[mmaimcal] Comparison of ALMA Antenna specs on offset pointing

[Jeff Mangum]

Hi Al,

At some point you mixed the FS spec with the pointing spec. Following 
are some gory details...

The US prototype antenna technical specification for offset pointing says:

The pointing error is specified for two different kinds of pointing, 
"offset" and "absolute" pointing. For "offset" pointing the contribution 
to the error budget for slowly varying causes of pointing error, but not 
for wind induced errors, may be limited to the differential error over a 
solid angle of 2 degrees radius about the desired position, and then 
only the change in that differential error over a 15 minute period when 
tracking at sidereal rate. For "absolute" pointing no such limitation of 
slowly varying errors will be made to contributions in the error budget. 
The nonrepeatable pointing error under Primary Operating Conditions 
(3.3.2.2) for "offset" pointing shall not exceed 0.6 arcseconds root sum 
square (RSS) when tracking an astronomical source at sidereal rate.

The Production Antenna Technical Spec for offset pointing says:

For “offset” pointing and tracking the contribution to the error budget 
for slowly varying causes of pointing error, but not for wind induced 
errors, shall be limited to the accumulated error when the antenna is:
- pointed within 2 degrees from any starting position, and
- tracking over a 15 minute period at sidereal rate.
It shall apply:
The non-repeatable “offset” pointing and tracking error under Primary 
Operating Conditions (section 4.4.3) shall not exceed 0.6 arcsec RSS 
when computed according to the requirements of Section 5.3.2.3.

My RID (Review something Comment) on this issue, along with the relevant 
pages from the prototype and production antenna technical specs are 
attached. The difference is in the solid angle over which the spec 
applies; "2 degrees radius" versus "2 degrees from any starting 
position". I read this to mean that the production antennas will hold 
their offset pointing accuracy over a solid angle of radius 1 degree (2 
degree diameter) rather than the prototype spec of solid angle 2 degrees 
radius.

-- Jeff




On 10/3/06 5:40 PM, Al Wootten wrote:
> At the review, Jeff noted:
>  It looks to me like the production antenna offset
>        pointing spec was changed to half the region over which the spec
>        applies.  The US prototype spec said: "...differential error
>        over a solid angle of 2 degrees radius about the desired
>        position, and then only the change in that differential error
>        over a 15 minute period when tracking at sidereal rate".
>
> And the production spec said:
>    Fast switching phase calibration requires the antenna to move from the
>    target source to a calibration source up to 1.5 degrees away on the sky.
>    For design purposes, in the fast switching cycle to be considered here,
>    the antenna shall perform steps of 1.5 degrees on the sky and settle to
>    within 3 arcsec peak pointing error, all in 1.5 seconds of time. The
>    antenna shall then track and integrate on a calibration source during one
>    second, then it shall switch back to the target source with the same
>    requirements on switching time and settling accuracy. It shall then track 
>    on the target source. The time for a full cycle of target-calibrator-target 
>    observation shall be 10 seconds.
>
> As we discussed today, we should understand the ramifications of this change,
> which has gone unremarked for several years now.
>
> Comments?
>
> Clear skies,
> Al
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