[mmaimcal] Widgets

[Al Wootten]

John asked:
 the list of possible widgets:
                                     1) Ambient load.
The widget at the subreflector most likely--as discussed before, especially at
the lower frequencies this may produce problems with receiver gain if
at or near the receiver package.  However, as a widget for the prototype
antennas, this should be included near the windows, as a comparison for
the subreflector loads.
                                     2) Semi transparent load.
As Jeff pointed out, these may not be two separate widgets.  Basically,
the semi-transparent load eases the gain compression problem which the ambient
load may have at lower frequencies; it may offer some advantages in
calibration.  However, no implementation has been without serious drawbacks.
                                     3) Quarter Wave plate. ( maybe-for
                                        how many bands?)
Advantages:
1) 'roughly' circular polarization
2) SM: Q and U are derived homogenously from the RL and LR cross-products and
thus leakage is more easily calibrated.
3) SM: for the bulk of observations that want high-quality I measurements, 
a single polarization product RR or LL will suffice as a proxy in the absence 
of V(=L-R, thus poorly measured) signal.  With only linear polarization,
both linears must be measured to estimate I.

Disadvantages:
1) lossy
2) narrow bandwidth (compared to receiver band; but we have no definition
of 'narrow'--Crystal noted that the interesting Zeeman lines tend to lie
at the edges of the receiver bands).
3) it is a widget

I think disadvantages outweigh advantages.

                                     4) Solar Attenuator.
This widget will be needed.

     Is this a complete list as far as we know?
SM: Crystal noted that stability of the parallel hands V=RR-LL or 
Q=XX-YY is enhanced by having a transfer switch in the front end that
lets one swap the signal paths of RR and LL or XX and YY through the
electronics - the VLA can do this (though I wasnt aware of that mode)   
and seems to be necessary for high-sensitivity Zeeman work (which needs
V).  Has this been considered for ALMA?

Clear skies,
Al
+--------------------------------------------------------+
| Alwyn  Wootten   (http://www.cv.nrao.edu/~awootten/)	 |
| Project Scientist, Atacama Large Millimeter Array/US   |
| Astronomer, National Radio Astronomy Observatory       |        
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