[evlatests] Frequency offsets between calibrator & target

[Vivek Dhawan]

The earlier message is appended. Corrections are based on
recent tests.

The explanation we offered for why the VLA "worked OK in the
past" is not valid. We (ie Ken and I) now think that the VLA
would have always done this. If you believe otherwise, send me
the project code of archival data and I might take a look as
time permits. The cure in the present era is to observe with
the same frequency for cal and target  (preferably fixed for
the whole observation).

To summarize the tests done:

test 1: observe a given calibrator with various frequency offsets,
(few KHz), verify that there is a phase offset between settings.

test 2: observe as before, with the VLA round-trip LO correction
disabled; no change in behavior.

test 3: I tried the correction that I suggested previously -

For the VLA the WGDELAY is in the parameters database, and for
the EVLA the corresponding FIBERDELAY. I extracted these numbers,
turned them into antenna phase corrections with the recipe given
in the previous email, and applied them in CLCOR opcode 'PHAS'.
In my test data, with upto 10KHz offsets, this removed the effect.
The VLA and EVLA had to be corrected with opposite sign (we think
we understand why, I won't go into it here).

Trying this recipe on Crystal's data (on a pair of calibrators
observed ~60KHz apart) did not work too well, I suspect because
I know the outbound LO delay only to ~1usec, insufficient to
correct her data with ~MHz offsets.

end of round 2.


-------- Original Message --------
Subject: [evlatests] Frequency offsets between calibrator & target
Date: Tue, 29 Jan 2008 17:48:41 -0700
From: Vivek Dhawan <vdhawan at nrao.edu>
Organization: NRAO
To: evlatests at nrao.edu, cbrogan at nrao.edu


Summary of problem: Crystal Brogan reported that in AC904 (6.7GHz,
B array, 2007 Oct 31) where calibrator and target were observed at
slightly different frequencies, phase transfer from calibrator to
target did not work - the images were highly corrupted.

Following discussion with Ken and a test observation, the current
state of understanding is as follows:

VLA and EVLA antennas all show a change of phase between 2 settings
with small frequency offset d_f,  of

d_phi = 2pi * L * d_f / c,

where L is the electrical path length in LO transmission to the
antenna pad. A baseline formed by two antennas equidistant from the
LO distribution point has no phase jump; this has been verified.
Celestial geometry (w term) is not relevant, but L is of the same
order of magnitude.

So, for example, for a 4kHz difference between calibrator scan and
target scan (e.g. on the same source, geometry unchanged), the phase
difference is 50deg at the end of B array (EVLA, glass fiber) or
~30deg for VLA (air).

How the E/VLA differ:

The EVLA does the fine tuning at the antenna (L302) using a reference
that has been distributed over fiber. The VLA does the Doppler tuning
in the Flukes, not in the antenna, and the Fluke reference signal is
not subject to the distribution path length. In the MODCOMP days, the
round-trip phase correction for changes in the LO path was done by
carefully excluding the Fluke part. Now it includes the Fluke and so
the VLA has the same behaviour as the EVLA, and is worse than the old
VLA system. (If cal and target are at the same frequency, this problem
goes away.)

Fixing it:

1. Online: Conceptually the right thing (I think) is to account for
     the LO path length correction as a delay instead of a phase.

2. Post-processing: Knowing the path lengths (pad positions) and
     frequency offset, a post-processing correction should also work.
     I have not yet figured out how to accomplish it in AIPS, but will
     think about it.
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