Advanced equipment and processes have made it possible to investigate phenomena with dimensions measured in nanometers. For example, phase-shifting optical interferometers can now measure surface roughness with a resolution of about 1 nm. In the semiconductor field, integrated circuits with submicron line widths are now being produced. Applications like these have created the demand for equally innovative vibration isolated support structures which provide the stability needed for such measurements and processes. Successfully meeting this level of stability requires careful attention to the problems of maintaining extremely precise relative alignment of the various elements of the system.
The Problem: Relative Motion
Vibration can be a limiting factor in the performance of a wide variety of research and production applications. Consider the task of photographing a highly magnified image. The microscope and camera optics together determine where on the film plane each point of the object is imaged. During the exposure time, if every point of the optical system; illuminator, sample, microscope optics, camera optics and film plane; moves exactly together so that there is no relative motion, the image will be clear. If, on the other hand, there is relative motion of the sample with respect to the objective lens, the image will be blurred.
The Goal: A Perfectly Rigid Structure
Since it is not possible to completely eliminate the sources of vibrational disturbances, the goal is to reduce relative motion between different elements by connecting them with a structure that is as rigid as possible.
In a perfectly rigid body, which exists only in theory, the distance between any two points remains constant in time. In other words, the size and the shape of the body do not change while it is undergoing force inputs from vibrations, static forces or temperature changes. If all of the elements are mounted together to form an ideal rigid body, the different elements will not move relative to each other and system performance will not be impaired.
Reality: Environmental Effects
Since it is impossible to create a perfectly rigid structure, an effective vibration isolation system must take into consideration these factors:
Dynamic Forces (Vibration)
Dynamic forces cause structural deformations that vary with the frequency of the driving force. Structural resonance can amplify the relative motion between optical components.
- Connect all of the critical elements together in a dynamically rigid structure that is designed to eliminate (damp) structural resonances.
- Isolate the system from vibration with mechanical filters or active cancellation technology.
Static forces cause deformations that are constant in time. However, the addition or movement of equipment in the system will change the static forces and cause misalignment of system elements.
- Build a statically rigid structure that deforms as little as possible under the application of external forces.
Non-uniform temperature changes usually cause a slow bending of the structure, with time constants of one hour or more. The key techniques for reducing thermal effects are:
Newport table systems minimize the relative motion of a large array of sensitive scientific and production equipment.
- Control the environment to reduce temperature variation.
- Design the structures to be as insensitive to temperature as possible.
Newport Optical Bench SystemsThe Most Rigid Isolated Structures Available
Newport invented the honeycomb optical bench and continues to produce the finest vibration control systems available for nano-technology applications. Newport advanced isolation systems offer the best vibration filter systems available. Super rigid honeycomb structures further minimize relative platform motion due to dynamic, static and thermal forces. Unsurpassed attention to detail and quality places Newport optical benches in a class of their own. If your system carries the Newport name, it is the best.
Super Rigid Honeycomb Core Construction
Optical tables provide a rigid platform for high-precision optical experiments and systems. They are designed to eliminate errors caused by relative motion between optical components in the beam path. Rigidity is the primary consideration in optical table design. Table rigidity can be quantified in terms of static or dynamic rigidity.
- Static rigidity describes the ability of an optical table to resist deflection when the static; or quasi-static; load distribution is changed. Static rigidity defines the table performance when stages move across the table or equipment is relocated, added or removed.
- Dynamic rigidity describes the ability of an optical table to resist deflection in response to mechanical excitation. Dynamic rigidity defines the table performance in response to floor vibration, acoustic noise and mechanical sources on top of the table.
Honeycomb is commonly used to produce very low weight, highly rigid structures. Reduced weight dramatically improves the dynamic rigidity of the structure by moving structural resonance modes to higher, less detrimental frequencies. The structural resonance modes are the frequencies at which the platform deflects, thus causing relative motion across the optics mounting surface. For a given input vibration force, the deflection will be reduced as the mode frequency increases. This is the primary reason why steel honeycomb has replaced granite in most high end optical applicationssince granite is relatively heavy, the resonance modes occur at lower frequencies and therefore produce higher amplitudes of surface deflection.
