For mounting Red Dot Sight, adapt Weaver Rail to Arca-Swiss compatible clamp?

[KirbyKrieger]

Hi.  Posted on DPR's Sony Alpha forum, but didn't get enough info.  Perhaps someone here can help out …

I just purchased a red-dot sight (RDS).  These are generally used by people when shooting guns.  I would like to adapt it for use when taking photographs of fast-moving birds with my telephoto lens (Sony 70-400G).  Many have done similar.  (For those who don't know, a RDS projects a small LED-generated image on a glass plate.  One sets the RDS so that this "dot" shows the exact center of the lens' focal area, and then frames the shot by by super-imposing the dot on the target.  The RDS _does not_ project any light into the scene.  It is _not_ a laser.)

The sight comes with a hot-shoe adapter.  These generally allow too much movement.  Since I use Sony cameras, I need an additional Sony/Minolta hot-shoe to standard hot-shoe adapter.  I don't think end result will be satisfactory.

My idea is to take advantage of the tripod collar on the lens, and mount the RDS to that (after rotating the collar 180° so the foot is on top of the lens).  This would be, of course, for hand-held shooting.

I keep an Arca-Swiss compatible plate on the tripod collar.  The RDS comes with a 4" long "20mm Weaver" rail (I think).  In standard use, this rail is screwed front and back to a rifle (afaict).  What I would like to do is attach the RDS to the Arca-Swiss plate on the tripod collar.

One way to do this is to mount the rail somehow on an Arca-Swiss compatible clamp.  (Such as this 1" Kirk clamp sold by B&H.)
Another way would be to find a "Weaver-to-Arca-Swiss" adapter.

It would save me trouble to do this with stand-alone parts, rather than have to pay someone to machine any parts I purchase.

Has anyone done such a thing?
Have any suggestions?

Thanks.

[MrSmith]

Not something I would have to do myself but if you get a countersunk 1/4 screw long enough to go trough an arca plate and into the tripod collar then you just need to drill and tap the plate to take a weaver base.
Any decent gunsmith should be able to source the mounts and do the drilling/tapping. I guess you would have to source the tripod screw.

[fennario]

Not sure the degree of precision you require, however, there will be a parallax error similar to using a rangefinder.  Red dot will need to be zeroed to a certain distance and the error will increase as the subject moves away from the zero point.  That said, not sure how much of a real-world effect this will have on focus/framing accuracy.

[KirbyKrieger]

Thanks.  I'm going to try it with a single screw.  If the RDS works well for my needs, I'll likely have a second screw put in.

[KirbyKrieger]

Not sure the degree of precision you require, however, there will be a parallax error similar to using a rangefinder.  Red dot will need to be zeroed to a certain distance and the error will increase as the subject moves away from the zero point.  That said, not sure how much of a real-world effect this will have on focus/framing accuracy.

Others have reported success in capturing birds-in-flight (even swallows) with an RDS mounted.  All of the RDS's have 2-axis correction.  As with you, I don't know how well it will work in practice.  I suspect (iow, I have no idea) that the useful range of distances (far enough to not fill the frame; not so far that the subject doesn't fill enough of the frame) falls within the offsets created by the difference between the sighting optics and the camera lens. The role of the RDS is in _framing_ the sighted object — the advantage is being able to center the camera using a larger view of the scene (wider angle), while the camera keeps its smaller telephoto view.  Actual focus must be done using the camera's auto-focus.  This is best done, according to what I've read, using the "wide" AF mode, though I'm sure it is somewhat camera-specific.

Once I get the rig mounted and I try it out, I'll post my "real-world" experience.