Special EVLA Proposal Opportunity

Expanded Tuning Range 4.2-7.7 GHz

March 30, 2007

We have recently determined that the 'interim' EVLA C-band receivers are capable of tuning, with good sensitivity, over most of the frequency range from 4.2 to 7.7 GHz. Because these antennas are not yet equipped with their wide-band orthomode transducers (OMTs), the polarization purity of the correlation products is quite poor outside of the 4.5-5.0 GHz VLA range -- nevertheless, the retrofitted EVLA antennas should provide adequate sensitivity and imaging for non-polarimetric observations in total intensity.

Because of this unexpected early capability, the NRAO is circulating a special call for exploratory proposals which use the expanded tuning range. This call is specifically for proposals to use the current D configuration (through mid-May 2007), the upcoming A configuration. and the move period from D to A configuration. Users wishing to utilize this new tuning capability within this time frame must submit proposals by 17:00 MDT (23:00 UT) on April 9, using the VLA Proposal Submission Tool at http://www.vla.nrao.edu/astro/prop/vlapst/ . Please ensure that your "Proposal Type" is labeled as "Rapid Response--Exploratory Time."

There are currently seven EVLA antennas outfitted with the interim C-band receivers. This number will rise to eight by mid April, and nine by early May. The 'K' value sensitivities (K=0.12 T_sys/eta_a, where T_sys = system temperature in Kelvin and eta_a is the antenna efficiency; see page 8 of the VLA Observational Status Summary, or http://www.vla.nrao.edu/astro/guides/vlas/current/), proportional to the system noise, are roughly as follows:

Frequency Range	K
4.2 - 4.5	10
4.5 - 5.0	6
5.0 - 5.9	10
5.9 - 6.1	> 20
6.1 - 6.2	10
6.2 - 6.4	> 20
6.4 - 6.5	> 12
6.5 - 7.5	12
7.5 - 7.7	15
>7.7	>20

The frequency ranges 5.9 - 6.1, 6.2 - 6.5, and above 7.7 GHz are strongly affected by resonances within the interim OMT. We are currently establishing the usable frequency ranges in the vicinity of resonances, and hope to have this information available on the web by about April 4. Plots of the sensitivity and polarization purity vs. frequency can be obtained from the 'EVLAreturns' web site at http://www.vla.nrao.edu/astro/guides/evlareturn/ .

(added on April 5, 2007) We have now determined the K-value as a function of frequency over the range 5.9 - 6.7 GHz. This plot shows the location of the resonances in more detail.

Potential users are advised that these will be 'shared risk' observations. We cannot guarantee success, but will provide reasonable assistance. JObserve cannot be used to set up observations in the new tuning ranges. For further information concerning technical capabilities, contact Gustaaf van Moorsel (gvanmoor@nrao.edu) or Rick Perley (rperley@nrao.edu). For information on this special announcement, contact Claire Chandler (cchandle@nrao.edu) or Jim Ulvestad (julvesta@nrao.edu).

--- Jim Ulvestad, on behalf of NRAO VLA/EVLA Staff ---