Ordinary small-cell coreNewports high-performance trussed core
- Trussed Core DesignNewport innovated the trussed core design to maximize the tables rigidity to weight ratio. The trussed core design uses an additional steel member to bridge across the center of the honeycomb cell. This extra mechanical component significantly stiffens the cell with very little increase in weight. The truss improves both the static and dynamic rigidity and increases the structures point load carrying ability. Most other manufacturers use a traditional open cell construction and cannot match the static rigidity of a Newport table.
Trussed core design offers superior rigidity.
- Vertically Bonded CoreAll Newport tables are vertically bonded along the height of the honeycomb core. Most other manufacturers forego this extra step because of added expense and time required to apply adhesive to each individual core member. However, vertical bonding is the most important step to maximize the tables rigidity-to-weight ratio. An additional benefit is the introduction of constrained layer damping effects into the core construction.
- Triple Core InterfaceThe trussed core design enables a triple core interface at each honeycomb cell. Whereas open core designs only have two structural members at each interface, the Newport table has three. Since most manufacturers use 0.010 thick steel sheet, their core interface has a thickness of 0.020 inches. The Newport design offers a 50% increase in local rigidity by using a 0.030 inch support member. Additionally, the three sheets are fully bonded for the full table height producing greater stiffness and constrained layer damping.
- Custom CoresNewport offers the greatest experience and core selection available. Newport commonly builds trussed cores in several sizes, open aluminum cores for very lightweight structures and perforated cores for high vacuum or cleanroom applications. Other constructions include egg crate and tubular structure designs. This allows extending Newports capabilities beyond optical tables to build honeycomb towers for drawing optical fibers, vertical systems for large laser beam lines, pedestal flooring systems for semiconductor manufacturing equipment and a large variety of instrument platforms, gantries and bridges.
Superior Flatness/Thermal Stability
Beam path stability is a primary requirement for many optical experiments and processes. Newport meets the challenges of leading edge nano-technology by offering the best surface flatness and thermal stability available in a honeycomb structure. Whereas other manufacturers seek to make tables more quickly, Newport takes the extra steps to focus on making the best.
These precision platens permit manufacture of large tables on a single, ultra-flat surface instead of multiple granite platens, which compromise end-to-end flatness.
- Precision Bonding PlatensMetrology quality steel assembly platens are used to ensure the superior flatness of our optical tables. The platen supports the top surface of the table during the assembly and bonding process and imprints the top surface with the same flatness characteristics. Routinely verified with optical interferometers, these platens are the largest and flattest in the industry. These platens allow Newport to build the worlds largest optical tables.
- Gravity Pressure BondingApplying constant pressure across the optical table during the bonding process ensures that the top surface conforms to the platen flatness. Newport uses gravity pressure bonding to control the load across the table surfaces and to ensure process consistency from table to table. Other manufacturers use hydraulic presses to speed up the assembly process. However, the hydraulic pressure must be monitored for consistency and may apply uneven forces across the table top.
- Low Thermal Stress BondingSlow curing of the table bonds at room temperature is critical both to maintain table flatness and ensure long term stability of the table structure. Fast curing adhesives heat the core and lock thermal stresses into the table. Some manufacturers apply heat to the table to improve their production cycle times. Temperature induces stress causing the table to bow when it is returned to room temperature. Temperature curing can also induce long term bowing, and in extreme cases delamination of the core, top or bottom surface. Newports slow cure method takes longer but allows production of the flattest tables available. Newport also provides a lifetime guarantee against delamination with each table.
- Super Invar and other Advanced MaterialsNewport has more experience with advanced materials than anyone else in the optical table industry. Newports long relationship with the Aerospace industry and advanced labs all over the world has given the Newport Vibration Control team unsurpassed materials capabilities. Newport is the premier supplier of Super Invar tables for the absolute leading edge of optical research. Super Invar exhibits almost no thermal expansion near room temperature. Newport also has extensive experience with very light, highly stable carbon structures. For the ultimate in flatness, Newport can also provide granite platforms, granite/-epoxy composites or granite/honeycomb composites.
