Real time active aiming?( was Re: [Laser] Laser comm stuff)

[Derek Weston]

> > It'd be interesting to experiment using one of the precision retro
> > reflectors used by surveyors.  Does anyone have an idea of the % of
> > incident light these return and the typical angular difference between
> > incident and returned beams?
> 
> At a guess 80%+. They are coated on the front face and have pretty good mirror
> coatings. That's what I had in mind. The angle is an arc-second or two, at most.  You
> could position 3 around your aperture. Similar items are available surplus.
> 
> Another possible solution would be to introduce a partial mirror (with a hole for the
> detector light, or partially silvered) to reflect in your telescope's optical path.
> This is much harder to align, though, since you need to tweak one end, then the other,
> and iterate.
> 
> -John

All points taken.

However, the retroreflector route seems to me inefficient (more
hi-precision HW req'd) if the link is inherently capable of data
transfer. Why add "stuff" when you don't need to? Some microcontroller
code is much easier (for me) and cheaper than precision optics and
mechanical (including thermal movement) difficulties. 

I guess also that my thinking is colored by my experience with a
neighbourhood laser-linked network I set up a few years back. It worked
well when aligned, but frequent realignment was necessary. To get the
necessary range the beams were narrow, typically less than an inch
diameter at the target, so a retroreflector offset by an inch from the
photodetector would not have been be useful.