Polaris® Low-Distortion Kinematic Mirror Mounts for Ø2" Optics

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POLARIS-K2F Ø2" Mount Cutaway Diagram

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Spring with Indexing Tab

Features

• Machined from Stress-Relieved Stainless Steel with Matched Coefficients of Thermal Expansion (CTE)
• Spring-Loaded, Indexed, Three-Point-Contact Plate Secures Optic
• Hardened Stainless Steel Ball Contacts with Sapphire Seats for Durability and Smooth Movement
• Matched Actuator/Body Pairs Provide Smooth Kinematic Adjustment
• Extensive Testing Guarantees Less than 2 µrad Deviation after Temperature Cycling (See Thermal Test Data Tab for Details)
• Passivated Stainless Steel Surface Ideal for Vacuum and High-Power Laser Cavity Applications
• Custom Mount Configurations are Available by Contacting Tech Support

The Polaris^® Low-Distortion Kinematic Mirror Mounts (US Patent 9,599,786) provide long-term pointing stability while minimizing optic surface distortion introduced by the mount. The mount featured on this page is designed for Ø2" optics; other sizes of Polaris low-distortion mounts available from stock are listed in the table to the lower right.

Optic Retention
Each mount features a three-point-contact faceplate to mount the optic to reduce optical distortion compared to standard setscrew or retaining ring optic mount designs. An indexed retention spring between the optic and retaining ring eliminates bending moments on the optic and ensures the force on the optic remains constant over large temperature changes. See the Low Distortion tab and the Optic Installation tab for more details.

Polaris optic bores are precision machined to achieve a fit that will provide optimum beam pointing stability performance over changing environmental conditions such as temperature changes, transportation shock, and vibration. Each mount is designed to be used with a Ø2" optic that is within +0/-0.005" of the specified diameter; using an optic outside of this range will lead to diminished performance. For inquiries about mounts for other optic sizes, please contact Tech Support.

Design
Machined from heat-treated stainless steel, Polaris mounts utilize precision-matched adjusters with ball contacts and sapphire seats to provide smooth kinematic adjustment. As shown on the Thermal Test Data tab, these mounts have undergone extensive testing to ensure high-quality performance. The Polaris design addresses all of the common causes of beam misalignment; please refer to the Design Features tab for detailed information.

Post Mounting
These Polaris mirror mounts are equipped with #8 (M4) counterbores for post mounting. The Ø2 mm alignment pin holes beside the mounting counterbores allow for precision alignment when paired with our posts for Polaris mirror mounts. See the Usage Tips tab for more recommendations about mounting configurations.

Cleanroom and Vacuum Compatibility
All Polaris mounts, retaining rings, lock nuts, and locking collars sold on this page are designed to be compatible with cleanroom and vacuum applications. See the Specs tab and the Design Features tab for more information.

Installing an Optic into a Polaris^® Ø2" Low-Distortion Mount

Ensure that there is no grease, dirt, or dust in the optic bore or on the optic itself. Remove any particulates with clean compressed air and/or clean with methanol, acetone, or ethanol. The optic, indexed retention spring, and retaining ring are installed from the back of the mount, as shown in the animation below. Be sure the index tab is in the key way and touching the retaining ring; reversing the retention spring orientation will result in greater distortion of the optic. Gently turn the retaining ring using a spanner wrench, such as Item # SPW604, until the optic makes contact with the three fingers on the faceplate.

Securing an Optic by Turning Back the Retaining Ring
Continue to tighten until the spring is fully compressed into the pocket of the retaining ring. Once the retaining ring makes slight contact with the back of the optic, loosen it by 1.25 turns so that the spring pressure alone holds the optic in place. This will put the appropriate force on the optic, resulting in an additional distortion of a few hundredths of a wave for a mounted optic compared to the optic in its unmounted state, as shown in the graph below.

We do not recommend using a torque wrench for mounting an optic in our Ø2" Polaris mounts. The increased diameter of the optic cell makes it difficult to get an accurate reading; this can lead to mounting an optic with too much force, resulting in increased distortion of the optic face, or too little force, leaving the optic loose in the mount.

Optical Distortion
The graph below shows the effect of varying the amount of turns back of the optic retaining ring on the surface of a 12 mm thick mirror mounted in a Ø2" Polaris mount. The shaded region below illustrates the acceptable range of turns back. Minimal turns back should be used for setups that will be exposed to vibrations, while stationary setups may be able to accommodate more turns back. All flatness measurements are taken with a ZYGO interferometer, which uses a 633 nm beam. Please see the Low Distortion tab for more information on optic face distortion as a result of torque.

