[mmaimcal]Over the top motion and axis intersection K correction for Antenna RFP

[Min S. Yun]

Hi Al,

I think there are two separate issues being discussed here.  The 
over-the-top spec of 125 degree (or whatever) is related to 
determining the cos(EL) dependence of the K-term, and this means
it helps only if you go far enough in EL to see the sign change in
cos(EL), well in access of the scatter in the pointing measurement data.
Thus, I would not think going over to just 100 degree in EL helps much.

The spec on the K-term is a manufacturing requirement.  It is 
never perfect, of course, and you are doing this over-the-top
observations to MEASURE the magnitude of this error.  Obviously
you want to keep this as small as possible since it just one more
possibly degenerate term that can affect your pointing solution.
Although not terribly well determined, both OVRO and SMA antennas
seem to have K-term of order 1mm or less, so this should not be a
challenging spec.  Another reason for keepping this spec tight is
that even the over-the-top measurements may not be
very accurate if the reflector does not behave
correctly as it goes over the top -- it is rumored that the VLA
antennas flex differently when it goes over the top.

You might want to have a chat with Barry or Rick over this particular
issue and their experience with the VLA.


					-- Min 

-----------

Min S. Yun				e-mail: myun at astro.umass.edu
Assistant Professor			phone:  (413) 545-2215
Astronomy Department			FAX:    (413) 545-4223
University of Massachusetts             http://www.astro.umass.edu/~myun
Amherst, MA 01003



On Thu, 18 Sep 2003, Al Wootten wrote:

> Folks,
> 
> Several questions have come up as the Antenna IPT moves toward its RFP 
> which they want to finalize by tomorrow.  In particular the following 
> question has arisen:
> 
> Specs on over-the-top:
> Currently the spec is 125 degrees over the top.  My understanding is 
> that over-the-top motions help in tracking objects near the zenith, and 
> that it makes calculation of the 'K-correction', the intersection of the 
> axes offset, more reliable.  On the present prototypes the spec on the 
> intersection of the axes is that the error shall not exceed 3mm.  In the 
> draft RFP this has been decreased to 2mm.  One school argues that this 
> should be even tighter, 1mm.  Another school argues that the telescope 
> we are building is quite stiff and that the spec is already too tight as 
> it will be unlikely to vary by much.
> 
> Questions:
> Can the over the top spec be relaxed to as little as 100 degrees without 
> compromising determination of the offset of the intersection of the axes?
> 
> What should the spec be on the determination of the offset of the axes.
> 
> According to my notes, we discussed this on or about 14 December 1998 
> but I don't have my notes of that meeting here with me in Richmond and 
> apparently my workstation in Charlottesville has been turned off in 
> anticipation of disaster.
> 
> Thanks for your help.
> 
> Clear skies,
> Al
> 
> _______________________________________________
> mmaimcal mailing list
> mmaimcal at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/mmaimcal
>