You run the risk of the radar possibly frying the electronic internals of the light, as well as it causing a blind spot in your radar return. As is, my tests show that spotlights mounted on the roof will cause a HUGE BLINDSPOT when it lights up your rails!
Wanna see for yourself? Put a kid up there at night with a handheld lying on a cardboard box, at the same height you want to mount it. Then have them pan it around and up & down. I guarantee you that you will NOT be able to see anything off the bow with all that glare bouncing off your shiny SS rails!
With that sad, you "could" put it off a roof, "if" you designed some kind of deflector or 'cone' or shield (that could be added for when in use) that would minimize any reflection off the rails, as they WILL LIGHT up if in any part of the beam .. and light up enough to destroy your night vision and impede what you could otherwise see.
Here's what I did ... I bought some starboard and made a quick-detach mount for the pulpit. I used the wired version from Go-Light, < $100 shipped, see
http://www.golight.com/ . The mount that affixes in seconds to the FAR end of the pulpit, so when in use there is
zero glare sent back towards the helm. Think about it, nighttime running might be 10% of your overall use and you should know ahead of time you may need the light. So just hook it up when needed. And with it out of the elements, it will last for YEARS!
FYI, I am told the 'pan/tilt rate on the wireless one is ideal, whereas I find the motor movements on the wired one a tad too fast, causing overshoot even when you get the hang of it. With my wired one, the wiring just comes in thru the front window and then plugs into the ubiqitous cigarette lighter that powers any needed auxilary item (as no butts are allowed on board :evil: my boats, unless they're female and in a tiny, tiny thong
).