[asac] Propositions for Japan participation to an enhanced ALMA project.

[Al Wootten]

  Propositions for Japan participation to an enhanced ALMA project.
  
            Draft by S.Guilloteau
  
  From the December 4, 1999 meeting of the ALMA Liaison Group  in Grenoble.
  Revised after the February 16, 2000 meeting of the ALG in Mitaka. 
  
  This document present a number of possible contribution of Japan to an enhanced ALMA
  project. The enhancements resulting from the various proposition are quite varied :
  adding new capabilities (e.g. supra-THz capability),  shortening the project duration,
  increasing the sensitivity, simplifying the maintenance or reducing global cost in some
  areas. Each proposed contribution is presented in 4 steps : a description, the expected
  improvement on ALMA, the method proposed to value the contribution, and a short term 
  consequence on Phase I activities.
  
  In addition to the specific items mentionned below, which result in visible improvements,
  it is of course also expected that Japan shares the basic infrastructure and running costs of
  ALMA.
  
     1)     An addition of a number of 12-m antennas.
  
     Description :
     Involvement of Japanese industry is essential for Japan participation in the project.
     Japanese industry could build up to 1/3rd of the total number of 12-m antennas. These
     antennas would be built to the same or better specifications than the US-European
     antennas, with a possibly different design, but plug-in compatible in the same
     stations. 
     
     ALMA benefit :
     Antennas of similar performances would increase the ALMA sensitivity (or speed).
     Antennas of better performances would significantly increase the highest frequency
     capabilities of ALMA.
     
     ALMA value :
     Unless antennas performance are significantly increased, it is proposed to evaluate
     this contribution on an equal value for each antenna, unrelated to its origin.
     
     Phase I implication :
     Coordinate antenna specifications and interfaces, including foundation specifications.
     
     2)     An addition of a number of smaller, high accuracy, antennas in a compact array.
  
     Description :
     Japan could built a compact array of small, but high surface accuracy, antennas.
     This array could take the form of e.g. a compact hexagon with 7 antennas of 6 to 8 m
     in diameter. The outer antennas could be moveable on rails to allow fast
     reconfiguration in order to tailor the shape of the array to the source declination, to
     avoid shadowing effects.  The antenna mount and receiver cabin could be identical to
     those of the 12-m antennas, allowing them to move on larger configurations also, and
     to have the highest compatibility for receiver interfaces. Expected antenna surface
     accuracy is of the order of 15 microns rms or better.
     
     ALMA benefit :
     Such a compact array would enhanced ALMA capabilities for short spacing
     measurements, specially at the highest frequencies where the 12-m antenna
     performances and the atmospheric properties make the problem most difficult.  It
     would also allow to operate the 12-m antennas with an under-illumination pattern at
     the highest frequencies to select a better compromise field-of-view  and pointing
     performance versus sensitivity. The reconfigurable option would open the possibility
     for ALMA to explore the highest frequencies, perhaps even above 1 THz, with
     appropriate field of view and angular resolution. 
     
     ALMA value :
     Assuming the same complement of receivers as the 12-m antennas, we would give
     each small antenna the same value as a 12-m antenna. The increased complexity of
     the proposed ® quick ¯ repositioning system compensates the savings expected from
     the smaller dish diameter.
     
     Phase I implication :
     Watch out mount and foundation design to allow close packing of 6 to 8 m antennas.
     
     3)     A participation to the junction effort
  
     Description :
     A new facility for SIS junction production is being developed in Mitaka. This facility
     could be used for ALMA junction production. Japan is also developing innovative
     technologies for junctions (e.g. the distributed junction scheme from Dr. Noguchi)
     
     ALMA benefit :
     This contribution could alleviate a possible bottleneck in the ALMA project
     
     ALMA value :
     This contribution would be evaluated on the basis of US-EU agreement for junction
     production value.
     
     Phase I impact :
     Developments should be integrated in ALMA Phase I, to be incorporated in receiver
     design and production plans.
  
     4)     Fabrication of one/several receiver frequency channels
  
     Description :
     Japan could provide e.g. the 490 GHz receivers for ALMA (or other bands).
     
     ALMA benefit :
     This could speed up the completion of the ALMA project, and avoid excessive retrofit
     actions to the receiver packages.
     
     ALMA value :
     This contribution would be evaluated on the basis of US-EU agreement for receiver
     band value.
     
     Phase I impact :
     Japan receiver experts should become involved in the Joint Receiver Design activity
     if they are going to be suppliers of  some frequency modules. 
  
     5)     Cryogenics
  
     Description :
     Japan is probably the biggest supplier of cryocoolers, with proven reliability. 
     
     ALMA benefit
     Basic contribution
     
     ALMA value :
     Market prices
     
     Phase I impact :
     Implementation of a real scale test at 5000 m is very valuable.
     
     6)     Photonics
  
     Description :
     NTT is actively developping high frequency photomixers, which may be suitable for
     the full photonic LO system for ALMA
     
     ALMA benefit :
     Photonic LO system is simpler than the photonic reference approach.
     
     ALMA value :
     Value based on estimated cost of the photonic reference + multiplier solution and
     purely photonic approach, whichever is the most expensive. The gain in simplicity
     justifies extra cost (if any).
     
     Phase I impact :
     Evaluation of photodetectors needs to be performed actively. Good liaison between
     the Tucson group and NAOJ is important here.
  
  
     7)     Correlator
  
     Description :
     Japan is developping a wideband, 128 000 channels FX correlator. The goal is
     ultimately to cover the full 2 GHz bandwidth (perhaps even 4 GHz) with this number
     of channels. This removes the extra complexity of input filtering (analog or digital).
     
     ALMA benefit :
     Line surveys could be carried out more effectively. Serendipitous discoveries are to
     be expected (e.g. molecular masers in stars). Multiple-line observation and continuum
     subtraction from narrow lines could be executed more precisely and effectively.
     Heavy molecules could be detected by pattern-matching integration of the line forests.
     
     ALMA value :
     The value should be based on the observing time savings that such a correlator will
     provide for the astronomy projects requiring narrow lines observations. The induced
     computing cost resulting from the huge number of channels should be evaluated and
     accounted accordingly. The effective sensitivity should also be asserted. 
     
     Phase I impact :
     Invite Japanese expert to correlator PDR in January. Develop scientific evaluation
     based on typical observing scenarios.
  
     8)     More digital bandwidth 
  
     Description :
     Japan could provide enhanced transmission bandwidth from antenna to central
     building, and from central building to Operation Support Facility in San Pedro.
     
     ALMA benefit :
     Current nominal bandwidth of standard fibers does not allow 3-bit samples to be
     transferred on single fiber from the antenna. This would become possible. Ultra high
     speed link with OSF would allow to relocate the correlator and all its associated
     computing equipment in San Pedro.
     
     ALMA value :
     Based on cost.
     
     Phase I impact :
     Maintain contacts for information.
  
  
  
     9)     Other proposals mentionned
  
     A few other options were discussed, among which a large single-dish antenna, a
     super-computer for enhanced data mining capabilities, etc  These were felt
     impracticable, or to far from the scope of an ® Enhanced ALMA ¯ project.