Table Design Theory

Newport Optical Table shown with a breadboard and active damped legs.

Introduction

Cost, size, weight, stiffness, ease of mounting component and portability are all important things to consider when designing a holography table.

Size

The size you choose is very important. If you make the table too small you will not have room to make more elaborate setups. If you make the table to large it wastes lab space and make it more cumbersome to align optics. You can make creative setups on small tables but for an open table layout consider the largest size of film you will use. You will need a collimation mirror capable of filling that mirror. You will also need the distance from your pinhole in your spatial filter to be the diameter of the mirror times the focal ratio of the mirror. ie. 12.5" dia mirror at f 4.5 needs to have 56.25 inches from the spatial filter to the mirror. Add some room for the mirror mount, the spatial filter objective and the steering mirror and "68 inches is about as small as you would want to make this table. It will easily allow an 8 x 10 film size.

Flattness

A flat table helps to allow the moving of components with out having to re-align them. Also if there are local variations in height components can rock. It is important to watch metal tables for small surface damage caused by droping sharp objects. This can allow a component to rock.

Stiffness

A table for holography needs to be stiff enough so when moving components around the bench you don't change the alignment of the other optical tables. Also, stiffness helps in making the amplitude of any resonances smaller and higher in frequency. This equation from LEOT has proven quite useful in table design.

• P = Force exerted by the point load (in lbs)
• L = Length of the panel (in ft)
• b = Width of the panel (in ft)
• H = Thickness of the pane (in ft)
• T = Thickness of the skins (in ft)
• E = Young’s modulus for the skin material (in lb/ft^2)
• G = Shear modulus for the core (in lb/ft^2)

With these units, the deflection will come out in ft. Of course you can change units if you keep your units consistant.

A proven rule of thumb is to allow 10 wavelengths of light deflection in a lab and 20 wavelengths of light deflection for art holography when adding a 100lb point load to the center of a table.

Young's Modulus (lb/ft^2)

• Steel 4,180,000,000
• Aluminum
• Granite 1,296,000,000
• Concrete 576,000,000

Shear Modulus (lb/ft^2)

• Honeycomb 32,400,000 (varies with product)
• Blue Foam 96,336
• Balsa Wood 2,001,600 (varies in each tree)
• Duocell 633,600
• Steel
• Aluminum
• Soda Cans 230,000 (best guess)

Attachments

Since the table is there to hold your optics stable and in position the attachment of you optics should be considered early on in table design.

Gravity Bases

The simplest and cheapest method for attaching optical components is to make them heavy. Filling a base with sand, lead or other heavy substance can filled into a base. A base can also be fabricated from a heavy material such as steel or concrete.

Magnetic Bases

Magnetic bases require the surface of the table to be ferrous. They are cheap and can be positioned anywhere. The disadvantage is the cheaper bases have a 8mm x 1 thread and the commonly available posts have a 1/4"x20tpi thread. This can be solved by machining an adapter. The bases have a rotating knob to allow the magnetic field to be turned on and off.

Tapping the table survace allows very rigid mounting of optics. 1/4" x 20 TPI Holes on 1" centers make attaching 2" diameter rods quite easy. The disadvantage is the cost of drilling and tapping all of the holes.

Isolation

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The ground is always in motion. It is important to keep the motion from the ground from moving the relative positions of your optics. One form of isolation it to geographically isolated from ground noise. Working on the concrete floor in a quiet location has proven to be a good choice for holographers.

Another form of isolation is to rest the table on an air spring. A partially inflated inner tube has proven to be a usable choice.

Sorbothane works well down to 20hz if properly loaded but below that it is not very effective.

When money is no object there are commercially designed legs to isolate tables. They come in both passive and active varieties.

When designing an isolation system it is important to consider the resonant frequency of the table. A low frequency resonance is more difficult to damp out, but very large and stiff table often have very low resonant frequencies.

Table Materials

There are a large number of materials that can be used as a table. Tables have been built from materials as different as cans, doors and pavers as well as many other commonly found items. Below is a list of commonly used materials. The suggested sizes are tables proven to work for making holograms.

Granite

Granite Optical Table from Kinetic Systems

Suggested Size: 4'x8'x12"

• Easily obtained in large sizes
• Can be ground quite flat
• Very stiff in larger thickness
• Low thermal expansion coefficient

• Not magnetic
• Quite heavy
• Expensive

Composite

Suggested Size: 4'x8'x12"

• Lighter than Granite.
• Has high dampening coefficient. Resonant energy is dissipated quickly.

• Not as stiff as granite.
• Complicated to manufacture
• Raw materials are difficult to acquire for amateurs.

Concrete

Suggested Size for art holography: 4'x8'x6" or 3'x5'x3.5"

• Inexpensive
• Can easily be made flat by an experienced craftsman
• Can be made in any size

• Not as stiff as granite per pound.
• Heavy.

Sand

Suggested sizes: 4'x4'x12" built on plywood resting directly on the ground with inner tubes. 4'x8'x2' built on a 4'x8'x3.5" concrete base resting on 6 inner tubes.