Measuring the offset of sat dishes with no offset in the specs
 

At 13:21:12 Tue, 3 Nov 2009, Java Jive wrote:
Thanks for the first sensible piece of advice John ...
Now write a small program which will allow you to repeatedly plug in the
unknown factors until you find the best possible match to the equation
of a parabola of the form y = a.x^2 where 'a' is a constant equal to
the reciprocal of four times the (as yet unknown) focal length.

Although there are already around in the public domain such programmes
I could copy, the method you suggest depends on a series of fairly
accurate measurements by the user of my site. I'd rather keep it
simple, though, of course that may not prove possible. I'm taking the
attitude that I'll only cross that bridge when I find I have come to
it.

The problem can be simplified if the vertex of the parabola is assumed
to be located on the lower rim of the dish. Only one measurement is
then needed from the straight-edge to the surface of the dish, say at
the exact centre. This leads to an equation which I think should be
solvable without using some iterative procedure, though my maths doesn't
seem to be quite up to it at the moment...

While the experts here may say that knowing the offset angle of a dish
doesn't really matter, the instructions that came with my dish didn't
even mention that there was such a thing, saying simply that the dish
should be pointed at the sky at the angle of elevation - and helpfully
giving a long list of the elevations for Astra 28E for places all over
the UK. So I started off by tilting the dish at an angle of 25 degrees.
Luckily, I had also read somewhere that it was best to begin by pointing
the dish towards the horizon. Of course, anyone who's installed more
than one dish will have a pretty good idea of what the angle should be
in their area.