The Ultra-Sensitive Array

These enhancements can be made entirely within the existing infrastructure at the VLA Site:

• New Cassegrain receivers: lower noise temperatures and much wider bandwidth performance (up to 8 GHz in each polarization channel) in existing bands; add 2.4 GHz and 33 GHz bands; complete the outfitting at the 40-50 GHz band; extend the 1.4 GHz band downwards to 1.1 GHz or lower.
• New prime focus receivers: rebuild the focus-rotation mount so that the subreflector can be removed, and add at least three low-frequency feeds at the prime focus.
• A fiber-optic data transmission system to transmit the broad-band signals and monitor data, replacing the original waveguide.
• A new correlator, able to support at least 40 antennas, with up to 8 GHz in each polarization and up to 8,000 spectral channels per baseline per polarization product, and the ability to independently process up to 8 different frequencies from one or two frequency bands.
• Improved antenna performance by adjusting panels for higher efficiency and developing techniques to permit better pointing.
• New antenna stations for a ``super-compact'' E configuration to enable fast mosaicing of large fields.
• New On-Line Computing, monitor/control, and archiving capabilities to permit much more flexible observing modes and user interface to the array.

  
Figure: The current (+) and predicted (o) continuum (left) and spectral line (right) sensitivity of the VLA after the upgrade. For both panels, a 12-hour observation with 27 antennas and with the efficiencies and system temperatures listed in Table 2.3 are assumed. The continuum sensitivity assumes bandwidths given in Table 2.3, while the line sensitivity is based on a bandwidth equivalent to 1 . (Note the different vertical scales.) For the A+ configuration and 37 antennas, the sensitivity would be improved by a further factor of 1.33. To illustrate the relative sensitivities of the proposed new bands for nonthermal and thermal objects, the left panel also shows the slopes of a typical synchrotron spectrum (dashed) and of an optically thick thermal spectrum (dotted). The vertical placement of these spectra is arbitrary.

The continuum sensitivity will improve by more than an order of magnitude in some bands. Figure 2.1 shows the current and expected sensitivity for both line and continuum observations, and demonstrates how these various improvements will impact all observing frequencies. Typical spectra for an optically thin object, and an optically thick thermal object are shown to demonstrate how the proposed S and Ka bands will be the best for work on these objects. New and powerful spectral line observations will be possible and significantly more frequency choices will be available.

Note also that although the sensitivity improvements per spectral line shown in Figure 2.1 are modest below 10 GHz, the proposed new correlator will allow several different transitions to be observed simultaneously. Some multi-transition spectral-line experiments will be more sensitive because longer integration times are available per line.