radio astronomy: why the fuss?
During the RFI measurement campaign in the Karoo, the RFI measurement system (which is much less sensitive than a radio telescope) picked up these static discharges, and the nylon chairs were subsequently replaced with canvas equivalents. Nylon chairs weren’t the only culprits. A portable gazebo, used to provide protection for vehicles, consisted of steel pipes strung together by a combination of metal chain and elastic cord that was threaded through the pipes. It was found that the contact of the links of the metal chain with the inside of the pipes and with each other caused RFI, especially when the wind made the frame shake. The gazebo had to be removed from the site.
The 26 m diameter dish at HartRAO.
Image: M Gaylard/HartRAO
So where do we go? It seems RFI is everywhere. Even the Antarctic isn’t immune from RFI from satellites. Then there are airplanes, criss-crossing the sky while radiating merrily. There are very, very few places left on Earth that are radio quiet. One such place is the Karoo. It is the (relative) lack of RFI, more than anything else, that has led to the decision to build a radio telescope array in such a deserted place. Not because there is no light-pollution from nearby cities (only optical telescopes care about light-pollution). Not because it is cheaper to build (due to the lack of roads, power lines and other infrastructure, it will be much more expensive). Not because we love the Karoo (we do, but it is not the reason). No, the reason is the (relative) lack of RFI. Virgo A, also called the Messier 87 galaxy.
Image: Hubble Telescope/Wikimedia Commons
amplifiers are saturated. 2. The receiver may not introduce any inter-modulation distortions. Therefore strong sources outside the bandwidth also need to be regulated. 3. The above two issues require RFI transmissions to be reduced, even if they cannot be removed entirely, i.e. even if they are still detected by
the radio telescope and (hopefully) flagged by the processing software. However, it is also feasible that some RFI transmissions can be avoided altogether, or shielded to levels below the radio telescope sensitivity. This should be the aim wherever possible. 4. The radio astronomy protected bands must remain completely ▲ ▲
Keeping it clean There are many reasons why it is important to devote so much effort to keeping the Karoo site as RFI-clean as possible, some of which are: 1. The receiver has to remain linear, i.e. there may not be any amplifier saturation. A strong RFI source, even if it only occupies a tiny fraction of the spectrum, will corrupt the data across the whole spectrum if the
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