[mmaimcal] Re: [alma-site] location of terahertz array

[John Richer]

Hi Simon,

The size and location of compact array for ALMA, or indeed if it
should be built at all, are all important and interesting issues.  In
the small number of meetings where I've heard this discussed, I've
never seen a clear statement of the scientific goals of the ultra
compact array.  I'm assuming the argument goes like this

  1) we need an array of small dishes because we are nervous about
     'homogeneous mosaicing' producing complete spatial frequency
     coverage, particularly at high submm frequencies and in the
     continuum.

  2) So we build a small array of dishes to deal with this problem and
     use ALMA receivers to make it relatively cheap. 

  3) But the marginal cost of specifying this array for THz operations
     is small, so let's do it; and similarly the marginal cost of the
     high mountain site is small.

There is a logic to all this, but the key issues are obviously

- do we believe we need this array for the short spacing information?
  I haven't seen the simulations yet to show this one way or another.
  Of course, everyone has a different view on how effective
  'homogeneous' mosaicing will be, but I think at low frequencies and
  for narrow band signals it should be very good;  arguably, this will
  cover most science goals which are probably line observations of
  extended gas clouds.

- What is the science increment by going to >1THz at very high angular
  resolution?  I know there are some lines out there, but what
  specifically drives the need for super high resolution when so
  little is known about these lines anyway?  

I think this all needs looking at in the context of this wider debate,
but they are fascinating issues to chew over.   Can anyone point me to
the science cases for >1THz, or tell me about simulations involving
the compact array?

Cheers,

John

--
John Richer 
Astrophysics Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE
Tel: +44-1223-337246  Fax: +44-1223-354599  http://www.mrao.cam.ac.uk/~jsr/