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The dashed line indicates 0.10λ for reference. 1.25 turns back from optic contact is recommended for secure mounting with minimal additional distortion caused by the mount. The error bars signify the standard deviation of the test results at every 0.25 turn interval. The spring will start to disengage from the optic at 3.25 turns back

Simulation Results for POLARIS-K1F Ø1" Low-Distortion Mount

Mounting stresses are responsible for the strain on an optic that results in optic distortion. Minimizing distortion effects is crucial; any distortion to the optic is transmitted to the light that is reflected from it.

Placing the 3-point mounting forces perpendicular to the face of the optic and indexing them directly opposite each other minimizes bending moments on the optic, thus reducing optic distortion. Typical 360° face mounts and side-located setscrew mounts create bending moments that cause optic distortion as seen in the drawings below. Spring loading the optic allows one to gradually tune the retention force, dampening stresses caused by temperature changes, and it provides a soft means of compensating for variances in manufacturing that can further produce stress on the optic. The finite element analysis results shown below show the displacement, stress, and strain applied to the optic for 20 oz-in of torque applied to the retaining ring of our POLARIS-K1F mount and 10 oz-in of torque applied to the setscrew of the equivalent standard setscrew mount. The Ø2" low-distortion Polaris mounts also have a similar design and therefore will show similar performance to the POLARIS-K1F mount.

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Strain Experienced by a Ø1" Mirror Installed in the POLARIS-K1F Mount

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Results of finite element analysis comparing the displacement experienced by a Ø1" mirror installed in a POLARIS-K1F mount with 20 oz⋅in of torque applied to the retaining ring (left) to the same optic installed in an equivalent standard setscrew mount with 10 oz⋅in of torque applied to the setscrew (right). Note: the magnitude of the worst distortion of the Polaris mount is less than the smallest distortion on the standard mount, even though the false color map utilizes the same hue.

Optical Distortion Testing Using a ZYGO Phase-Shifting Interferometer

Procedure
A BB2-E02 Ø2" Broadband Dielectric Mirror was installed into a POLARIS-K2F Low-Distortion Kinematic Mount using the procedure detailed in the Optic Installation tab; that is, the retaining ring was tightened until the spring was fully compressed, and then the retaining ring was loosened incrementally. The mount was then secured to a Ø1" stainless steel post using an 8-32 cap screw tightened to a torque of 16 in-lb. Results of each measurement are provided in the table to the right.

Results
As seen in the table to the right, the wavefront distortion was found to remain at or below 0.133λ.

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Retaining Ring Turned Back a 1/4 Turn

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Retaining Ring Turned Back a 1/2 Turn

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Retaining Ring Turned Back a 3/4 Turn

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Retaining Ring Turned Back 1 Turn

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Retaining Ring Turned Back 1.25 Turns

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Retaining Ring Turned Back 1.5 Turns

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Retaining Ring Turned Back 1.75 Turns

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Retaining Ring Turned Back 2 Turns

Positional Repeatability After Thermal Shock

This testing was done to determine how reliably these mounts return a mirror, without hysteresis, to its initial position so that the alignment of the optical system is unaffected by the temperature shock. During the testing phase, each Ø2" Polaris low-distortion kinematic mirror mount was attached to a Ø1" stainless steel post secured to a stainless steel optical breadboard in a temperature-controlled environment. Each mount was secured to a post by an 8-32 cap screw with 16 in-lb of torque. The retaining rings of these mounts were backed out 1.25 turns from optic contact; please see the Optic Installation tab for why we recommend against using a torque wrench with our Ø2" mounts. The beam from an independently temperature-stabilized diode laser was reflected off each mirror’s surface onto a position sensing detector.

Procedure:
Each mirror mount tested was heated, and the elevated temperature was maintained in order to soak the mount at a constant temperature. Then the temperature of the mirror mount was returned to the starting temperature. The results of these tests are shown below.

Results:
As can be seen in the plots in the expandable tables below, when the Polaris low-distortion mounts were returned to the initial temperature, the angular position (both pitch and yaw) of the mirrors returned to within 2 µrad of the initial position. The performance of the Polaris mounts was tested further by subjecting each mount to repeated temperature change cycles. After each cycle, the mirror’s position reliably returned to within 2 µrad of its initial position.

