSS grant. Early on in the PIPSS grant period, unfortunately for us the requirements for new airport instrumentation changed; no moving parts can be used in new transmissometers. This caused us problems, as the original design used a shutter to mask the LED. We have therefore had to change the system, including all the software, which has taken more time than we originally anticipated, though has resulted in a better instrument all round. However, the extra work meant that we missed the window for field trails starting in October 2008 (field trials have to be conducted in winter). We are therefore asking for a further 6 months funding to enable us to complete the field trials starting in October 2009 and to give AGI some time to learn about the new instrument so they can take over manufacture.