[mmaimcal]comments on Y+ vs. ring configuration notes from stephane

[John Conway]

Hi,

 On the issue of astrometric accuracy for ring and Y+ arrays 
 I include a postscript file as an attachment where
 I discuss the issue of resolution and astrometric accuracy
 in a simple way (hopefully nit too simple).

 It seems to me making the simplest assumptions then  
 both the astrometric accuracy and FWHM both depend 
 on the mean square basline length - hence 
 two arrays with the same FWHM are likey to have 
 very similar astrometric accuracy.

  A number of assumptions are made in the above 
  argument (see attachment), specifically that 
  the phase noise (or for low SNR when fitting small position 
  differences between actual and correlated position, the
  imaginary part) has a noise which is independant 
  of baseline length. If at reasonable SNR the 
  atmosphere makes a signifcant contribution cf 
  additive noise then this may have to be modified
  slightly, but for the long baselines we are talking 
  about here the atmospheric structure function is fairly 
  flat with baseline length, so I don't see it will have a 
  big effect.

  There may be slight differences in astrometric 
  performance betwen rings and Y+ for the above 
  reasons - but I don't see any first order differences
  due to differences in the general baseline length 
  distribution as implied by the argument below.

     John 



On Thu, 6 Jun 2002, Stephane Guilloteau wrote:

> > > B) Better astrometric performance
> > >
>     I had  a subtle argument in mind when saying so. That argument may be
> wrong. Here it is:
> The advantage of the ring array for astrometry is not resolution. It is he
> accuracy of the delay determination.
> This accuracy is better than for an equivalent (in terms of angular
> resolution) Y+ array, because in the ring
> any antenna has essentially the same number of short baselines than of the
> longest baselines. The antenna
> phase being determined using all baselines, this may give a small advantage
> to the ring...
> 
>     Quantitatively, it may be peanuts...
> 
>     Any comment ?
> 
>     Stephane
> 
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