ALMA Scientific Advisory Committee

1
Antenna Procurement

• S.Guilloteau
• ex-AIPS (Alma Interim Project Scientist)
• VAPS (Virtual Alma Project Scientist)

2
Antenna Procurement Problem

• Need to follow the general schedule, including
  expenditures.
• Need to follow the rules, including procurement
  rules for major items.
• Risk of conflict between
• US expenditure schedule (funding profile rises
  steeply, so placing large contracts early helps
  ....)
• and ESO procurement rules (large contracts must
  be through competitive call for tenders)
• Risk of ending with two different antenna designs
  if potential conflicts are not resolved at the
  Board level.

3
Science Implications

• What happens with two different antenna "designs"
• common mode errors dont cancel
• But differences may help (?)
• cost (construction, commissioning, operation)
• other ?

4
Science Implications

• Pointing
• in compact configuration
• WIND Not so true because of wind "shadowing
• SUN Not so true at sunrise
• GRAVITY True, but both designs are essentially
  rigid
• in other configurations
• WIND differs over the site
• SUN GRAVITY remain constant over the site

5
Science Implications

• phase / pathlength / focus
• as pointing, but a more subtle effect.
• Axis non-intersection is likely the dominant
  effect on pathlength (baseline) prediction, and
  has no common mode error
• Other mechanical deformations would benefit from
  identical antennas (e.g. gravity common mode)
• Phase effects due to fiber length
• Fiber run to antenna is dominant in effective
  length change (but if monitored and corrected, no
  common mode)
• polarization matching and primary beam shape
• determined by quadrupod leg design (shadowing of
  quadrupod legs, but exact shape plays a minor
  role too)

6
Summary (?)

• If quadrupod layout is identical, advantage of a
  single design exist, but is rather limited
• ? 32 excellent antennas 32 good antennas is
  better than 64 good antennas
• ? 64 excellent antennas is even better
• Cost
• 2 designs
• 2 software interfaces (different pointing models)
• 2 commissioning
• Cost IS Science
• (better get a few Post-Docs for the same price)

7
non-Identical Antennas