[evlatests] spectral line tests

[cchandle at aoc.nrao.edu]

Report on spectral line tests with Doppler tracking, from Friday night,
Jan 30 2005.

The aim of these tests was to investigate the phase on VLA-EVLA baselines
at the beginning of each scan, which is predicted to be random because
of the VLA Flukes changing frequency when Doppler tracking.  The tests
comprised:

1) Observations of a strong astronomical SiO maser at 43.1 GHz for 1
hour total using 2 min scans, with Doppler tracking.

2) Observations of a strong OH maser at 1665 MHz for 1 hour, 2 min scans,
with Doppler tracking.

3) Observations of a strong H2O Maser at 22.2 GHz for 1 hour, 2 min scans,
with Doppler tracking.

The tests revealed the following:

o Strong fringes on all VLA-VLA baselines.
o No fringes on VLA-EVLA baselines.
o Strong fringes on the EVLA-EVLA baseline.

It turned out that there was a frequency offset between the sky
frequency calculated by the VLA online system and that requested for
the EVLA antennas by obs2script, sufficient to wash out all fringes
between the VLA and EVLA antennas.  At 43 GHz this offset amounted to
f(EVLA)-f(VLA)~200Hz, at 1665 MHz the offset is f(EVLA)-f(VLA)~10Hz,
and at 22 GHz the offset is f(EVLA)-f(VLA)~65Hz.  The frequency offset
is not the same for each scan and suggests a fundamental difference in
the way the sky frequencies are calculated between the VLA online system
and obs2script.  No cross-correlations between VLA and EVLA antennas
with Doppler tracking will be possible as long as both sets of antennas
are driven by these different software systems in their current states.

I suggest it is low priority for obs2script to be fixed to match the
VLA online system, although it would be useful to understand where the
differences in calculated sky frequencies arise in order to make sure
the calculations for the EVLA are correct.

Further tests will be needed to ensure that spectral lines can be detected
on VLA-EVLA baselines observing with fixed frequency (no Doppler tracking)
throughout, although since Rick and others have successfully done this
in multi-channel continuum mode I do not see why it should not work...

Claire