[mmaimcal] Text of AI on antenna quardupod design from Beasley

[Ed Fomalont]

it may be a show-stopper for differential astrometry.  some of us here 
at the AOC have been talking a bit about that.  VLB is also potentially 
impacted (harder to phase up the array), but VLB is out anyway with no 
maser...

Hi Bryan,

     I don't think that there will be much impact on differential 
astrometry with an array with two telescope types.  The sources are 
generally near the center of the primary beam and the differential 
geometry between the mounts can be calibrated.   Although the EVN, with 
a multitude of telescope types, does not do as well astrometrically as 
the VLBA, the difference is probably not related to the different 
telescopes.   However, there could be small differential effects which 
are peculiar to each telescope design which could be difficult to 
remove.  But, the differential tropospheric phase scatter is likely to 
dominate the positional error.

     And, in my discussions with the ACA group, I mentioned that ALMA 
was not planning VLB support .  Since the ACA array is relatively small, 
it can be more easily phased-up and they are definitely considering VLB 
support, ala a maser.

Ed


Ed