Polaris™ Low Drift Kinematic Mirror Mounts
|Item #||POLARIS-K05||POLARIS-K1 &|
|Optic Size||Ø1/2" and Ø12.5 mm||Ø1" and Ø25 mm|
|0.08" (2 mm)|
|0.08" (2 mm) Min|
|Adjusters||Three, 130 TPI||Three, 100 TPI||Two, 100 TPI|
|Resolution||~11 mrad/rev||~7 mrad/rev|
- Ø1/2" & Ø1" Mirror Mounts Provide the Ultimate in Thermal and Dimensional Stability
- Fabricated from Heat-Treated Stainless Steel
- Ball Contacts with Sapphire Seats for Durability and Smooth Movement
- Matched Actuator/Body or Actuator/Bushing Pairs Provide Smooth Kinematic Adjustment
- POLARIS-K05 and POLARIS-K1-2AH Have an Integrated Tapped Steel Body Design
- POLARIS-K1 and POLARIS-K1-H Have Bronze Bushing Inserts
- Extensive Testing Guarantees Less than 2 μrad Deviation after 12.5° Temperature Cycling (See the Test Data tab for Details)
The Polaris™ Kinematic Mirror Mounts are the ultimate solution for applications requiring stringent long-term alignment stability. Fabricated from heat-treated stainless steel, Polaris mounts utilize precision-matched adjusters and incorporate ball contacts and sapphire seats at all contact points. The epoxy used to bond the sapphires in place is baked using a NASA-approved low-outgassing procedure. The adjuster grease is ultra-high-vacuum-compatible low-outgassing PTFE grease.
During the design phases of these Polaris kinematic mirror mounts, extensive modeling and testing were used to determine the materials, components, and dimensional specifications that would result in optimum performance. The Polaris design addresses all of the common causes of beam misalignment, including temperature-induced hysteresis of the mirror position, drift, and backlash. Please refer to the Design Features tab for detailed information.
The Polaris mirror mounts are equipped with #8 (M4) counterbores for post mounting. We recommend using a stainless steel post such our Ø1" Posts. We also recommend using a torque driver to accurately install the optic and prevent optical surface distortion. Please see the Usage Tips tab for more information and other usage recommendations.
|Optic Size||Ø1/2" and Ø12.5 mm||Ø1" and Ø25 mm|
|Optic Thickness||0.08" (2 mm) Min||0.08" (2 mm) Min|
|Adjusters||Three, 130 TPI|
Matched Actuator/Body Pairs
|Three, 100 TPI|
Matched Actuator/Bushing Pairs
|Two, 100 TPI|
Matched Actuator/Body Pairs
Mechanical Resolution per Adjuster
|5 µrad (Typical); 2 µrad (Achievable)|
|Measured Adjuster Lock|
Mechanical Resolution per Adjuster
|Resolutiona||~11 mrad/rev||~7 mrad/rev|
|Front Plate Translation (Max)||5 mm||6 mm||N/A|
|Mechanical Angular Range||±5°||±4°|
|Front Plate Separation at Pivot Adjuster||1.5 ± 1 mm||3 ± 1 mm||3.175 mm|
|Beam Deviationb After Thermal Cycling||<2 μrad|
|Recommended Optic Mounting Torque||6 - 10 oz-in for|
6 mm Thick Optics
|6 - 10 oz-in for 6 mm Thick Optics|
|Maximum Front Plate Torque from Mounted Components||N/A||N/A|
|Mountingc||#8 (M4) Counterbore|
|Vacuum Compatibility||Grease Vapor Pressure: 10-13 Torr at 20 °C ,10-5 Torr at 200 °C|
Epoxy Meets Low Outgassing Standards NASA ASTM E595, Telcordia GR-1221
|Operating Temperature Range||-30 to 200 °C|
Polaris™ Mirror Mounts Test Data
All of the Polaris Low-Drift Kinematic Mirror Mounts have undergone extensive testing to ensure high-quality performance. After mounting the Polaris to a Ø1" stainless steel post, the mirror and post assembly was secured to a stainless steel optical table in a temperature-controlled environment. The mirror was secured using the flexure spring, not glued; 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.
For Comparison: To get a 1 µrad change in the mount’s position, the 100 TPI adjuster on the Ø1" Polaris 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 6 µrad.
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 so that the alignment of the optical system is unaffected by the temperature shock.
Procedure: The temperature of each mirror mount tested was raised to 37 °C. The elevated temperature was maintained for 60 minutes (soak time). 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 above, 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 best result achieved with a competitor’s mount (above center) was significantly worse. 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.
