Let me start off with a statement of the obvious! Everyone on this site knows about solderless breadboards. What I’m going to describe in this article though is their Big Brother optical equivalent. Way back in the early 2000s I was playing around with some home-built spectrographs that I hooked up to both linear and array CCD detectors. (This was great fun and in fact I hope to publish a second short article on a couple of home-built spectrographs that I made during that time for all the amateur spectroscopists out there.) In order to decide on what configuration and type spectrograph I wanted to build at that time, I first decided to try my hand at constructing a simple optical table.
An optical table (or optical bench) is a very useful accessory to have for anyone who experiments with lenses, prisms, gratings and other optical components, enabling them all to be securely fixed to a rigid base. These components can be quickly unscrewed and relocated elsewhere on the table in order for you to try out different layouts during your experimenting.
Now commercial optical tables are available of course but they are all horrendously expensive, when you consider the relatively simple function they are required to perform, i.e. holding things still! Even a small table, say 3ft x 4ft (0.9 x 1.2m), will set you back hundreds of USD/GBP/EUR and possibly several thousand dollars depending on size and stability. These tables are also very heavy, requiring specialised installation and support. Amateur spectroscopists have more important things to spend their money on, and in any case, it is fairly easy to make one for yourself, with simple tools and easily obtainable materials. A table that I built and used regularly for my optical experiments several years back is described below. The following is for the construction of a 3 ft x 2 ft (0.9 m x 0.6 m) optical table. This size is compact enough to fit readily on a writing desk or kitchen table and is large enough to accommodate a number of components for designing spectrometers, and experiments in optical interference, fibre optics and so on.
You will need the following materials:
- A piece of chipboard or MDF (Medium Density Fibreboard) 3ft x 2ft by 0.75 inch thick. MDF should be your first choice since chipboard tends to splinter when being drilled. Plywood is another good alternative. The thickness of ¾-inch is a good compromise between weight and stiffness, ½ inch being a little too thin for a bench of these dimensions.
- A piece of acrylic sheet 5/16” (8 mm) thick, of the same size as the base board. Acrylic plastic sheet is very strong for its weight and is easier to work with than metal. If the 5/16” thickness is unavailable, ¼” will work fine.
- Metal sheet to go on top of the acrylic sheet (optional).
- Some standard DIY tools – electric hand-drill and bit, centre punch, ¼”-20 tap for threading the holes, contact adhesive, spray paint and lacquer, rubber feet.
Baseboard and Top Surface
Cut the wood base to size ensuring that the edges are square and sand the edges and both sides. Cut the acrylic sheet to size (or preferably have it done at the time of purchase). When purchased, the acrylic plastic will be supplied with two protective peel-off paper sheets that should not be removed until ready to use. The acrylic surface will be glossy, and the side to be bonded will require a good sanding with medium, and then fine, sandpaper so as to form a good bond with the baseboard when glued. Using a paintbrush, apply a thin layer of acrylic contact adhesive (Bostik™ or similar) to both the wood and plastic sheet and carefully place the two pieces together making sure they are precisely aligned. Contact adhesive forms a bond immediately so there is no room for error. If you mis-align here, they will be very difficult to separate afterwards. Practice first on gluing together two smaller pieces and check the adjustments and movement you can make.
With my design I wanted to add a metal top to the optical table. The main choices were sheet metal or aluminium. Steel is the cheapest option, but aluminium is easier to work and drill. In the end I chose aluminium, and I was able to obtain some 12-gauge aluminium printing plates. These have a dull, grained and anodised upper surface and a shinier un-grained back surface. Unfortunately, 12-gauge printing plates are too thin to be used alone. To increase thickness, I glued three identical plates together with contact adhesive to give 36-gauge. When dry, this laminated aluminium sheet was glued to the acrylic sheet.
Drilling the Holes
And now for the “fun” part…if fun is the right word here! You’ve got to drill and tap a large number of holes. Using a large carpenter’s set-square, a matrix of lines was drawn out at 1-inch (25mm) intervals across the whole table. If you prefer you can make holes at half inch centres, but this means you will have to drill four times as many holes. For a 3 ft by 2 ft table, with 1-inch centres and allowing for a 1-inch border around the edges, a total of 805 holes need to be drilled! With a centre punch carefully mark all the positions for the drill bit. This takes time! Care and attention at this stage is paramount since this will determine the final precision of your optical table! So, take your time; take frequent breaks; go and watch your favourite TV show for a while; you get the idea.
The hole size I chose for my table was ¼ inch Imperial (very common in the USA). This was because many standard optical bench components such as lens holders and supports all tend to use this size and because I already had some second-hand lens holders and other fixtures of those dimensions. These components generally have ¼-20 threads. A metric hole-size can of course be used, particularly if you want to make your own optical holders and mounts at a later stage. Clearly it is essential to maintain the hand-drill vertical at all times to ensure the final component will be held perpendicular to the table. Using a drill press or some form of mounting for the drill is not an option in this situation as you slowly work your way around the table. The other alternative is to have the drilling carried out at a professional metal working shop on a jig, but this will add to the cost. With care and attention most, if not all, of the holes can be successfully drilled vertically by hand.
The most time-consuming and laborious stage comes next - tapping all those holes you have just drilled. Make sure that you use a pre-tap followed by the final tap to achieve a good thread for each and every hole. All the holes should be drilled and tapped to a minimum depth of about half an inch (12 mm). This is sufficient to hold optical components securely in place with the fasteners which will typically be hex-head screws and bolts. Tap through the top sheet (if used), through the acrylic layer and through the wooden baseboard of the table. Again, take your time here and don’t rush things.
I undercoated the drilled and tapped table was with spray paint, followed by a minimum of 2 coats of semi-matt black paint, again best applied from a spray can. I added an additional spray-coat of semi-matt acrylic lacquer for increased abrasion resistance. For vibration isolation, 4 Sorbothane™ anti-vibration feet were attached to the underside of the table at the 4 corners to absorb and dampen unwanted vibrations (available from Edmund Optics).
My own optical table has served me well for many years experimenting with optical equipment and spectrometer designs. If I was doing the project again, however there is at least one change I would make: I have noticed with time some delamination of the top aluminium sheet around the holes. This was due to my use of multiple layers to increase the thickness of the top sheet. With the frequent screwing and unscrewing of all the optical components, areas around some of the holes are de-laminating and producing small raised domes. If you can find a thicker aluminium sheet (at least 1/8" or 3/16"), this would be ideal. Alternatively, just using the acrylic sheet as the final top layer might be a better solution.
Good luck with your own designs!