What the two profiles actually specify
Two trapezoidal bar profiles, known everywhere by the names of the manufacturers who popularised them, and sold as V-size and D-size. The narrow profile, generally called Vixen-style, is commonly quoted at about 44 mm across and is what almost every small and medium telescope carries. The wide profile, generally called Losmandy-style, is commonly quoted at about 75 mm — three inches — carries heavier tubes and resists flexing far better across a long one.
Both figures are trade conventions rather than published dimensions, which is exactly why measuring your own saddle slot beats reading a listing.
A saddle on a mount is machined for one or the other, and an increasing number are cut to accept both. That dual saddle is the reason many owners never discover that two profiles exist until they change mounts.
What the convention actually fixes is a shape and an approximate width. It does not fix a length, a hole pattern, a thickness or a tolerance.
What it does not promise
A matching profile does not promise a matching hole pattern. The threaded points on a tube or a set of tube rings differ between makers, and a bar with two fixed holes is far less adaptable than one with a long slot.
It does not promise rigidity. A thin bar of the correct profile flexes under a long tube, and that flex appears during a guided exposure as movement the software corrects in the wrong direction.
And it promises nothing about safety in the clamp. A bar without stops at its ends can slide straight out of a saddle the moment the clamp is loosened, which is the most predictable accident in the hobby and the cheapest to prevent.
What it works with
The dovetail is the single joint between everything above the mount and everything below it. It carries the tube, and the upper face of a bar frequently carries a guide scope or a second accessory of its own.
On this site it appears on dovetail bars themselves, on every mount family, on guide scopes that attach through one, and on the telescopes that arrive with a bar already fitted — which is worth checking before buying either.
The questions that keep arriving
Will a narrow bar work in a wide saddle?
Only if the saddle is a dual design built to accept both, and many are. A wide-only saddle cannot grip a narrow bar securely, and improvising with packing pieces puts an expensive tube on an unreliable joint.
Which profile should I prefer?
The one your mount already accepts. Where there is a genuine choice — a new tube and a dual saddle — the wider profile flexes less under a long tube and matters most for imaging; the narrower is lighter and entirely adequate for visual use on a small refractor.
How do I tell which one my mount has?
By measuring the width of the saddle slot rather than by looking at photographs. About 44 mm is the narrow profile and about 75 mm the wide one — obviously different once measured, and easy to confuse in a listing image where nothing gives the scale.
Why do bars from different makers not always fit?
Because there is no published standard. The angle and the width are conventional rather than specified, and small differences between manufacturers are common. A clamp that closes fully on one bar can bottom out on another of the same nominal profile.
Last reviewed 17 September 2026