When you think about radio telescopes, most people’s perception is a dish type antenna.
What you may think, when you think of a “dish” is possibly ones for satellite television, or possibly the large dishes from something like the movie “Contact”. The SKA’s dishes, one of the three types of receivers that will be used (the other two being low-frequency aperture array dipole antennas and the mid-frequency aperture array antennas) will be built and deployed on a scale never before seen, using new and ground breaking techniques.
Existing radio telescopes
The largest single dish telescope in the world is the Arecibo radio telescope, which is only partially steerable, located in a natural bowl in Puerto Rico. At 305 metres across it has been providing invaluable scientific research since the early 1960s, and as a pathfinder for the SKA, is still at the cutting edge of radio astronomy.
The SKA telescope, using multiple 15 metre wide dishes as an interferometer will exceed the single dish sensitivity and capabilities of telescopes such as Arecibo, even though individually they are much smaller,
The combination of several thousand of these “smaller” 15 metre wide dishes will form a substantial part of the SKA, giving the telescope its high frequency capability. The high frequency dishes will be fully steerable too, unlike the aperture arrays, which rely on computer technology to “steer” them towards targets. This means they have to be built to very exacting engineering standards.
Many aspects of the SKA dish-design challenge are without precedent, not only because of the large numbers of dishes required, and the engineering accuracy that entails, But also because of the huge sensitivity and the amounts of data that will result from this vast collecting area. Current radio telescope arrays like the VLBA in New Mexico use telescope dotted all over the United States for example, but the amount of data these generate will be dwarfed by the SKA.
The dishes for the SKA will be made from carbon fibre composites, with an accuracy of their figure (the shape) unrivalled in radio astronomy. Capable of withstanding high winds, and all sorts of intense thermal and environmental stresses, the SKA’s dishes will be unique in the world of radio astronomy.
Although sensitivity is a strong driver of the SKA system design, there is also a need to carry out surveys of large regions of the sky, something which the SKA will be able to conduct thousands of times faster than any previous radio telescope.
A small number of large dishes could provide high sensitivity but many smaller dishes with larger field of view also offer the prospect of faster surveys of the sky.
This trade-off has resulted in the adoption of dishes with a diameter of 15 metres to give both a good field of view whilst also allowing high sensitivity at a suitable cost to enable the SKA to come in within its target budget.
Considerations for dish design
In developing several thousand dish telescopes, multiple design, science and development factors have to be taken in to consideration. The following list is just some of the major requirements that are being addressed as part of the dish design process:
- Imaging dynamic range – The signal range in terms of intensity over which the SKA will be able to observe. It also means that the dishes have to have extremely stable pointing and accuracy in harsh desert environments.
- Design for mass manufacture – Making the most efficient and technically outstanding design choices, whilst keeping to a budget that comes within that set out by the SKA Organisation
- Low operating cost per dish – Once built, how to maintain them and operate the telescopes on the most cost effective budget. The reliability of the dishes will be a critical factor here
- Rapid installation with minimum manpower and equipment – The scale of the task at hand means that installation of each dish, when they will number in the thousands has to be efficient.
- Feed flexibility – Ensuring maximum flexibility from both single pixel and phased array designs
- Maximum sensitivity per dish – Ensuring each dish delivers the maximum sensitivity and that this is consistent between all of the dishes in the array
No existing radio telescope design has matched these requirements in combination, and hence the SKA will be attempting to address technical challenges which will make it unique in the field of radio astronomy, under some of the most inhospitable locations on our planet.