On 10 GHz, 3dB beamwidth of a 3m dish is about 0.6 degrees. That means if you miss a point source by 0.3 degrees, you're 3 dB down! Although the Moon is 0.5 degrees across, it's stil quite a chore to follow it, especially on transmit, where its noise cannot be used, and if you don't have a very precise angle readout. Because of Earth rotation, it will escape well out of your beam during the 2.5 min TX sequence.
So, despite the fact that it's cloudy here most of the time (especially during those March/November contests!), I decided to include video tracking into my setup.
Although old camcorders are available at flea markets for rock-bottom prices, I find them too heavy and clumsy to mount and precisely aim. Therefore, I went on to design a special Moon-tracking camera.
I based my design on one of those miniature 'naked' (bare PCB) 'spy' video cameras usually used in front door intercoms, baby monitors, security etc. Their price is now (end 1998) about $60 (new) and falling.
They usually come with an miniature wide-angle fish-eyish lens, that is unsuitable for Moon tracking, because the Moon will be only a tiny speck on the screen. This lens can be easily removed by unscrewing. Usually there is a teeny-weeny setscrew you have to loosen first. (NOTE: while unscrewing the lens, the focus changes from infinity to very close - it's easy to blow up a wasp across the whole screen - a cheap 'video microscope'!!!)
I replaced the lens with an 300mm achromat doublet, and the Moon almost fills
(it can be seen here:)
equipment on rear side of the dish (JPEG 15kB)
This is fine for ultra-precise tracking and for watching the craters and
mountains on the Moon, but it makes it difficult to FIND the Moon, HI!
I think a 120-150mm achromat would be just fine. You can get such a lens from
and old binocular (a real one - not some plastic toy!) where it's used
as the objective lens. (Because of the tiny size of the CCD in the camera,
a real anastigmat (photographic type) lens is not necessary.) Check the
focal distance of the lens by trying to burn a piece of paper in the Sun.
(The aperture of such a lens is usually too slow to really burn white paper.)
The distance between the lens and the paper when the image of the Sun is
smallest and brightest is a good approximation of the focal length.
I mounted the lens and the camera at the two ends of an aluminium tube. The length and diameter of the tube are chosen to suit the lens. The distance between the lens and the camera chip must equal the focal distance of the lens. The inside surface of the tube must be matte black, to get good contrast. I burned a few pieces of black car tire rubber inside, to generate soot. A ligt baffle (black hard paper ring) inside the tube also helps improve the contrast. The aperture of the baffle must be such, that from the viewpoint of the camera chip, the whole lens can just be seen. The baffle must fit snuggly into the tube, so that you don't get glare around the rim. It also helps to mount the lens a little deeper into the tube, to provide a kind of 'lens hood'. In that case, the tube must be cut accordingly longer, of course, to keep the rigt distance between lens and camera.
To hold the lens, I have cut two 5mm long pieces of the same tube, and removed a piece of them (to get a kind of 'C" shape), so that they can be squezzed into the tube on both sides of the lens.
The camera (or the lens, for that matter) must be mounted so that it can be easily moved a little along the tube, to adjust focus.
A light-tight box must be built around the camera. Just use anything suitable. Check for light-leaks by plugging the lens end of the tube, shining a torchlight from all directions onto the camera, and watching the monitor.
The whole tube assembly with camera and lens is then mounted on a support rod, so that its aiming can be precisely adjusted with four screws. The camera end of the tube is connected to the support rod via a flexible metal strip, and the lens end goes through a ring with the adjustment screws.
The support rod is then attached rigidly to the rim of the dish. (it can be seen in the upper right corner of this photo:) old 3m dish on Mk I mount (JPEG 28kB)
Front wiew of the camera
Side wiew of the camera