[mmaimcal] alma OTF

[Jeff Mangum]

"Min" == Min Yun <myun at aoc.nrao.edu> writes:

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

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

Actually, 3.3 measurements per beam is close to marginal for the total
power OTF case.  When taper is taken into consideration, the minimum
sampling rate becomes something like 2.5 points per beam.  Remember
too that it is good to have some margin for error given that
telescopes do not track a given course perfectly (due to wind, for
example), so the true sampling obtained may be far from ideal.  This
is not to say that we need to touch up the OTF spec, if the ALMA
antennas perform as designed.

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

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

You could have gone faster as you are well under the minimum sampling
for an observing frequency of 115 GHz.

Cheers,

Jeff