At the antennas, the project involves: i) improving receivers at existing observing bands; ii) adding new observing bands; and, iii) modifying the antenna structure for improved operation.
The VLA receivers have been upgraded gradually since the early 1980s. Initially, better low-noise amplifiers were used in existing receivers. More recent systems have used the VLBA design, in which the receiver is attached directly to the feed and the polarizer is cooled in the cryogenic dewar. This design reduces the noise contribution from the polarizer and eliminates long, ambient temperature waveguide runs that added to the system temperature.
The ``VLBA-style'' receivers are now used at 1.4, 8.4, and 40-50 GHz. These receivers will remain with perhaps only minor modification. The greatest improvement in system temperature can be made in the 5, 15, and 22 GHz bands using the VLBA-style receivers and modern HFET amplifiers. Completely new receivers will be built for these bands, and should reduce the system temperatures by up to a factor of 3. The new receivers will also provide GHz bandwidth per polarization channel (needed for continuum sensitivity) and will tune over a wider frequency range (to include spectral lines, e.g., methanol, whose astrophysical significance was unknown when the VLA was built). Current plans call for bandwidth ratios of order 1.4-1.5:1 in many bands.
At present, only ten VLA antennas are outfitted for 40-50 GHz operation; this band will be made available on all antennas.
Two new receiver systems will be added at the Cassegrain focus: 2.4 GHz and 33 GHz. A stand-alone 2.4 GHz feed will fit on the feed ring at the Cassegrain focus and will give higher performance than, e.g., an 1.4 GHz/2.4 GHz dual-band feed such as that used on the Australia Telescope. The 2.4 GHz band is potentially the highest sensitivity VLA band. It will also let the VLA participate in bistatic planetary radar observations with Arecibo Observatory. The 33 GHz band can also support bistatic radar experiments with the Goldstone 70 m antenna and would allow imaging of many interesting molecular lines, including red-shifted CO. Table 1.1 summarizes the proposed new and upgraded VLA Cassegrain observing bands.
Table 1.1: Proposed VLA Cassegrain Observing Bands
Plans for new prime focus receiver systems are less well defined. A 330 MHz system is currently located at the prime focus, as is a low-performance, dismountable 74 MHz system on 8 antennas. The presence of the 74 MHz dipole feed degrades performance of the 1.4 GHz band by about 10%. Hence, if 74 MHz feeds are deployed on all antennas, it must be possible to dismount them (or otherwise move them from the prime focus position by mechanical means).
Other specific proposals for prime focus systems include:
Table 1.2: VLA Sensitivity
Table 1.2 compares the continuum sensitivity of the current instrument to that we hope to achieve. We assume a maximum useable bandwidth with RFI excision at the lower frequencies, and add an atmospheric contribution where relevant. The number under refers to the continuum sensitivity in microJy/beam achieved in 12 hrs integration, summing over two orthogonal polarizations. The total bandwidth assumed for 0.33, 0.60, 1.0, 1.4, and 2.4 GHz is 50, 100, 200, 500, and 1000 MHz, respectively. All other estimates assume 2 GHz net bandwidth.