In photonics since 1999. Benelux technical support, answer within one working day.
Kinematic mirror mounts and where the drift comes from
A kinematic mount constrains an optic at exactly six degrees of freedom, so it returns to the same place after adjustment. Most alignment drift is not the mount itself: it is thermal expansion of the posts, an over-tightened optic, or a beam path that is longer than the bench is stiff.
THE SHORT ANSWER
A kinematic mount constrains an optic at exactly six degrees of freedom, so it returns to the same place after adjustment. Most alignment drift is not the mount itself: it is thermal expansion of the posts, an over-tightened optic, or a beam path that is longer than the bench is stiff.
| Degrees of freedom | 6, exactly constrained |
|---|---|
| Typical adjustment | Two axes, tip and tilt |
| Adjustment range | Around plus or minus 2 degrees |
| Fine screw resolution | Sub-arcminute |
| Drift sources | Thermal, mounting stress, post length |
| Locking | Clamp, without shifting the beam |
A kinematic mount holds an optic with exactly the six constraints needed to fix it in space, no more. Adding a seventh constraint is what makes a mount non-kinematic, and it is why an over-clamped optic does not return to the same position when you release and retighten it.
Where drift actually comes from
When alignment moves overnight, the mount is rarely the culprit. Four causes account for most of it.
Thermal. Aluminium expands roughly 23 micrometres per metre per kelvin. A post 200 mm tall moving through two kelvin changes height by about nine micrometres, which is nothing for imaging and a great deal for interferometry.
Mounting stress. An optic clamped too hard deforms. This shows up as astigmatism in the wavefront rather than as movement, and it is the most frequently misdiagnosed problem on an optical bench.
Lever arm. An angular error at a mirror becomes a positional error downstream, multiplied by the distance. A path twice as long is twice as sensitive to the same mount.
Screw hysteresis. Fine adjusters do not retrace their own path. Approaching a setting consistently from one direction removes a surprising amount of apparent instability.
Practical measures
Keep posts short, keep the beam low, tighten optics only until they stop moving, and let a newly built setup sit for a few hours before you trust an alignment. If drift persists after that, measure the room temperature before buying better mounts.
When to use it
- Any beam path that has to hold alignment over hours or days
- Optics that will be removed and replaced repeatedly
- Interferometry and other phase-sensitive work
- Long beam paths where small angular errors become large positional ones
What to watch out for
- Over-tightening a retaining ring bends the optic and shows up as astigmatism, not as drift
- Long posts amplify angular error at the far end of a long beam path
- Aluminium expands about 23 micrometres per metre per kelvin: a two-degree room swing is measurable
- Adjuster screws have hysteresis; approach a setting from one direction consistently
Still deciding?
Tell us the wavelength, the aperture and what you are measuring. We answer with a part number, a price and a lead time, usually the same working day.