Drift After an Angular Adjustment Test
Purpose: To determine the long-term drift effects of making an angular adjustment. See the Drift and Backlash discussion in the Beam Misalignment tab for why this drift occurs.
Procedure: The pitch adjuster on the POLARIS-K1 was rotated 125°.
Plot Interpretation: The initial displacement at t = 0 min was greater than 2500 µrad with a correction back in the other direction to get close to zero on the position sensing detector. In order to show the long-term effects, the vertical axis scale is zoomed in. The initial displacement to start the test shows up as a vertical red line at the start of the plot.
Results: As can be seen from the plot to the right, the deviation in the beam propagation direction was less than 5 µrad of pitch rotation and less than 2 µrad of yaw rotation.
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. In addition, the high-precision, matched adjusters show very little drift even after making a large adjustment. As a result, the Polaris mounts are ideal for use in applications that require long-term alignment stability.
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.
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.
Considering how the mirror is secured in the mount is another important design factor for the Polaris. Other premium ultra-stable mirror mounts require the user to glue the mirror into the mount to achieve the best performance. In contrast, the Polaris mounts offer excellent performance without the use of adhesives. These mounts use a flexure spring that is pressed onto the edge of the mirror using a setscrew. Setscrews, when used by themselves to hold an optic, tend to move as the temperature changes. In contrast, the holding force provided by the stainless steel flat spring is sufficient to keep the mirror locked into place regardless of the ambient temperature.
We also offer extra low-optic distortion Polaris mounts with a flexure retaining ring optic mount.
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.
Click to Enlarge
POLARIS-K05 with mirror, mounted on Ø1" RS Series Post (mirror and post not included with mount)
Click to Enlarge
At zero torque, the sample mirror's flatness was λ/20 over the clear aperture (λ = 633 nm).
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.
Due to its relatively low coefficient of thermal expansion, 303 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.
Use a Wide Post
The Polaris' performance is optimized for use with a Ø1" optical post. These posts are made of 303 stainless steel and provide two planes of contact with the mount, which help confine the bottom of the mount during variations in the surrounding temperature, thereby minimizing potential alignment issues.
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. The graph to the right shows surface distortions that result from increasing torque values delivered by the TD24 torque wrench for a POLARIS-K1 with a Ø1", 6 mm thick BB1-E02 mirror mounted in it. The test was stopped once the distortion was greater than 0.3 waves.
Front Plate’s Position
Although the mount is designed to allow adjustments of up to 9.5°, 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.
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-K1 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 polishing pad for the top and bottom of the post as well as the bottom of the mount.
We do not recommend taking the adjusters out of the bushings, 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.
Thorlabs offers several different general varieties of Polaris mirror mounts, including standard low drift, SM-threaded, low optic distortion, and piezo actuated. The table below compares the features of all of our Polaris mirror mounts.
|Item # Suffixa||-K05||-K05T6||-K1||-K1-H||-K1-2AH||-K1T8||-K1T8H||-K05F6||-K1F6||-K1PZ|
|Mirror Mount Type||Standard Low-Drift||Low Optic Distortion||Piezo Actuators|
|Optic Size||Ø1/2" and Ø12.5 mm||Ø1" and Ø25 mm||Ø1/2" and|
|Ø1" and Ø25 mm|
|Optic Mounting||Leaf Spring||SM05-Threaded Bore||Leaf Spring||SM1-Threaded Bore, 8.6 mm Deep||Low-Distortion Ø1/2" Mount||Low-Distortion Ø1" Mount||Leaf Spring|
|Number of Adjusters||Three||Two||Three|
|Adjuster Type||3/16"-130 TPI Matched|
|1/4"-100 Matched Actuator/Bushing Pairs||1/4"-100 Matched|
|3/16"-130 TPI Matched|
|Adjuster Knobs||None||Three, Ø0.60" (15.2 mm)||None||Three, Ø0.60" (15.2 mm)||None||Three, Ø0.60" (15.2 mm)|
|Adjuster Lock Nuts||Not Compatible||Included||Compatibleb||Not Compatible||Compatibleb||Included|
|Mounting Holesc||Two # 8 (M4) Counterbores|
|Alignment Pin Holes||None||Two, Ø2 mm for DIN-7M6 Ground Dowel Pin|
at Each Counterbore
|None||Two, Ø2 mm for DIN-7M6 Ground Dowel Pin at Each Counterbore||None|
|Yes||With Bottom Two|