Excellent Damping Properties
Experimental or process disturbances caused by relative motion between optical components generally occur at the structural dominant bending or torsional modes. In addition to pushing these natural modes to higher, less detrimental frequencies, a major advantage of honeycomb over granite is the high level of damping present in the honeycomb structure. Damping attenuates the amplitude of the natural modes and reduces the relative motion across the table surface. In very precise applications such as optical interferometry, maximizing the tables damping properties is absolutely critical.
Three types of damping are used for optical tables: Narrow Band Tuned Damping, Broadband Tuned Damping and Constrained Layer Broadband Damping. Newport is the only company that offers all three forms of damping in a single optical table.
Newport tuned dampers (left) concentrate damping where its needed most, at the frequencies of resonance modes. Since broadband dampers (right) are designed to provide moderate damping over a wide range of frequencies, they are not as effective at damping the modes of table vibration.Undamped tableNarrow band tuned damping
- Narrow Band Tuned Damping Using Vibration AbsorbersNewport is the only manufacturer to build narrow band vibration absorbers into an optical table. These vibration absorbers allow tuned damping of specific modes. Narrow band tuned damping is the most effective means for eliminating structural resonances. These dampers selectively cancel vibration modes to minimize the Dynamic Deflection Coefficient and make the table behave more like an ideal rigid structure. For very large structures, such as optical tables over 15 feet in length, that exhibit structural resonances below 150Hz, narrow band techniques are the only effective means of damping.
Narrow band tuned dampers can only be found in Newport RS Series tables. Newport carries in stock over 200 individual dampers plus damp modes from 20480 Hz. A trained vibration engineer tests each table to determine the natural modes of the structure.
Narrow band vibration absorbers are then selected to eliminate these modes and installed in the highest amplitude position (usually the corners). Different grades are available to offer varying levels of damping. In applications where heavy payloads can significantly effect the structural modes of the table, Newport can simulate the load and optimize the tuned damping. When damping doubler systems, where two or more tables are rigidly connected, the tables are assembled and tuned as a monolithic structure.
- Broadband Damping: Tuned Vibration AbsorbersBroadband damping is less effective than narrow band techniques. However, Newport tables offer a variety of broadband damping mechanisms that further improve table performance. The dash pot system contained in Newports tuned vibration absorbers offers higher frequency broadband damping. This broadband damping extends over several octaves above the tuned frequency of the vibration absorber.
Undamped breadboardConstrained layer damping
- Broadband Damping: Constrained Layer Core InterfaceConstrained layer damping structures usually consist of two or more metallic sheets separated by a compliant material. The Newport vertically bonded trussed honeycomb core consists of three metallic sheets separated by an adhesive. Although the adhesive is rigid, its damping factor is much higher than that of steel and introduces substantial damping into the core. This construction produces substantial constrained layer damping at each core interface.
- Damped Working SurfaceThe top of a Newport table is bonded to a polymeric material that serves to seal the honeycomb core and damp the working surface. The polymer experiences the same bending and shear stresses as the stainless steel top. Since the damping factor of this material is much higher than that of steel, considerable damping is introduced into the work surface. This design eliminates the skin resonance problems found with undamped table to designs.
- Damped Table SidesThe sides of a laboratory grade Newport table are made of a highly damped, epoxy sealed wood composite. Similar to the panels used in high end audio speakers, composite wood materials are acoustically dead. Compared to the metal sides used by most other table manufacturers, the composite wood sides offer significant damping to the structure and eliminate another source of resonance. Knock on the side of a Newport table to feel the difference. Newport offers steel sides on cleanroom and vacuum compatible products.
Optical laboratories and high technology industries require high levels of product cleanliness.
