[mmaimcal] Total Power Mapping

[Steven T. Myers]

Sorry if youve gone over this before - I guess I should go back through
the old minutes to get up to speed...

After voicing my concerns with Min and Michael after the meeting, I
thought about the total power mapping some more.  It is doable, of course,
though how long it takes and how you do it is the tricky part.

There seem to be 2 big questions:

Chop pattern - what to you make the drives do to trace out the same AZ/EL
pattern on one chop cycle and not drive the telescope off pointing or
into resonance.  This will determine the chop period also, which may
be longer than you would like given the atmosphere.

Gain Stability - where is the 1/f for the receiver, which will determine
how well the chop cycle does for gain fluctuations, and thus the effective
sensitivity.

The first is under our control, the second a function of the amplifier
design.  In general, at least for HEMTs, my impression is that the better
the HEMT the worse the 1/f is.  SiS might be OK.  Anyway, we need the info
from the CDL guys, or do a test rig ourselves, or preferably info from
people who do this (eg. the MAT and MAP gang).

To mimimize resonance, you usually try to arrange for the acceleration
to be zero at the chop ends.  A sinusoidal chop does this.  Unfortunately
for a line chop this means you spend the maximum time out on the ends.
You could do out-of-phase sine chops on each of the AZ and EL axes, which
gives a circular pattern on-sky, which can then be swept (or drifted)
across the target sky patch.  This has the advantage of uniform rate along
the circle, but you have to process the data to make images (its been
done, no too hard).  Because the telescope is always in motion, maybe this
has stability benefits.  I think there are other drive ramping
schemes that have better center weighting if you want to stick to a linear
rastering chop.

You should probably fill me in on whats been worked out already so I dont
keep bothering y'all with silly stuff...

  -steve

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|:| Steven T. Myers                      |:|  Associate Scientist      |:|
|:| National Radio Astronomy Observatory |:|                           |:|
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