For Comparison:
To get a 1 µrad change in the mount’s position, the 100 TPI adjuster on the POLARIS-K1F mount needs to be rotated by only 0.05° (1/7200 of a turn). A highly skilled operator might be able to make an adjustment as small as 0.3° (1/1200 of a turn), which corresponds to a 6 µrad change in the mount's position.

Thermal Test Data

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3-Adjuster Low-Distortion Mount

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The plot above shows the final angular position of the POLARIS-K2F mount for 20 consecutive thermal shock tests. The change in temperature is the difference between the starting temperature and the temperature at the end of the test and includes factors such as the variation in room temperature.

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2-Adjuster Low-Distortion Mount

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The plot above shows the final angular position of the POLARIS-K2F1 mount for 20 consecutive thermal shock tests. The change in temperature is the difference between the starting temperature and the temperature at the end of the test and includes factors such as the variation in room temperature.

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Design Features of Ø2" Polaris Low-Distortion Mounts

Several common factors typically lead to beam misalignment in an optical setup. These include temperature-induced hysteresis of the mirror's position, crosstalk, drift, and backlash. Polaris mirror mounts are designed specifically to minimize these misalignment factors and thus provide extremely stable performance. Hours of extensive research, multiple design efforts using sophisticated design tools such as finite element analysis software, and months of rigorous testing went into choosing the best components to provide an ideal solution for experiments requiring ultra-stable performance from a kinematic mirror mount.

Thermal Hysteresis
The temperature in most labs is not constant due to factors such as air conditioning, the number of people in the room, and the operating states of equipment. Thus, it is necessary that all mounts used in an alignment-sensitive optical setup be designed to minimize any thermally induced alignment effects. Thermal effects can be minimized by choosing materials with a low coefficient of thermal expansion (CTE), like stainless steel. However, even mounts made from a material with a low CTE do not typically return the mirror to its initial position when the initial temperature is restored. All the critical components of these Ø2" Polaris low-distortion mirror mounts are heat treated prior to assembly since this process removes internal stresses that can cause a temperature-dependent hysteresis. As a result, the alignment of the optical system will be restored when the temperature of the mirror mount is returned to the initial temperature.

The method by which the mirror is secured in the mount is another important design factor; these Polaris mounts offer excellent performance without the use of adhesives. Instead, they have a three-point-contact faceplate, an indexed retention spring, and a stainless steel retaining ring that hold the optic in place. This design minimizes distortion on the optic surface while maintaining beam pointing stability.

Crosstalk
Crosstalk is minimized by carefully controlling the dimensional tolerances of the front and back plates of the mount so that the pitch and yaw actuators are orthogonal. In addition, sapphire seats are used at all three contact points. Standard metal-to-metal actuator contact points will wear down over time. The polished sapphire seats of the Polaris mounts, in conjunction with the hardened stainless steel actuator tips, maintain the integrity of the contact surfaces over time.

Drift and Backlash
In order to minimize the positional drift of the mirror mount and backlash, it is necessary to limit the amount of play in the adjuster as well as the amount of lubricant used. When an adjustment is made to the actuator, the lubricant will be squeezed out of some spaces and built up in others. This non-equilibrium distribution of lubricant will slowly relax back into an equilibrium state. However, in doing so, this may cause the position of the front plate of the mount to move. The Polaris mounts use adjusters matched to the body that exceed all industry standards so very little adjuster lubricant is needed. These adjusters have a smooth feel that allows the user to make small, repeatable adjustments.

Cleanroom and Vacuum Compatibility
All Polaris mounts sold on this page are designed to be compatible with cleanroom and vacuum applications. They are chemically cleaned using the Carpenter AAA passivation method to remove sulfur, iron, and contaminants from the surface. After passivation, they are assembled in a clean environment and then double vacuum bagged to eliminate contamination when transported into a cleanroom.

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Polaris Mounts are Shipped Inside Two Vacuum Bag Layers

The sapphire contacts are bonded into place using a NASA-approved low outgassing procedure. In addition, DuPont LVP High-Vacuum (Krytox) Grease, an ultra-high vacuum compatible, low outgassing PTFE grease, is applied to the adjusters. These features provide high vacuum compatibility and low outgassing performance. When operating at pressures below 10^-5 Torr, we highly recommend using an appropriate bake out procedure prior to installing the mount in order to minimize contamination caused by outgassing. Please note that the 8-32 and M4 cap screws included with the Polaris mounts are not rated for pressures below 10^-5 Torr.

