[amsat-bb] Re: Camera on antenna
domenico.i8cvs at tin.it
Wed Sep 12 08:51:10 PDT 2012
----- Original Message -----
From: "Louis McFadin" <w5did at mac.com>
To: <amsat-bb at amsat.org>
Sent: Wednesday, September 12, 2012 3:37 AM
Subject: [amsat-bb] Re: Camera on antenna
> I use a camera mounted on the support strut of the dish feed. It is
> pointed at the center of the dish.
> Then I simply point the dish at the sun and make corrections to az and el
> until the shadow is exactly in the center of the dish.
> Then use those corrections in MacDoppler.
> Works great! No risk to the camera by looking at the sun.
> Lou McFadin
> w5did at cfl.rr.com
Hi Lou, W5DID
Using the Sun light and point a camera on the Sun is not a good procedure
to calibrate the pointing of a dish for radio frequency signals because it
is possible that the radio frequency feed is mounted shifted a fraction of
degree making a big unwanted squint angle.
Depending on the squint angle of the radio frequency feed with respect to
the dish focal point it make a wrong pointing so that using the Sun light
and a camera you believe that your dish is looking exactly at the Sun but
in reality it can be not exactly pointed to get a radio frequency signal
source like the Sun Noise.
Particularly using large high gain dishes with sharp radiation lobe the
correct procedure is to calibrate the pointing vector of the dish to the
Sun using the Sun Noise and not a camera pointed to the Sun light.
The correct procedure is to use the Sun Noise to calibrate antennas and Sun
light to calibrate optical telescopes.
Also to work EME with high gain dishes with narrow main lobe the Azimut
and Elevation indication must be made not using position potentiometers but
more sophisticated and precision devices like position resolvers, or
position absolute encoders or selsyn indicators who permits high and
constant resolution of fraction degrees for many years.
For my 1.2 meters dish at 2.4 and 10.5 GHz I use not position potentiometers
but 2" surplus position selsyn motors originally used to point guns on board
of ships during the WWII
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