[mmaimcal] THz observations and local oscillators

[Darrel Emerson]

The talk of the ACA and possible operation above 1 THz reminds me of the
following detail:

   The baseline spec. for the ALMA interferometer local oscillator
coherence (i.e. fast phase jitter, too fast to be corrected by
calibration) is set such that 90% coherence is obtained between an
interferometer pair at 950 GHz.  This spec. is analagous to the antenna
surface accuracy spec;  the frequency dependence is the same as for the
Ruze law applied to antenna surface errors.  This 90% coherence at 950
GHz spec. is extremely difficult to reach, whether by multipliers or by
a direct photonic system.  Of course atmospheric phase noise adds to
this loss of coherence.

   For reference, if the same local oscillator system were to be used at
1.5 THz, the coherence would become 77%, while at 2.5 THz it becomes
48%.  So, with the current specs, a system using even perfect antennas
would be degraded to less than half the theoretical sensitivity when
used as an interferometer with this local oscillator system at that
frequency.  If serious interferometer operation above 1 THz is
contemplated one day, either this loss has to be accepted, or at least
parts of the LO system will have to be re-engineered - if indeed that's
technically feasible.

   Another detail, that's probably too obvious to mention: the
sensitivity calibration in single dish mode, such as made with some
hot/hotter load system or whatever, has to be tied somehow to the
sensitivity in interferometer mode, which will always be degraded by
finite LO coherence compared to single dish sensitivity.  The baseline
LO coherence spec implies a 1% degradation at 293 GHz, which needs to be
taken into account if an overall absolute calibration accuracy of 1% is
to be obtained.

    Just a detail.

		Cheers,
			Darrel.