Cleanroom-Compatible Packaging
Each vacuum-compatible Polaris mount is packaged within two vacuum bag layers after assembly in a clean environment, as seen in the image to the right. The vacuum-tight fit of the bags stabilizes the mount, limiting translation of the front plate due to shocks during transportation. The tight fit also minimizes rubbing against the bag, preventing the introduction of bag material shavings that would contaminate the clean mount.

In the vacuum-sealing process, moisture-containing air is drawn out of the packaging. This eliminates unwanted reactions on the surface of the mount without the need for desiccant materials. The vacuum bags protect the mount from contamination by air or dust during transport and storage, and the double-vacuum bag configuration allows for a straightforward and effective cleanroom entry procedure. The outer bag can be removed outside of the cleanroom, allowing the contaminant-free inner bag to be placed into a clean container and transferred into the cleanroom while retaining the benefits of vacuum-bag packaging. Inside the cleanroom, the mount can be removed from the inner bag when ready for use.

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POLARIS-K2F Ø2" Mount on a Ø1" Stainless Steel Pedestal Post

Polaris kinematic mirror mounts are specifically designed to provide excellent performance under thermal changes and vibrations. Below are some usage tips to ensure that the mount provides optimal performance.

Match Materials
Due to its relatively low coefficient of thermal expansion, stainless steel was chosen as the material from which to fabricate the front and back plates of these Polaris mounts. When mounting, we recommend using components fabricated from the same material, such as our Ø1" stainless steel posts.

Mounting Options
If post mounting a Polaris mount, we recommend using a stainless steel post such as our Ø1" stainless steel posts or Ø1" post extensions with our POLARIS-CA1 Clamping Arm. By choosing components fabricated from the same material as the mount itself, all expansions and contractions will occur at the same rate. In addition, it is best to use the shortest post possible. Alternatively, mount the Polaris directly onto a flat surface such as a breadboard using a 1/4"-20 to 8-32 thread adapter (Item # AE8E25E) or M6 to M4 thread adapter (Item # AE4M6M), which results in even better performance. Ensure that the mounting surface is highly flat, polished, and free of debris or scratches.

Mount Alignment
During alignment, minimized deflection will be achieved if the mount's front plate is kept parallel to the back plate. This ensures even thermal expansion of all the adjustment screws, causing the mounted mirror to translate in the Z direction as opposed to rotating during temperature changes.

Front Plate’s Position
Although the mount is designed to allow adjustments of up to 6.8°, to achieve the best performance, it is recommended that the front plate be kept as parallel as possible to the back plate. This ensures the highest stability of the adjustments.

Polish and Clean the Points of Contact
We highly recommend that the points of contact between the mount and the post, as well as the post and the table, are clean and free of scratches or defects. For best results, we recommend using a polishing stone to clean the table’s surface and a polishing pad for the top and bottom of the post as well as the bottom of the mount.

Not Recommended
We do not recommend taking the adjusters out of the body, as it can introduce contamination to the threading, which can consequently reduce the fine adjustment performance significantly. Also, do not pull the front plate away as it might stretch the springs beyond their operating range or crack the sapphire seats. Finally, do not over tighten the retaining ring that secures the indexed spring that holds the optic in place; only slight force is required to secure the optic in place. For more information, please see the Low Distortion tab. Do not adjust or remove the spring-loaded ball contact in the faceplate; the correct position is set at the factory.

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Adjuster Lock Nuts
Our Ø2" Polaris low-distortion mounts are compatible with the POLARIS-LN1 lock nuts, which are available separately below. The beam can be held on target with the adjuster thumbscrew or hex key while lightly tightening the lock nut by hand or with a thin-head wrench or cone wrench. For applications that require frequent tuning of the adjusters, the lock nuts only need to be lightly tightened by hand to a torque of approximately 4 to 8 oz-in (0.03 to 0.06 N·m). To avoid cross threading the lock nut, place it against the adjuster and "unscrew" the lock nut until you feel a slight drop; then thread the lock nut onto the adjuster. Each lock nut is pre-greased with the same ultra-high-vacuum-compatible, low-outgassing PTFE grease as the Polaris mounts and has been tested for adjuster fit.

Low Incident Angles
The recessed edge of our Polaris Low-Distortion Mounts make them ideal for applications requiring low angles of incidence. The illustration to the right shows two Polaris Low-Distortion Mounts with Ø1/2" mirrors (green) used at a near-normal incident angle.