- Pre-tapped and Washed WorksurfacesAll Newport worksurfaces are precision tapped and ground flat with a non-glazed finish before bonding to the sealed honeycomb core. They are then cleaned with automated industrial washing systems to ensure that all cutting fluids and metal particles are removed before bonding. Cleanroom products undergo a patented process to further protect the work surface before it is bonded to the honeycomb structure. Other manufacturers tap the grid on their optical tops after assembly and introduce contamination into the core or sealed hole cups. Even after extensive cleaning of post tapped tables, substantial residues still remain.
- Countersunk Mounting HolesAll tapped holes in a Newport table are countersunk and deburred. This step ensures easy thread engagement and eliminates a possible contamination problem. Tables without countersunk holes can shed metal particles as mounting bolts strip burrs from the thread. Non-countersunk holes are also more prone to cross threading and damage.
- Non-Corrosive Individually Sealed HolesEach tapped hole in a Newport optical table is registered to the core and individually sealed with a non-corrosive conical cup. The cup seals the honeycomb cell to prevent laser dyes, cooling fluids or the occasional mislaid cup of coffee from contaminating the table interior. Sealing each hole individually eases cleanup by eliminating places for liquids to hide. The conical design of the sealing cup aids in removing any remaining residues. A polymeric cup material is used to prevent particle shedding and breach of the sealing system due to corrosion. The cup is impervious to acids, bases and common laboratory solvents.
- Newport cups are built into a complete sheet and vacuum bonded to the top surface to eliminate all air contaminants. This step eliminates the leakage problems found in competitive cup techniques.
- Cleanroom Compatible Bonding AdhesivesAerospace grade adhesives are used to bond the core of all Newport optical tables. These adhesives meet NASA low outgassing requirements and are approved for high vacuum use. They are cleanroom compatible and do not shed or cause particulate contamination. Newport uses automated measuring and mixing systems to ensure adhesive batch consistency.
- Hermetically Sealed Vibration AbsorbersTuned damping vibration absorbers are hermetically sealed and fully qualified for cleanroom and high vacuum use.
- Perforated Core DesignsPerforated cores are available for high vacuum or cleanroom use. Each honeycomb cell of these special designs is ported to allow a high vacuum to be drawn. Laminar flow designs for cleanroom floor structures (FabFloor) combine the perforated core with a table airflow system.
|Trussed core design
||Better rigidity-to-weight ratio
Better static rigidity
Better dynamic rigidity
Increased point load carrying ability
|Vertically bonded core
||Better rigidity-to-weight ratio
Better static rigidity
Better dynamic rigidity
Increased point load carrying ability
|Triple core interface
||Better rigidity-to-weight ratio
Better static rigidity
Better dynamic rigidity
Increased point load carrying ability
|Low thermal stress bonding
||Improves table flatness
Eliminates long term table bowing
|Narrow band tuned damping
||Most effective method of eliminating specific vibration modes
Much more effective than broadband at low resonance frequencies
||Reduces the amplitude of vibration modes across broader frequency ranges
More effective than constrained layer over specific frequency bands
|Damped top skin
||Reduces ringing of work surface
|Individually sealed holes
||Prevents liquid spill from moving to other honeycomb cell locations or contaminating the table interior
|Non-corrosive sealed holes
||Prevents long-term table degradation from corrosive liquid spills
Newport Optical Bench Isolation SystemsThe Most Effective Vibration Filtering Available
Newport invented the Stabilizer isolator and introduced a new level of vibration isolation to the scientific industry. The Stabilizer features Newports proprietary hybrid chamber and laminar flow damping system to provide isolation from even the lowest amplitude vibrations. When used with extremely rigid tuned damped tables, Newport optical bench systems offer unmatched performance. It is no wonder most leading edge researchers will only accept Newport in their laboratories.Section view of a Stabilizer I-2000 Isolator.
Pneumatic isolators filter vibration before the mechanical noise can reach the optical bench work surface. Improved vibration isolation reduces errors caused by relative motion between optical components in the beam path. Pneumatic isolators combine with the optical table and payload to form a mass/spring/damper system. Pneumatic systems are used instead of mechanical springs since they offer self-leveling and minimize the effect of varying mass on isolation. The performance of the isolator is defined primarily by its natural frequency and damping characteristics:
Transmissibility plot shows damping effects.
