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Test observations below 1200 MHz have recently been carried out
with the EVLA to assess the feasibility of pursuing spectral-line
observations for redshifted HI 21cm or other spectral-line
transitions at frequencies where the VLA has severe limitations
and/or simply cannot observe. The results of observing two known HI
21cm absorbers, one at z=0.2467 (~1139 MHz) and the other at
z=0.3127 (~1082 MHz) can be found here.
These test observations show that successful observations are
possible with the EVLA at frequencies that are believed to be
dominated by RFI due to aeronautical navigation transponders, at
least at night time and in A and B configurations. We expect that
daytime observations, and observations in C and D configurations,
may be much more difficult. It should also be noted that for
observations below 1200 MHz, only IF pair B and D can be used at
this time. Furthermore, the polarization performance below 1200 MHz
is severely compromised, with the polarization impurity reaching 40%
at 1 GHz.
Nevertheless, this new 1.0-1.2 GHz capability on the EVLA is now
available for users, and NRAO will consider exploratory proposals to
use it in the current configuration through the end of D
configuration, 15 September 2008. Please ensure that any proposals
are labeled as "Rapid Response--Exploratory Time." Details of the
expected sensitivity based on the tests described above can be found
at the EVLA returns
web page, and any further information or test results will also
be posted at this page. It will be assumed that all proposals
submitted are suitable for dynamic scheduling unless explicitly
justified in the proposal and in the "comments" field of the
sessions section of the cover sheet. At present this excludes the
use of subarrays, although this may change in the future. Please
consult the EVLA returns web page above for the latest information.
Potential users are advised that these are "shared risk"
observations. We cannot guarantee success, especially for daytime
observations that might be severely affected by RFI, but we will
provide reasonable assistance. Users are also reminded of the
aliasing of continuum emission in the bottom 0.7 MHz of baseband
that affects narrow-bandwidth observing with the EVLA antennas, and
should consult the EVLA returns web page for recommendations on its
mitigation. Note that proposals for A configuration should be
submitted as usual at the 2 June 2008 deadline.
Technical questions about observing below 1200 MHz should be
directed to Emmanuel Momjian
or the VLA helpdesk. For
information about this special call for exploratory proposals to use
the new tuning capabilities please contact Claire Chandler.
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