OSRO, March 2010 - September 2011
For a description of the enhanced OSRO capabilities we expect to offer starting with the D-configuration in September 2011 please refer to the appropriate document.
The VLA correlator was turned off in January 2010, and replaced by a subset of the EVLA's WIDAR correlator. The Open Shared Risk Observing (OSRO) program continues into the EVLA era the shared risk access NRAO has been offering for all users since the EVLA construction project began. It provides early access to a number of WIDAR capabilities and observing modes for the general user community that represent a significant improvement over the capabilities of the old VLA correlator. At the same time as the telescope has transitioned from the VLA correlator to WIDAR the direction of the configuration cycles has also changed, from A→B→C→D→A… to D→C→B→A→D…, to facilitate correlator commissioning and to limit data rates during Early Science. The VLA is therefore spending two trimesters in D configuration, one with the VLA correlator (October 2009 - January 2010) and one with WIDAR. The second of these was initially used for commissioning the OSRO observing modes with the full array. Astronomical observations with WIDAR began in March 2010.
Expected EVLA capabilities
Antennas
All VLA antennas have been retrofitted to EVLA specifications; 27 EVLA antennas are routinely available for astronomical observing. See the antenna retrofits page for the current status of antenna receiver availability.
Receivers
As of December 2010 new EVLA receivers are fully in place for C, Ka, K, and Q band. Installation of the new S and Ku band receivers is in progress. L-band receivers are being upgraded one by one from interim [] to full EVLA. X-band receivers are being replaced one by one from old, narrow bandwidth VLA to the wide bandwidth EVLA version. Until sufficient wideband EVLA X-band receivers have been installed to enable evaluation of the system performance and RFI environment between 8 and 12 GHz the bandwidth of the old VLA X-band receivers (8.0-8.8 GHz) should be assumed. Please refer to the EVLA Observational Status Summary for the latest information.
[] Interim receivers pair the new EVLA receiver with the old VLA-style orthomode transducer (OMT). The polarization purity and sensitivity of the interim receivers typically is good only over the traditional VLA tuning range
Figure 1 shows the expected rate of antenna retrofits and installation of the final EVLA receiver systems for the remainder of the EVLA construction project. Note that for certain bands some antennas that do not have final EVLA receiver systems typically do have VLA (X-band) or interim (L-band) receivers. The maximum bandwidth that can be brought back from the antennas to the correlator is currently 2 GHz; this will increase to 8 GHz at a later date when the antennas have been outfitted with fast 3-bit samplers.
Figure 1: Availability of EVLA antennas and final EVLA receiver systems as a function of time.
New receiver bands will be offered for general use when the performance of at least five antennas has been verified by EVLA commissioning staff.
WIDAR correlator
Initially we are providing options for configuring WIDAR for OSRO in the following ways:
- "OSRO1": A configuration for continuum applications, to comprise two independently tunable sub-bands, full polarization, each of which has 128 MHz bandwidth with 64 channels, with the possibility of smoothing in frequency to reduce dataset sizes or to improve spectral response offline. In this configuration it is also possible to decrease the bandwidth by powers of two, keeping the same number of channels, to provide the spectro-polarimetry capabilities in Table 1.
Table 1: Correlator capabilities per sub-band for full-polarization continuum applications and spectro-polarimetry (OSRO1)
Sub-band BW (MHz) Number of poln. products Number of channels/poln product Channel width (kHz) Channel width (km/s at 1 GHz) Total velocity coverage per sub-band (km/s at 1 GHz) 128 4 64 2000 600/ν(GHz) 38,400/ν(GHz) 64 4 64 1000 300 19,200 32 4 64 500 150 9,600 16 4 64 250 75 4,800 8 4 64 125 37.5 2,400 4 4 64 62.5 19 1,200 2 4 64 31.25 9.4 600 1 4 64 15.625 4.7 300 0.5 4 64 7.813 2.3 150 0.25 4 64 3.906 1.2 75 0.125 4 64 1.953 0.59 37.5 0.0625 4 64 0.977 0.29 18.75 0.03125 4 64 0.488 0.15 9.375 - "OSRO2": A higher resolution configuration for spectral line applications, to comprise one tunable sub-band, dual polarization, with 128 MHz bandwidth and 256 channels, again with the possibility of smoothing in frequency to reduce dataset sizes or to improve spectral response offline. It is possible to decrease the bandwidth by powers of two, keeping the same number of channels, to provide the capabilities in Table 2.
Table 2: Correlator capabilities for spectral line applications, dual polarization (OSRO2)
Sub-band BW (MHz) Number of poln. products Number of channels/poln product Channel width (kHz) Channel width (km/s at 1 GHz) Total velocity coverage (km/s at 1 GHz) 128 2 256 500 150/ν(GHz) 38,400/ν(GHz) 64 2 256 250 75 19,200 32 2 256 125 37.5 9,600 16 2 256 62.5 19 4,800 8 2 256 31.25 9.4 2,400 4 2 256 15.625 4.7 1,200 2 2 256 7.813 2.3 600 1 2 256 3.906 1.2 300 0.5 2 256 1.953 0.59 150 0.25 2 256 0.977 0.29 75 0.125 2 256 0.488 0.15 37.5 0.0625 2 256 0.244 0.073 18.75 0.03125 2 256 0.122 0.037 9.375
These capabilities will be provided with integration times no shorter than 1 second, and Doppler tracking will hopefully be available with these correlator configurations. Please consult the Current OSRO Restrictions web page for the most up-to-date information on current OSRO capabilities. The capabilities available to the OSRO program will be expanded as soon as they can be supported for general use. It is currently hoped that 2 GHz of bandwidth will be available by the time of the next D configuration in 2011. The OSRO program will run from March 2010 through the end of the EVLA construction phase. Access to the OSRO program will be via the existing time allocation process and regular calls for proposals.
It will be possible to reduce simple OSRO datasets using AIPS, although users are encouraged to use CASA, which will also be available.
It should be recognized that the EVLA will be undergoing commissioning throughout the duration of the Early Science programs, and that the quality of data taken during this time cannot be fully guaranteed. Nevertheless, NRAO will make every effort to ensure projects awarded time under the OSRO program do obtain data, subject to the availability of our resources.
