The Dish Is Not Enough

I just want to quickly point out something about China's awesome new radio telescope - it's not as big as you might think. Yes, the dish is enormous, and yes, it's much bigger than the now-previous record holder. But sheer dish size isn't everything. While most normal reflecting dish telescopes like the GBT, Jodrell Bank, Parkes, etc. can use all of their collecting area to detect radio waves, this isn't the case for the giants.

Arecibo rarely uses all of the collecting area of its 305m reflector. That's only possible when it's pointing directly overhead, since the dish is far too big to move. Instead it only collects radio signals from an area roughly equivalent to a circle 225m in diameter. The advantage to this is that the instruments above the dish can be moved, allowing it to point anywhere in an approximately 40 degree swathe of the sky. Which, overall, is much more useful than a slightly bigger telescope that can only observe whatever happens to be directly overhead.

Arecibo's main reflector is spherical, so it doesn't focus the radio waves to a point. This is corrected using other, smaller dishes. FAST has a different strategy - the reflector is normally spherical, but can be pulled into the shape of a parabola (which directly focuses light to a point). This can be done for an area equivalent to about 300m across. The method is different but the principle is the same : it sacrifices collecting area for sky coverage. So although in terms of sensitivity and resolution it isn't really that much bigger than Arecibo after all, it will be able to survey an area ~80 degrees across, twice that available to Arecibo.

FAST will be a fantastic instrument when and if it comes to maturity. Giant radio telescopes are not things you just build, flick a switch and wait for the science to come pouring out. Size of the dish is just one limiting factor - it takes a very long time to reach that limit, because developing the instrumentation to do so is an uber-specialised task, never mind the fact that the construction of the dish determines what frequencies you can observe. Essentially each telescope is its own prototype.

For the 2012 portfolio review the NSF stupidly decided to "divest", i.e. stop, funding the GBT, on the grounds that similar capabilities are offered by the Effelsberg 100m telescope. It's not really a fair comparison; the GBT has a far more sophisticated design. Among other things, the receivers are mounted in such a way that they don't block the incoming radio waves at all. This makes for very clean data with no ugly artifacts that can make the images hard to interpret.

Similarly, Arecibo's frequency range is more three times greater than FASTs. Plausibly that could be extended considerably further, whereas it's not yet clear if FAST's dish-deforming method will work for the planned frequency range, let alone extending it (this gets more difficult at higher frequencies since the precision of the deformation must be greater). Arecibo has the world's most powerful radar transmitter for asteroid observations (one of only two such facilities in the world) - no transmitter of any kind is planned for FAST, and it studies atmospheric physics to boot.

None of this should detract from FAST. It's just that anyone thinking that Arecibo is about to become obsolete is woefully mistaken.
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