[mmaimcal] alma OTF

[Min Yun]

Reading thru the ASAC report, I noticed the OTF spec of 
30' in 1 second with 1 second turn around time.  Since I am
doing OTF observations with the 12m telescope right now,
I did a quick sanity check.

The 850 GHz beam for ALMA is 6", so the spec calls for
300 beams/sec scan speed.  For continuum, the data dump rate
of 1 msec means we can achieve 3.3 measurements per beam,
which is very good.  In fact, we can get two measurements
per beam up to 1.4 THz, so this OTF spec has the continuum
OTF imaging in mind.

For the line work, the top slew speed has to be reduced by
the minimum data dump rate.  A 16 msec dump time corresponds
62.5 measurements per second, and thus 31.25 beam measurements
per second at best (top slew speed of 4.125 arcmin per second).
This does not sound too fast, but perhaps we are not limited
by the atmosphere nearly as much as the continuum case.
If the minimum dump time gets longer (i.e. phase switching
cycle), this will significantly slow down the required
slew rate -- something to keep in mind.  At the 12m, I am using
30"/sec slew rate to map CO emission in galaxies.  

This is not nearly fast enough, and my images are background
limited rather than thermal noise limited -- not clear if this
is due to the atmosphere or stability in the system, but we
will certainly be fighting similar effects.  Oh, the top
slew speed above should be 3.125'/sec rather than 4.125'/sec.





					-- Min