Careful mount design keeps mirrors aligned

26 Mar 2004

Picking the right mirror mount for your optical application could save you from hours of frustration. Colin Freeland gives a round-up of the different types of mount available.

From Opto & Laser Europe April 2004

When it comes to constructing optical instruments and performing experiments, a set of mounts that can reliably hold a mirror and precisely adjust its alignment is essential. Although there are many different models on the market, all mirror mounts fall into one of three basic categories - kinematic, flexure and gimbal. Of these, the kinematic mount is the most common.

All mounts are capable of making two types of angular adjustment which are required for beam steering - tip and tilt, also known as pitch and yaw. Some also provide linear translation along the optical axis.

The most appropriate mount for your needs can be determined by assessing a few simple criteria. The most important considerations are the precision of the application, the degree of adjustment required and the long-term stability needed. Performance in these areas must then be balanced with your budget. Another issue is whether the optic is to be set and left, or adjusted frequently.

Kinematic mounts Kinematic mounts consist of a front plate to hold the optic and a base that attaches to a mounting post or an optical table. In many kinematic models the front plate is held in place by a series of springs. The number and strength of these springs depends on the size of the mount and the weight of the optic to be held.

It is important that the design of the base and front plate limits any unwanted movement while still allowing manual adjustment. A carefully designed mounting base can also aid alignment by providing more options for the mounting position. A true kinematic mount will constrain each of its six degrees of freedom (x, y, z, roll, pitch and yaw) to offer maximum stability and prevent any unwanted motion that could misalign an optic.

To achieve this, kinematic mounts have three points of contact: a cone; a v-groove (or two parallel rods); and a flat surface (plane). Most often, the cone interfaces with a metal ball in order to constrain motion in the x, y and z directions but allow pitch and yaw rotations. By contrast, the v-groove and the plane interface with a spherically-tipped drive screw. The drive screw is often made from a hardened stainless-steel ball that is attached to a thread. The v-groove is designed to constrain pitch and yaw movements but allow linear translation, while the third contact point, the plane, constrains roll.

Adjusting the two drive screws that interact with the v-groove and the plane allows the tip and tilt of the mirror to be precisely adjusted to steer the reflected light beam.

Some kinematic mounts achieve linear translation along the mirror's optical axis by replacing the hardened steel ball that interfaces with the cone with a spherically-tipped drive screw. Adjusting all three drive screws together enables the mirror to be moved a small distance (perhaps up to 10 mm or so) along the optical axis.

Similar in design to the kinematic mount, flexure mounts have two drives that use solid flexure springs to constrain unwanted motion instead of the cone, groove and flat of the kinematic design. Range of travel tends to be less than for kinematic mounts. Flexure designs may typically have a travel of ±5° whereas kinematic models can give more than 10°.

Both kinematic and flexure mounts rotate about a point that is not directly on the surface of the mounted optic. This means that when a rotation (pitch or yaw) adjustment is made it results in some unwanted translation. As mentioned previously, some mounts have three adjusters so that this can be accommodated. In other designs, the rotation point is moved closer to the surface of the mirror, creating less translation. Where this linear motion cannot be tolerated, gimbal mounts should be used because they provide true rotation without any translation. Because they have their rotational axes centred about the front surface of the mirror, the optical path length remains constant during angular adjustments.

Mounting Once inside the mount, the mirror rests on a two-line contact, which is machined into the mount and sometimes has soft inserts to protect the mirror. A set screw, ideally with a soft tip made from nylon, for example, then holds the mirror in place.

Most mounts have the optic inserted from the front, but in certain configurations it is useful to be able to mount from the back. This has the benefit that if you are switching an optic for one of a different thickness, the mount position need not be adjusted because the front of the mirror stays in the same plane. Mirror-mount front plates come in many forms, with some models also allowing the mounting of transmissive optics such as beamsplitters. Mounts with offset optics are available where the mirror or beamsplitter has to be in a confined position.

