Wide-band receivers without OMT
All EVLA antennas (see here for the
most current list) now have new wide-band C-band receivers, which
(with restrictions, see below) can be tuned from 4.2 - 7.7 GHz. The
rms (in arbitrary units) as a function of frequency between 5.5 and
8.0 GHz is plotted here. The features at
6.0, 6.4, and 7.9 GHz are resonances which disappear when Ortho Mode
Transducers (OMTs) are installed.
A plot showing the K value against frequency is shown here at higher frequency resolution than
the figure referred to above. This plot covers in detail the range
from 5900 to 6700 MHz. This range was chosen since it is not only
there that the resonances occur, but it also contains the familiar OH
lines at 6031 and 6035 MHz and the Methanol line at 6668 MHz. See the
Observational Status Summary for a definition of the K-value.
To illustrate the opportunities this new frequency range offers, we
show scalar cross-power spectra on a maser in W3 on one of the EVLA -
EVLA baselines in the 6030.747 MHz OH line,
the 6035.092 MHz OH line, and the 6668.5192 MHz Methanol line. The flux density
scale is indicated below the plot; because of a lack of accurate
calibrator flux densities at these frequencies we did not perform any
flux calibration; we expect true flux densities to be about 40% higher
than the y-axes indicate.
Note that outside the traditional C-band range polarization
information is lost, as illustrated by this
figure showing the ratio RL/RR for two baselines for an
unpolarized source. Only between 4.5 and 5.0 GHz this ratio is close
to zero, as expected for an unpolarized source. Polarization
observations won't be able to exploit the extended frequency range
until the installation of the OMTs.
Wide-band receivers with OMT
Currently all C-band receivers are being retrofitted with Ortho-mode
transducers (OMTs). Their purpose is to remove the resonances (Figure
1), and to regain polarization purity across the new band (Figure
2). See the relevant section for the
current status.
Figure 1: Effective Tsys for three antennas
equipped with new OMTs over the full C-band frequency range
Figure 2: D-terms (in %) for four antennas equipped with new
OMTs over the full C-band frequency range
Special behavior of IF pair AC at low end of band
Test observations at frequencies between 3800 and 8200 MHz show that
the sensitivity of IF pair AC degrades significantly at frequencies
below 4400 MHz. During the transition period, observers interested in
using the EVLA with the current VLA correlator to observe at C-band
frequencies between 3800 MHz and 4400 MHz need to utilize IF pair BD.
If all four IFs are needed, the automated power adjustment for IF pair
AC needs to be disabled in the observing script. For the latter,
contact local staff. Please direct technical questions about
observing at these frequencies to Emmanuel Momjian.
Figure 3: Effective system temperatures for all four IFs of
EVLA antenna 2 between 3800 and 5600 MHz
|
National Radio
