[evlatests] Pointing Offset Results

[Barry Clark]

I believe that the value we use for the rotation vs elevation is
was simply determined by looking at the finite element model and
seeing how far from the best fit paraboloid the subreflector lies,
calculating the cubic term of phase across the dish, and calculating
the subreflector rotation needed to cancel that cubic term.

If the dish were perfect and the subreflector well aligned, the
rotation trick simply removes the subreflector sag term, and would
not change the elevation of the gain maximum.

My impression is that the elevation of maximum gain is coupled to the
rotation trick by the fact that antenna deformation as a function of
elevation also contains a cubic term.  This offers the interesting
possibility that by changing the value of that rotation constant
we could also cancel some of that deformation term, and further flatten
the gain curves.  I can't remember anybody having tried this.

George Moellenbrock wrote:
> Rick-
> 
>         1) Many antennas show a strong elevation dependency in the X-band
>     results.  This can only be due to a model error.  Ken, on the basis of
>     last night's pointing run, has found the origin, and inserted
>     corrections.
> 
> 
> Does this change affect what the gaincurves actually are for bands subject
> to the sub-reflector rotation trick?   (Should they be remeasured?  At least
> at high-freq bands where the differences  could be meaningful....)
> 
> I guess this is another way of asking how orthogonal nominal
> or referenced pointing and the sub-reflector rotation trick are.  The
> nature of the rotation-trick=OFF gaincurves seem to imply (or not
> rule out, at least)  that they are coupled to a considerable degree.  
> I think you allude to this sort of thing in your gaincurve email (trying
> to optimize at el~50 deg, etc.).      Is the subreflector rotation trick
> implemented  in an antenna-dependent manner?  (Different
> amounts for different  antennas, etc.)
> 
> I fully appreciate that the gaincurves' principal mandate is
> to merely remove elevation-dependence.   However, like the nominal
> efficiency, the coupling details noted above amount to (at least) scaling
> effects for which the precise nature of the factoring (and normalization)
> is important if we want to tacitly assume (or even advertise) that the
> _net_ nominal calibration (sw power, efficiency, gaincurve, opacity, etc.)
> is within a few % of true Jy, and doesn't mysteriously change 
> discontinuously
> (even at less than a few %) every maintenance day without explanation,
> or every time new gaincurves are published.
> 
> I personally would discourage such adverts, frankly, and encourage
> explicit external f.d. calibration, but I also recognize the convenience
> from a characterization point of view---e.g., stress tests, pipelines, and
> similar---of having a _stable_ net nominal calibration regime.
> This means understanding (and keeping track of, if necessary)
> how the various factors, including
> implicit ones like subreflector rotations (and focus, etc.), care about
> each other, and only update more than modestly coupled parameters
> in parallel, for example, especially when (only) some corrections are
> deferred to offline processing.   We can be as precise as we deem
> reasonable and useful in all of this, of course, considering available
> maintenance effort, etc. (about which I am agnostic), but we should
> at least be clear about what the downstream expectations should be
> (about which I care).    In particular, since some of the effects are 
> corrected
> offline, there will be fewer surprises in using offline s/w if the net
> nominal calibration model is well-defined and collectively
> understood and coded-to, and consistently adhered-to,  and the
> consequences for subtle but  deliberate and desirable changes in
> telescope parameters recognized.
> 
> -George
> 
> 
> 
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