- Natural Frequency (Natural Mode, Resonance)The pneumatic isolator is essentially a simple harmonic oscillator that uses the fast roll-off at higher frequencies to act as a low pass mechanical filter. Below the natural frequency of the harmonic oscillator the isolator is essentially rigid and vibration is passed directly to the platform. At the natural frequency vibration is actually amplified. Therefore a primary goal is to lower the natural frequency since this improves low frequency isolation and overall isolation bandwidth.
- DampingAnother primary goal is to damp the harmonic oscillator amplitude at resonance. This lowers the magnification of vibration at low frequencies and improves system stability. Unfortunately there is a compromise between the natural frequency and damping as damping is increased, the isolator natural frequency moves slightly higher, and higher frequency isolation is decreased.
Conventional isolators use a compliance chamber to act as an air spring and a damping chamber to increase system stability. The two chambers are connected through a thin tube or orifice.A conventional pneumatic isolator with damping.
Damping efficiency is proportional to the air flow and pressure drop through the damper. Because the hybrid-chamber design minimizes air volume between the piston and damper, there is a better linkage between piston motion and damper air flow. This generates a higher damping force for a given piston displacement for faster, more efficient damping of vertical motion.
- Compliance ChamberThe compliance chamber is sealed with a flexible diaphragm to form a piston and support the optical table on compressed air. If the piston is pushed further into the compliance chamber, the pressure of the gas increases and provides a restoring forcesomewhat like a soft spring. The isolation performance is primarily related to the volume of the compliance chamber. As the compliance chamber volume is increased, the natural frequency is decreased.
- Damping ChamberAfter the compliance chamber, air is pumped to the damping chamber through a flow restricterusually a thin tube or orifice. The restricter dissipates energy in the air and essentially damps the system. The design of both chambers and the restricter must be optimized to minimize the natural frequency/damping trade-off. When added together the volume of these chambers is relatively large and unresponsive due to the amount of air that must pass between them.
Stabilizer Isolator Design
Newports proprietary Stabilizer design offers a unique and greatly improved approach to isolator design. The Stabilizer provides significantly more damping of the natural frequency amplitude due to the closer coupling of the piston and damping element. The result is improved isolation and better stability when compared to conventional isolators.
- Hybrid ChamberThe primary innovation of the Stabilizer isolator is the Hybrid chamber system. The hybrid chamber configuration enhances damping efficiency by minimizing air volume between the piston and the damping elements. Rather than the conventional design that features two equivalently sized chambers, where the first volume is the compliance chamber and the second the damping chamber, the hybrid chamber uses the entire isolator volume for compliance. The initial chamber is reduced dramatically in size. This forces the isolator to use the second chamber as part of the compliance volume. The hybrid chamber offers much larger compliance volume for a given package size. The larger compliance volume lowers the frequency of the natural mode and improves isolation.
The natural frequency of a pneumatic isolator decreases as the compliance volume increases, thereby improving the isolation performance. The hybrid chamber design maximizes the compliance volume; Ultra-soft diaphragms reduce the stiffness limitation.Isolator natural frequency vs. compliance chamber volume.
Damping efficiency is enhanced by a laminar flow damping orifice. Instead of a single orifice, the damper element design contains thousands of tiny orifices that help maintain laminar flow for improved damping efficiency over a wider range of operating conditions.
- Laminar Flow DampingConnecting the hybrid chamber volume is a laminar flow element. This element is made of porous metal and adds resistance to the airflow as it is forced between the hybrid chamber volumes. The resistance added to the airflow significantly damps the amplitude of the natural mode with minimal effect of the isolation bandwidth. The laminar flow element also improves isolator settling time and responsiveness. The airflow volume can be reduced in the factory to add higher levels of damping.
