EVLA Project Status

M. M. McKinnon and the EVLA Project Team

Antennas continue to be converted to the Expanded Very Large Array (EVLA) design. A total of 20 antennas are now used for scientific observations. EVLA antennas accounted for 71.4% of all antenna hours used in scientific observations in February 2009. The mechanical overhaul of the 21st antenna is well underway.

Test observations with a prototype WIDAR correlator were made in the last quarter of 2008 in support of a production review of the correlator’s major circuit boards. Since then, a subset of the final WIDAR correlator (called WIDAR-0) has been populated with a limited number of circuit boards. The first on-the-sky fringe visibilities with WIDAR-0 were recorded on March 6, 2009.

A plan for the transition from the VLA correlator to the WIDAR correlator has been developed in support of the announcement for early science opportunities with the EVLA (see the February 2009 NRAO eNews article). The transition will occur in the first quarter of 2010. Two WIDAR configurations will be supported for general use, suitable for continuum and spectral line applications, with bandwidth and frequency resolution better than those obtainable with the VLA correlator. The number of antennas that can be used with WIDAR is set by the number of station boards. The station board provides delay tracking and digital filtering of the signals from the antennas. Since 25 EVLA antennas are expected to be available by January 2010, the correlator transition plan places a priority on station board delivery.

A problem with the power supplies on the WIDAR station boards was discovered in late 2008. The problem has been resolved, and the production order for the station boards was placed on January 23, 2009. The order represents a significant achievement for our Canadian colleagues at the National Research Council. The delivery of the boards is consistent with the correlator transition plan described above.

The WIDAR’s baseline boards contain the correlator chips where the correlations are computed and ultimately dictate the bandwidth that WIDAR can process. Testing of the baseline boards continues, and their production order will be placed in the next few months. All of the baseline boards are scheduled for delivery in advance of the time they are needed per the correlator transition plan.

A total of six, fully EVLA-compliant C-band (4-8 GHz) receivers are now installed in the array. The production order for the receiver’s wideband orthomode transducer (OMT) was received in February 2009. The interim C-band receivers installed in EVLA antennas are basically EVLA receivers that use VLA-style OMTs. The process of converting these to fully EVLA-compliant receivers is a matter of replacing the VLA-style OMT with the EVLA OMT. We expect the OMT replacement process to proceed quickly, giving as many as 17 antennas with EVLA-compliant C-band receivers by the end of 2009.

Eight new Ka-band (26-40 GHz) receivers have been installed in the array. These receivers provide a completely new scientific capability for the (E)VLA. A call for Ka-band proposals for the C-configuration was issued January 15, 2009 (see News for VLA/VLBA/HSA Proposers, January 15, 2009), along with a call for exploratory proposals for the current B-configuration. The EVLA’s new Observation Preparation Tool (OPT) must be used to configure Ka-band observations since current VLA-specific software (i.e. observe and Jobserve) cannot accommodate the new band.

The first, fully EVLA-compliant S-band (2-4 GHz) receiver was installed in an EVLA antenna in January 2009. Preliminary observations and laboratory tests indicate that the receiver is performing to specification. The production order for the S-band OMT was placed. As with Ka-band, the S-band receiver provides a completely new scientific capability for the (E)VLA. S-band observations with a limited number of antennas may be possible in late 2009, assuming production proceeds on schedule.

Excellent progress continues to be made with the fabrication of the feed horns for the EVLA receivers. The fabrication of the horns for the L, C, and Ka-band receivers has been completed. By late February 2009, 13 S-band horns have been assembled, well in advance of receiver production. The production order for the Ku-band horns was placed. The X-band horns have been designed and prototyped, and the production order will be placed in early FY2010.

Although the production quantities of the 3-bit, 4 Gsps digitizer chips were delivered to the NRAO in September 2008, the production of the sampler modules that contain the digitizers has been delayed due to a clocking problem encountered with a serial-to-parallel converter on the sampler board. The layout of the board has been revised, and testing of revised board is underway. If testing goes well, the production of the sampler modules could resume as soon as June 2009.

A science workshop on “The EVLA Vision: Galaxies through Cosmic Time” was held in Socorro on December 16-18, 2008, and 96 scientists from around the world participated. The workshop included numerous science talks on galaxy evolution and cosmology in addition to presentations on EVLA project status.

The science workshop was followed by a meeting of the Science Advisory Group for the EVLA (SAGE) on December 19-20. The SAGE was asked to make recommendations on the development of capabilities to be delivered to the community, possible topics for a 2009 science workshop, and a process to involve the community in proposing early science experiments for the EVLA. The SAGE was also asked to comment on a proposed program for resident shared-risk observing (RSRO). The prioritized growth path recommended by SAGE is to implement the EVLA’s wide bandwidth capability, improve spectral resolution using correlator recirculation, and enable high frequency operation of the instrument. The SAGE deemed that special correlator modes, such as phased array operation, pulsar capabilities, and burst mode, were of lower priority. The SAGE recommended that stars should be the topic of the next EVLA science workshop, which will be held on May 26-28 in Socorro. The SAGE endorsed the RSRO program and recommended a variety of methods for aggressively advertizing the EVLA’s capabilities.