Material selection Aluminium is the most commonly used material in optomechanical mirror mounts. It is easily machined and can be anodized to give a harder surface finish. Black anodizing can also reduce unwanted surface reflections.

Using a steel adjustment screw in an aluminium mount would produce excessive friction, leading to poor adjustment resolution and long-term wear. As a result, for specialized applications such as use in a vacuum, it can be preferable for the body of the mirror mount to be made of brass or stainless steel. An adjuster can then be threaded directly into the body of the mount. This is particularly useful where space is limited and the mounts are very small. This is normally only used in OEM applications.

However, for most commercially available mounts, a threaded brass bush or something similar is mounted into the aluminium body. A stainless-steel threaded adjuster can then be used with minimal lubrication. It is the quality of the machining of the stainless-steel adjuster, the closeness of thread fit to the brass bushing and the choice of lubrication that most affect the feel of the mount and its stability.

To be able to achieve the theoretical resolution of the mount the user must be able to easily set the adjuster. The feel of the adjuster is crucial for fine-tuning the position. This is where the methods and materials used in manufacturing have the most effect on the cost and performance of the mount.

If there is end-float or "play" in the threads, you will not be able to position as accurately as a precision-lapped thread. There are also implications for long-term stability. The lubricant used also has an effect on the feel of the adjuster. High-quality threads require little lubrication, but economy mounts may use a thick grease to fill gaps in the threads.

One low-cost method of improving a mount's stability and making it less susceptible to long-term drift or accidental misalignment is to have locking mechanisms on the adjusters. Unfortunately, locking mechanisms will always slightly move the mount's alignment but manufacturers try to minimize this in different ways. Higher-quality adjusters do not really need locking mechanisms because the play on the threads is minimized and prevents gradual drift.

Adjusters may come with removable knobs and hex key drives, which are useful as the user need not touch the mount directly. They have the added benefit that when the mirror is adjusted the knob cannot easily be turned by mistake. They also take up less space in an instrument or on a bench where many mounts are positioned.

Resolution The angular resolution of a mirror mount is expressed in degrees or arc seconds (there are 3600 arc sec per degree). The stated resolution can be somewhat confusing as different manufacturers adopt different descriptions, but fundamentally the resolution of a mirror mount is determined by the smallest movement possible on the adjuster. An accepted standard is that the smallest rotation of the adjustment screw that can be made is 1°. Therefore the resolution is determined by the pitch of the thread (the number of turns per inch or millimetre) and the smoothness of the adjusters.

Metric threads are described as 0.25 pitch, for example (0.25 mm per turn), whereas imperial threads are expressed in TPI or threads per inch. A common specification is 80 TPI but higher-precision mounts use 100 TPI - the same as 0.25 mm pitch thread.

A typical 1 inch mirror mount with 100 TPI or 0.25 pitch adjusters would have a resolution of 2 arc sec. For the highest-resolution applications differential actuators are available. Where differential adjusters are used, resolutions down to less than 0.1 arc sec are achievable.

Most manufacturers offer a range starting with economic kinematic mounts with simple adjusters, and continuing up to the most expensive mounts with high-precision hand-lapped threads. Depending on the quality of adjustment required and the long-term stability of the optical set-up, a suitable mirror mount should be available for everyone.

Jargon buster Kinematic mount: A mount that uses the interaction between a series of set screws and a cone, groove and flat surface to make angular and translation adjustments.

Flexure mount: A mount that uses a series of solid springs to constrain movement of the front plate holding the mirror. As the springs are very good at constraining unwanted twist, this gives improved positioning accuracy.

Gimbal mount: Both kinematic and flexure mounts suffer from slight unwanted translation because the axes of rotation are not centred on the surface of the mirror.

In contrast, a gimbal mount provides angular adjustment without any translation. It is generally used for the most precise beam-steering applications.

Resolution: The smallest linear or angular adjustment that a mount can make. Resolutions down to 0.1 arcsec are possible with top-quality mounts equipped with differential actuators.