- Ultra-Soft DiaphragmsOne of the limitations on pneumatic systems is the diaphragm stiffness. The stiffness of the diaphragm limits the isolation improvements when using a larger compliance volume. Newport uses custom molded ultra-soft diaphragms to maximize the benefits of the hybrid chamber design.
High Accuracy Leveling Valve Systems
Pneumatic isolators commonly use valves to stabilize and relevel the optical table after disturbance. Three two-way valves are supplied with each Newport system to control the platform in three degrees of freedom (vertical, pitch and roll). The valve is mounted to the isolator and senses the floating platform height with a lever arm. As the table is disturbed the lever arm moves and actuates the leveling valve to add or exhaust air in the pneumatic isolators as required to maintain platform height. Air is only supplied when the system is releveling.Newport valves are designed specifically for optical table systems.
Newport manufacturers its own custom valve designed specifically to meet the needs of optical table users. Other manufacturers use less than optimal off-the-shelf valves and cannot offer high accuracy releveling. Only 3 leveling valves are required regardless of the number of isolators in the system.
- Airflow Control ValvesNewport provides needle valves to control the airflow from the leveling valve to the hybrid chamber. This system offers a much higher level of user control and allows the system response to be optimized for any specific optical table system.
- EAR Lever Arm ConnectionNewport uses a specially designed EAR damping material to connect the lever arm to the optical table. This material prevents high frequency vibrations from traveling through the arm to the table. Other manufacturers use common foam for this purpose, however, the foam increases releveling errors and degrades over time.
- Pressure GaugesPressure gauges are included on the leveling valves to monitor the pressure inside the isolator.
Very Low Amplitude Horizontal Isolation
The pneumatic isolator offers isolation in the vertical direction; horizontal isolation is achieved through a mechanical filter system. Many manufacturers use some form of a bearing surface to provide a pivot or rolling surface for vibration filtering. However these contacting systems always have mechanical defects and surface roughness between the bearing surfaces. The bearing surface introduces frictional noise and limits the isolation of very low level vibrations that commonly disturb high accuracy systems.
Trifilar PendulumNewport uses a frictionless trifilar pendulum system for horizontal vibration isolation. The natural mode of the pendulum is very low in frequency, typically 1 to 2 Hz depending on the length of the pendulum. Above the natural frequency the pendulum isolates vibration. Since the system operates by flexing, it offers no frictional resistance to motion and can isolate very low amplitude mechanical vibration.Diagram of Newports patented horizontal isolation piston.
Viscous DampingThe natural frequency of the pendulum is damped using low vapor pressure oil. This system offers a significant decrease in the amplitude of the natural mode without adding higher frequency frictional noise.
Superior Mechanical Design
Newport has refined our laboratory grade pneumatic isolator for many years and offers a variety of features not found in imitative products.
Interchangeable spacer bases of different heights permit convenient cost-effective adjustment of working surface height or retrofitting of caster systems.
- Self-Centering (US Patent 5,071,108)Only Newport offers a self-centering device to ensure unrestricted piston motion. This patented feature eliminates problems caused by shorting out the vibration isolator during table set-up.
- Height AdjustmentLaboratory grade pneumatic isolators include a mechanical height adjustment system to allow improved table leveling, height control and compensation for non-level floors.
- Modular Support BasesAll LabLeg support systems are built on modular support bases. These support bases offer seven standard heights to choose from. Each system can be modified to a new height by simply ordering new support bases. Rigid legs can be upgraded to pneumatic systems by simply ordering the top assembly.
The tie-bars on ordinary tables can actually amplify floor vibrations.
- Freestanding DesignNewport legs come standard without any mechanical connection between them. Freestanding legs are preferred for unlevel floors and produce better decoupling from vibrations. Tie-bar systems can introduce significant structural resonances in the 25100 Hz frequency range. Newport only recommends tie-bar systems if the table will be moved frequently and requires a caster wheel system.
