Polaris® Side Optic Retention Mount for Ø19 mm Optics

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Each Polaris mount undergoes extensive thermal testing to ensure high-quality performance. Please see the Test Data tab for additional test results.

Features

• Machined from Heat-Treated Stainless Steel with Low Coefficient of Thermal Expansion (CTE)
• Hardened Stainless Steel Ball Contacts with Sapphire Seats for Durability and Smooth Movement
• Matched Actuators and Back Plate Provide Stability and Smooth Kinematic Adjustment
• Extensive Testing Guarantees Less than 2 μrad Deviation after 12.5 °C Temperature Cycling (See 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
• Patented Optic Bore Design with Monolithic Flexure Arm

Polaris^® Low-Drift Kinematic Mirror Mounts are the ultimate solution for applications requiring stringent long-term alignment stability.

Optic Retention
This Polaris mirror mount features a patented optic bore design with a monolithic flexure arm to hold the optic. This design provides high holding force and pointing stability while allowing quick and easy installation of the optic. For more details, please see the Test Data tab.

Polaris optic bores are precision machined to achieve a fit that will provide optimum beam pointing stability over changing environmental conditions such as temperature changes, transportation shock, and vibration. This mount's performance has been tested and verified with Ø19 mm optics that have a diameter tolerance of up to +0/-0.1 mm; performance will be diminished if the mount is used with optics that have an outer diameter tolerance greater than zero.

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 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
The Polaris mirror mounts are equipped with #8 (M4) counterbores for post mounting. Select mounts also include Ø2 mm alignment pin holes around the mounting counterbore, allowing for precision alignments when paired with our posts for Polaris mirror mounts. See the Usage Tips tab for more recommendations about mounting configurations.

Cleanroom and Vacuum Compatibility
The POLARIS-K19S4 is designed to be compatible with cleanroom and vacuum applications. See the Specs tab and the Design Features tab for details.

Polaris Mirror Mounts Test Data

All of the Polaris Low-Drift Kinematic Mirror Mounts have undergone extensive testing to ensure high-quality performance. Thermal Shock testing confirms the exceptional stability of the mounts and demonstrates that they reliably return to their initial position after a transient temperature shift. Interferometric wavefront distortion testing demonstrates the ability of Polaris mounts to secure an optic without significantly distorting the optical surface.

Vibration Testing

Purpose: This testing was done to determine how reliably Polaris mirror mounts behave when subjected to intense physical vibrations.

Procedure: A pair of POLARIS-K19S4 mirror mounts were mounted on Ø1" Posts for Polaris Mirror Mounts and secured to a stainless steel breadboard with POLARIS-CA1 clamping arms. Laser beams were reflected from the mirrors onto two position sensing detectors, located on the same breadboard. The entire platform was vibrated with a variable frequency and amplitude and the displacement of the beam on the detector was recorded. The two beam paths were oriented at right angles so that the vibrational motion was in a direction parallel to the face of one mount and perpendicular to the face of the other. See the video to the right for a demonstration of this vibration test.

Results: When subjected to vibrational frequencies as high as 100 Hz and accelerations as high as 6 g, the POLARIS-K19S4 mounts remained mechanically sound. The angular position of the mounts remained stable within about 10 µrad for both parallel and perpendicular vibrations.

Conclusions: Our POLARIS-K19S4 Ø19 mm Polaris mirror mounts provide exceptional performance, even under rugged operating conditions. As a result, these mounts are ideal for use in systems that require the greatest degree of stability when vibrational noise is expected.

Positional Repeatability After Thermal Shock

Purpose: This testing was done to determine how reliably the mount returns the mirror, without hysteresis, to its initial position. These measurements show that the alignment of the optical system is unaffected by the temperature shock.

Procedure: After mounting the Polaris to a Ø1" Post, the mirror and post assembly was secured to a stainless steel optical table in a temperature-controlled environment. The mirror was held using the flexure mechanism; see the Usage Tips tab for additional mounting recommendations. A beam from an independently temperature-stabilized laser diode was reflected by the mirror onto a position sensing detector. The temperature of each mirror mount tested was raised to 37 °C. 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 below, when the Polaris mounts were returned to their initial temperature, the angular position (both pitch and yaw) of the mirrors returned to within 2 µrad of its initial position. The performance of the Polaris was tested further by subjecting the 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 130 TPI adjuster on the POLARIS-K19S4 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.

Conclusions: The Polaris Mirror Mounts are high-quality, ultra-stable mounts that will reliably return a mirror to its original position after cycling through a temperature change. As a result, the Polaris mounts are ideal for use in applications that require long-term alignment stability.

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Thermal Repeatability for Ø19 mm Polaris Mirror Mounts

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The plot above shows the final angular position of the POLARIS-K19S4 for 10 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

Optical Distortion Testing Using a ZYGO Phase-Shifting Interferometer

Mounting stresses are responsible for the strain that results in optical surface distortion. Minimizing distortion effects is crucial; any distortion to the optic affects the reflected wavefront. Our Ø19 mm Polaris mount features a monolithic flexure arm that is designed to provide maximum stability while minimizing optic distortion. 

