[Laser] Satellite dish as a light antenna?

[W2MXW]

It would work very well as long as you can make the surface reflective 
enough, which may be too difficult/costly. There are better options for a 
"light dish". You'd have to strip off the paint and have the bare steel 
plated with chromium, or some other reasonably reflective metal (polishing 
it alone won't make it better than about 50% reflective and it'll rust) and 
forget "aluminum" or "silver" spray paints, I did heavy research into that 
some time ago, the shiniest of them are none too great. Another option is 
to cover it with aluminum foil tape (which won't exactly be "optical" 
quality but we don't care about imaging here) although all of these methods 
are none too great .
You'd be far better off getting a purpose-made reflector; Edmund Scientific 
sells beautiful highly polished parabolic aluminum reflectors in 12", 18" 
and 24" diameters all with 0.25 f/d ratio (prime focus feed, although 
easily made into Cassegrainian feed as well. I have one of these in 12" 
diam. and it is my fave light antenna, it's what I'll be using for a 
cloudbounce beacon and for attempts at RX AO-40 laser among other things. 
These are also great for micro/millimeter wave use of course. Prices start 
at $30 for the 12" last check, and I think the 24" was $60.
For the 12" size you can get matching radomes of clear acrylic (which I got 
for outdoor use) also from Edmunds but they're about $50.
Another slightly cheaper option although more elbow grease is the parabolic 
antenna sold by SHF microwave, they are aluminum and therefore can be 
polished easily with a buffing wheel loaded with black emery bar to a high 
finish (remember, we want relative smoothness, as shiny as we can get it 
within reason so as much of the light goes to the same point as possible, 
but don't need "imaging" quality so don't worry about getting a true 
"mirror" finish like for a telescope!!) The SHF dishes are 12" diam., f/d 
of 0.6 and cost $25.
I made for shorter range portable use and LIDAR experiments a "mini" dish 
using a 4" dia., ~0.6 f/d aluminum parabolic reflector (sorry, proprietary 
source, local elec. store just happened to have them in surplus, no longer 
do and they don't sell online anyway) mounted to be an offset feed (even 
though designed for prime focus) just like a DSS satellite dish, but in 
miniature, and am using it with a PD-4 photodiode. It works very well 
despite being not designed for offset, since the PD is so large, the beam 
distortion resulting from this "ersatz offset" doesn't matter since almost 
all the energy is still captured by the huge PD, which means I get the full 
4" clear aperture with nearly full efficiency. This may even be capable of 
RX AO-40 laser.
With any of these, if you are using visible light alignment is easy, you 
can eyeball it. If IR, then use a vis. emitter of the same type first and 
directly sub. the IR after alignment; the property of reflectors which I 
love, is that unlike lenses, the focal length is same at all frequencies. 
Align detectors by looking for smallest spot from a distant light source.

73 de Jon W2MXW


At 02:07 PM 2/3/02 -0800, you wrote:

>I am wondering how a 2 foot satellite dish (Dish network or similar) would
>work as a light collector? Of course the surface would have to have a
>reflective coating in place of the matt finish. The detector/laser pair
>would be mounted at focal point of the dish. The thought is to use an audio
>source for alignment, then switch over to light for the contact??
>
>Does anyone have any thought about this?
>
>73,
>Steve
>AD6HT
>
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