- SafeLockSafeLock mounting brackets offer a safe and reliable connection of the isolator to the optical table. The brackets use a slotted connection to make installation easier. Numerous SafeLock secured systems have survived earthquakes throughout California and Japan. Earthquake restraints are available as a table accessory.
|Hybrid chamber design
||Lowest natural frequency offers maximized isolation bandwidth
Optimized damping capabiity
Compact design is 30% smaller than conventional isolators
Improved isolator responsiveness
Improved high center of mass stability
|Laminar flow damping
||Less amplification at resonance
Improved setting time
Very little effect on isolation bandwidth
Adjustable damping for special applications
||Improved low frequency isolation
Improved isolation of low amplitude vibrations
|High accuracy leveling valves
||More accurate leveling of table
Improved repositioning of table after disturbance
Allows table-to-table beam pointing applications
|Airflow control valves
||Allows optimization of pneumatic control
|EAR gain arm interface
||Decouples sensor arm vibration from table
Better leveling accuracy
Does not degrade like foam pads
|Trifilar pendulum horizontal isolation
||Zero-friction design enables isolation to much lower vibration levels
Improved re-positioning after table disturbances
||Ensures isolator alingment
Elimnates horizontal shorting
||Adaptable to uneven floors
Allows alignment of separate tables
|Modular support bases
||Allows system height to be field modified
Allows field integration of tie-bar caster system
|Free standing design
||Better isolation performance
Elminates structural resonances found on tie-bar systems
Adaptable to uneven floors
Frequently Asked Questions About Optical Table Design
Is there a difference in honeycomb core designs?
Yes. Newport uses a trussed core design that, although lighter than competitive cores, actually exhibits better static rigidity. The higher rigidity is due to the vertical bonding provided at the triple core interface. Other manufacturers forego the production expense of bonding along the height of their core resulting in a core that is both heavier and less rigid. The lighter Newport trussed core is also dynamically superior due to the higher stiffness-to-weight ratio.
Which is more effective, tuned damping or broadband damping?
Narrow band tuned damping techniques are far more effective than broadband dampers. Narrow band dampers selectively eliminate the vibration mode. Different dampers can be used for different modes and are effective for damping mode harmonics. Narrow band dampers are individually chosen for the table geometry and are not affected by table loads up to 25% of the table mass. Tables with heavier loads can be custom tuned at Newport with the specified load in place. Narrow band techniques add less weight than competitive dry dampers that can actually increase vibration problems by placing more load at the table ends and decreasing the stiffness-to-weight ratio. Dry dampers incorporate semi-rigid adhesive layers which can degrade over time. As a result, dry dampers are not as reliable as the low vapor pressure, hermetically sealed oil damping techniques used by Newport.
Are figures of merit useful for comparing table tops?
Yes, if properly applied. All figures of merit; such as the Maximum Relative Motion and Dynamic Deflection Coefficient; are extracted from a combination of measured data and assumptions. The methods of calculation and assumptions should be clearly stated by the manufacturer or the data may not be comparable. Newport uses these figures of merit conservatively for the benefit of its customers when comparing among the many tables offered by Newport. Manufacturer data claiming maximum compliance levels and maximum static deflection should be discarded unless the computational method is clearly stated. In any case, the best evaluation is the direct comparison of compliance curves measured at the proper table location.
The most important data shown on compliance curves are the table resonance frequencies and the Q of each resonance. Higher resonance frequencies result in lower table deflection. Minimum Q; the ratio of peak compliance to the ideal rigid body compliance at the same frequency; indicates high table damping and reduced bending due to vibration.
Does side construction matter?
Yes. Newport offers two types of side walls: wood composite for laboratory use and damped metal sides for cleanroom and vacuum applications. The wood sides are acoustically superior to any competitors metal sidestap on them and you can hear the difference! The composite wood will not corrode and is epoxy sealed against liquid contaminants.
Should an optical table use freestanding legs or tie-bars?
Freestanding legs offer better isolation than legs coupled together with tie-bars. Tie-bar systems can introduce vibration modes directly into the leg. They also offer a false sense of security in earthquake zones. Tie-bars should only be used if rolling casters are required. In earthquake zones, approved Newport earthquake restraints should be used.
Table Isolator Set-up Schematics