[mmaimcal] Q-band on ALMA

[Frazer Owen]

	The Q-band system on ALMA in not about point source sensitivity,
although it may be used for detections of southern sources. Even above the
VLA declination limit, the efficiency drops off significantly, so ALMA
may be significantly better for much of the sky below Dec=0. 

	The main driver for Q-band on ALMA has always been surface
brightness sensitivity, primarily for SZ and cosmology. This was hurt by
going to the bigger dishes. Since it is uncertain how well the total
power ONF imaging will work, this is a big issue. Also the most compact
configuration is critical for this work. However, there will still
be a range of experiments which ALMA can address which the VLA cannot 
because the D-configuration is too big, the VLA antennas are marginal at
Q-band and the VLA pointing is not good enough.

	There also are a range of synchrotron experiments which the
VLA cannot address well because of the same reasons. These involve larger
sources: radio galaxies and SNR's where one is interested in studying
the highest energy particles one can image. These experiments put constraints
on particle lifetimes and thus reacceleration/source history.  ALMA, even with
the bigger dishes should be better at this than the VLA. ALMA should be
able to mosaic easily at these frequencies while it is inbetween a struggle
and impossible with the VLA at Q-band.

	Of course, VLB will be very useful with ALMA at 7mm. 

	Also the larger beam of ALMA and the speed of movement of
the antennas should make surveys possible at Q-band more easily than wit
the VLA. The gain in the time to cover a given field should be like
[1.2*(25/12)^2]^2 (using Brian's 20%) = 27, ignoring any problems moving
antennas with the VLA. This should, of course, carry over to spectral
line experiments.

	Finally, perhaps the most important consideration, is that there
will be a significant fraction of the time when ALMA cannot operate at
any band but Q, due to poor weather conditions. Many of the synchrotron
experiments and surveys could use this time. My memory is that the
fraction of time this is true is fairly high. Do we want to let
ALMA sit idle for even 10% of the time because we did not put all
the receivers on it we should ? One should review this. 
Furthermore the VLA works well at Q only during the winter months, mainly at
night. We were able to get M87 Q-band observations at A-array during the
winter but were never able to get B and C since they were in poorer weather
seasons. ALMA has a lot of poor weather conditions when it can operate at
Q-band (partly just due to weather and partly due to its faster switching times)while it is only during the best conditions that the VLA can use
Q, where it has a lot of pressure for experiments at other bands as well. 

	Thus it would be a shame if ALMA loses Q-band. But that would not be
the first decision about ALMA that I don't agree with.


---Frazer