[mmaimcal] Correlator reconfiguration time

[Jeff Mangum]

Mark's point about OTF being the great savior seems right to me.  Our
experience with OTF on the 12m is consistent with his note about a
point at which you start wasting too much time turning around.  That
is why I have wanted to implement some other scanning modes which are
more complex, but more efficient, than the simple raster scan mode.

Cheers,

Jeff


"Mark" == Mark Holdaway <mholdawa at tuc.nrao.edu> writes:

>> Trying to beat down the quasi- 1/f noise in HEMTS, or sky noise, in
>> total power mapping.  And, if there is a switching secondary, something
>> related to chopping.
>> 
>> We should probably have a contest.  I can't claim familiarity with all
>> the possible modes that might demand fast dump times, but they always
>> turn out to be less than what you planned for.  For the GBT, the new
>> standard continuum back end was originally designed to have a ~ 1 s 
>> cycle time.  Try using that to switch out 1/f noise.

Mark> I agree with Harvey: my simulations indicated that the atmosphere was
Mark> actually pretty easy to take out compared to 1/f. My strategy for beating
Mark> 1/f noise was to observe as fast as possible with OTF ==> very small
Mark> integrations per beam made the thermal noise higher and the 1/f noise
Mark> lower.  We WANT to be thermal noise limited; to get better SNR required
Mark> doing many scans across the source.  What limits you here is if you go TOO
Mark> fast, you spend all your time turning around and you have epsilon
Mark> integration on source... so there IS an optimal slew rate and dump time
Mark> (ie, a point beyond which faster may be worse). Even if 1/f noise
Mark> ultimately limits the sensitivity, it should still average down, as the
Mark> residual (ie, high frequency part of the) 1/f noise after subtracting a
Mark> temporal baseline should still average down.  (Argue with me if you think
Mark> this is wrong.)

Mark> Anyway, this didn't require any new correlator setups between OTF scans,
Mark> just God-awful correlator dump times which I don't remember.  Harvey's
Mark> advice is good: faster, faster, faster: while there may be an optimal
Mark> speed/dump rate, we might have the calcs off by a factor of 1.4 or 2.
Mark> Bryan Butler is the man now.

Mark> 	-M