[evlatests] interband collimation corrections

[Tom Frost]

to answer Vivek's question about holography on # 14 & 16;

Ramon Gutierrez & I looked thru all the Antenna Mechanic
documentation concerning Holography....we could not find
any records of holography being done, other than VLA Test
Memo No.169 dating from 1993 when all 28 VLA Antennas were
done....Ramon thinks that #14 & #16 have not been done since
then...

Vivek Dhawan wrote:

> Sorry - this initially went to only a subset of evlatests.
> ------------------------------
> 
> I was talking about the subreflectors all being in slightly
> different places, variations of 3-5mm. The corresponding
> equivalent displacements of feeds in the focal plane is proportional to
> magnification M, so about 1-2 inches on the VLA. (The axial
> or focus position shift is  ~M*M/2)
> 
> The interband colimations are not affected - a shift (i.e. across the
> optic axis) of the subreflector makes all the bands change colimation by
> the same amount. Similarly, the sag under gravity affects all bands the
> same, but is accounted for as belonging to the antenna, I believe.
> The different subreflector positions can be seen in the different
> antennas (Q-band) peaking in gain at different elevations.
> 
> However, a change in subreflector tilt moves the colimations by the
> same magnitude at all bands, but the vector points around a circle as
> the subreflector is rotated around the bands. This does affect the
> interband colimations. So also a change in FRM rotation encoder offset.
> These are easiest to see if pointing offsets changes from many bands are
> plotted on a circle each at the position of the corresponding feed,
> right after a change in the subreflector position.
> 
> I guess the question I have is, was holography used to set the
> subreflectors of antenna 14 and 16 (and others) when they were VLA 
> antennas, before conversion to evla?
> 
> 
> V.
> 

-- 

Tom Frost
Servo Group Leader
National Radio Astronomy Observatory
VLA Site--Plains of San Augustin
New Mexico, USA
web page    www.aoc.nrao.edu/~tfrost/index.htm
e-mail	    tfrost at aoc.nrao.edu