1. ALMA Scientific Advisory Committee Grenoble, Apr 2003 Antenna Procurement S.Guilloteau ex-AIPS (Alma Interim Project Scientist) VAPS (Virtual Alma Project Scientist)

2. ALMA Project ASAC 2-3 April 2003 Antenna Procurement 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. ALMA Project ASAC 2-3 April 2003 Antenna Procurement Science Implications What happens with two different antenna "designs" –common mode errors don’t cancel –But differences may help (?) –cost (construction, commissioning, operation) –other ?

4. ALMA Project ASAC 2-3 April 2003 Antenna Procurement 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. ALMA Project ASAC 2-3 April 2003 Antenna Procurement 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. ALMA Project ASAC 2-3 April 2003 Antenna Procurement 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. ALMA Project ASAC 2-3 April 2003 Antenna Procurement “non-Identical Antennas’’