To determine the amount of optic distortion exerted on the mirror by the flexure arm, a ZYGO Phase-Shifting Interferometer was used to measure the wavefront distortion at different torque values (see the images below to the left). Based on results of the tests seen below, we recommend a torque of 3 - 4 oz-in for our Ø19 mm Polaris mount, at which the optic wavefront distortion is ≤0.1λ.

Please note that the optimal optic mounting torque can vary by ±1 oz-in due to variations in optic diameter and tolerance buildup.

Procedure:
A broadband dielectric mirror was installed into a Polaris mount using the setscrew to clamp down the flexure arm. Measurements of the optic distortion were then recorded using the ZYGO interferometer. Once each measurement was complete, the amount of force needed to push the optic out of the mount was measured to check optic retention. The wavefront distortion values shown here give peak-to-valley distortion across the entire optic, representing the worst-case scenario; the center of the optic exhibits significantly less distortion than the edge.

Results:
As seen in the table below, the peak-to-valley wavefront distortion was found to be ≤0.1λ when 3 - 4 oz-in of torque was applied to the setscrew of the Ø19 mm mount.

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POLARIS-K19S4 Wavefront Distortion for Setscrew Torque of 3.5 oz-in (See Table to the Right for Other Setscrew Torques)

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Details of the Polaris Design

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, 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 the Polaris 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 for the Polaris; this Polaris mount offers excellent performance without the use of adhesives. Instead, it uses a monolithic flexure arm to hold the edge of the optic. The monolithic design is less sensitive to fluctuations in temperature and induces less distortion on the optic surface than a simple setscrew retention design.

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 or bushings that exceed all industry standards so very little adjuster lubricant is needed. As a result, alignment of the Polaris mounts is extremely stable even after being adjusted (see the Test Data tab for more information). In addition, these adjusters have a smooth feel that allows the user to make small, repeatable adjustments.

Cleanroom and Vacuum Compatibility
The POLARIS-K19S4 is designed to be compatible with cleanroom and vacuum applications. It is chemically cleaned using the Carpenter AAA passivation method to remove sulfur, iron, and contaminants from the surface. After passivation, it is 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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The shaded 5 - 7 oz-in region in this plot denotes the recommended optic mounting torque for a 6 mm thick optic in Ø1" Polaris mounts with monolithic optic retention. Over torquing the monolithic flexure arm past the recommended 5 - 7 oz-in range can result in a significant increase in the surface distortion of mounted optics.

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A POLARIS-K05 mount can be mounted to a surface using a Ø1" Post for Polaris Mirror Mounts and a Polaris Clamping Arm. Using a 1.50" long post, the optical axis is 2.0" above the table surface.

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POLARIS-K1E Mounted Directly to Breadboard using 1/4"-20 to 8-32 Adapter

Through thermal changes and vibrations, the Polaris kinematic mirror mounts are designed to provide years of use. 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 the Polaris mounts. When mounting, we recommend using components fabricated from the same material, such as our Ø1" Posts for Polaris Mirror Mounts and Polaris Clamping Arm.

Use a Wide Post
The Polaris' performance is optimized for use with our Ø1" Posts and our POLARIS-CA1 clamping arm. These posts are made of stainless steel and provide two lines of contact with the mount, which help confine the bottom of the mount during variations in the surrounding temperature, thereby minimizing potential alignment issues.

Optic Mounting
Since an optic is prone to movement within its mounting bore, all optics should be mounted with the Polaris out of the setup to ensure accurate mounting that will minimize misalignment effects. We recommend using a torque wrench when installing an optic in the Polaris mounts. Over torquing the flexure-spring optic retainer can result in dramatic surface distortions (see the graph to the left).

Front Plate’s Position
The Ø19 mm Polaris mount is designed to allow adjustments of up to 8°. To achieve the best performance, it is recommended that the front plate be kept as close to parallel to the back plate as possible. This ensures the highest stability of the adjustments.

Mount as Close to the Table’s Surface as Possible
To minimize the impact of vibrations and temperature changes, it is recommended that your setup has as low of a profile as possible. Using short posts will reduce the Y-axis translation caused by temperature variations and will minimize any movements caused by vibrations. Mount the Polaris directly onto a flat surface such as a breadboard using a 1/4"-20 to 8-32 thread adapter (AE8E25E) or M6 x 1.0 to M4 x 0.7 adapter (AE4M6M). For direct mounting, the POLARIS-K1E must have the bottom two knobs removed, as in the photo to the right. By doing so, the instability introduced by a post will be eliminated.

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 LF1P polishing pad for the top and bottom of the post as well as the bottom of the mount.

Use Polaris-Specific Adjustment Tools
We offer stainless steel knobs for most of our Polaris adjusters, either included with the mount or available separately. For Polaris mounts with side hole adjusters, we recommend using the SA1 Side Hole Adjustment Tool, which features a precision-fit tip designed for the side holes to allow backlash-free adjustment. The stainless steel material is chemically cleaned for use in clean setups. 

Not Recommended
We do not recommend taking the adjusters out of the back plate, as it can contaminate the threading. This can 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 screws that secure the flat spring that holds the optic in place; only slight force is required to secure the optic in place.