April’s talk

“ALMA”

Neil Philipson is one of those useful people to know, as he clearly has a passion for his topic.  He called himself a designer and telescope seller.

His pet subject is the ALMA setup in Chile, namely the Atacama Large Millimetre Array.

It was completed in 2013 and consists of 66 12metre radio dishes, and these are through a collaboration of Europe, USA and Japan/East Asia.  Europe and the USA provided 25 each, then Japan tacked on another 16 later; these are a grouping of four 12m dishes and 12 seven metre dishes and are called the ‘Compact Array’.  So this is now all called ‘Enhanced ALMA’

He calls it ‘supercomputer astronomy’ and says it’s the largest collection in history.

So why choose microwave collectors?  After all the wavelengths are a thousand times longer than the visual stuff, so the collectors have to be huge, in theory, to get the same resolution as our optical telescopes.  These millimetre and sub millimetre wavelengths radiate from stuff that is close to absolute zero.  This is the oldest stuff in the universe.  You can also include early stage galaxy formation and nebulae which are in the early stages of forming stars.  They radiate in these wavelengths.

Mr Philipson then came onto location.  Our atmosphere blocks most of the radiation at this wavelength so why wasn’t ALMA chucked into space?  (His words). Basically because of the expense, and at 5000m altitude, you can get above most of Earth’s atmosphere and you still get the resolution.

Extravagant suggestions were made for the original location.  After all, you had to have stable, very dry air, and it had to be cool and far away from people.

So, Greenland, the Himalayas and even the South Pole were among the original choices.

The Llano de Chajnantor was finally chosen, 16,500 feet up.  For its trouble, Chile gets 10% observing time.  (Japan gets 15% and Europe and the USA share the rest equally.)

So these 66 dishes have a fairly free rein up there, although they are posted in clumps and have parking pods too.  They are linked by fibre optics to an accuracy of a millionth of a millionth of a second.  (In the same way that we have the very long baseline interferometry of the longer wavelength radio telescopes such as the MERLIN or VLA complexes)  At their biggest separation they have a 16km diameter but can be gathered into a 250m diameter.  At their biggest separation they have a resolution down to 0.004” (arc seconds), which is ten times that of Hubble.  In compact formation, the field of view is 50”, at expanded, it is 1.25″

The dishes are figured to one fortieth of a wavelength of what they’re looking at.  The collecting receivers are in a hole in the middle of each dish and they have to be cooled to 4 kelvin, because the stuff they’re observing is so cold and you don’t want the receivers radiating heat.

Mr Philipson lamented that the achievements of ALMA are not publicised much, but ALMA has been able to see beyond  a very distant galaxy cluster to a time only 200 million years after the Big Bang.  He mentioned the ’Sunyaev-Zel’dovich effect’  which is seen in early galaxies interacting gravitationally, producing hot gas and distorting the cosmic microwave background.  (The CMB is the radiation from the Big Bang.)

Most intriguing is its observations of Betelgeuse, whose diameter has shrunk since it was possible for us to resolve the disc.  (It is about a half arc second across.)  It also has a plume of gas bigger than our Solar System, and the fact that is shrinking so quickly indicates that it is going supernova in the next 100,000 years, if it hasn’t done so already.

If anyone comes across any interesting finds, let us all know, especially those where Mr Philipson says multiple rotations of the Earth were needed to get enough information because of the faintness.  Also, decent pictures of the Array seem lacking.

Clear Skies.

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