Efforts are under way to significantly improve the sensitivity of the VLBA. The major element of this effort is the bandwidth upgrade that has been reported elsewhere. But there are also opportunities for significant improvement in some of the receivers, especially at the higher frequencies. Some of the most important science being done on the VLBA involves observation of faint water masers in the 22 GHz band, one of the bands where current state-of-the-art amplifiers are much lower noise than the original VLBA amplifiers. Spectral line observations do not benefit from bandwidths wider than what is needed to see all lines in a source, so the best way to improve the capability of the VLBA for the maser observations is to improve the receivers. Of course, any receiver improvement also benefits continuum observations in the same band. Therefore, during 2007, the amplifiers in the existing VLBA 22 GHz receivers were replaced with the much appreciated financial help of the Max-Plank-Institut für Radioastronomie. The final report for the upgrade is in VLBA Sensitivity Upgrade Memo 21.
The low noise amplifiers in the VLBA 22 GHz receivers were replaced with Cryo-3 amplifiers from the NRAO Central Development Laboratory. Most other parts of the system, including feeds, polarizers, and dewars were retained to contain costs. The receiver temperatures were reduced by about a factor of about 2.4. Taking into account the contributions from spillover and the atmosphere, the sensitivity of the array was increased by a factor of 1.6. The reduction in system equivalent flux density (SEFD) by about 38% significantly exceeded the project goal of 30%. The improvement is equivalent to increasing the integration time, or the bandwidth for a continuum source, by a factor of 2.5. The new amplifiers also made the effect of the atmospheric water vapor line at 22 GHz obvious so a new standard observing frequency was established at 23.8 GHz, away from the peak of the line. For continuum observations in winter conditions, the sensitivity at that frequency is about 1.8 times greater than with the old system at 22.2 GHz. The expected benefit in wetter seasons should be even greater. The upgrade was completed in January 2008.
The attached figure shows the zenith SEFD measured using most of the pointing and gain observations taken between October 2006 and January 2007, before the upgrade, and between January and April, 2008, after the upgrade. The SEFD is a good measure of the overall sensitivity of an antenna taking into account the telescope gain, the receiver temperature, and the additional contributions to system temperature from sources outside the receiver such as spillover and the atmosphere. For reference, the SEFD of typical VLBA receivers between 1.4 and 8 GHz, where the both amplifiers and the atmosphere make only small contributions, is around 300 Jy. Because of the fitting methods used in the data analysis, the results presented correspond to the performance under the best conditions encountered during the period of the analysis. The average across sites of the zenith SEFD at 22.2 GHz before the upgrade was 796 Jy. After the upgrade, the average zenith SEFD was 502 Jy. Off the center of the atmospheric water line, the SEFD is even lower. At 23.8 GHz, in the standard frequency that has been added in the middle of the 23.6-24.0 GHz protected band, the SEFD in good conditions is 441 Jy. The sensitivity at 23.8 GHz is now a factor of 1.8 greater than the pre-upgrade sensitivity at 22.2 GHz, which should significantly benefit continuum projects. Note that, in wetter conditions, such as summer, the absolute sensitivity will be less because of added water vapor. But the difference between 22.2 and 23.8 GHz will be even higher than the winter numbers shown in the Figure indicate, making it even more beneficial to shift continuum projects to 23.8 GHz.