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Optical Table





Table Mounts

Optic Mounts

Plate Holder



Table Mounts

Each optical and non-optical component of the hologram recording setup will need at least one table mount (Figure 18, parts A, B, & C). Some components, like the plate holder, will need two. This adds up to a minimum of 7 mounts for the simple single-beam transmission or reflection hologram setup in the section on Creating Transmission & Reflection Holograms. The most complicated optical setup, in the section on Creating Transmission & Reflection Holograms, uses 20-24 mounts. If you feel you need to use more than one mount for a component where I've used only one, please feel free to do so. You can't have too much stability!

This holography system uses gravity and the mass of the lead weight to keep the table mounts stable and in place on the surface of the optical table. This type of mounting system allows precise positioning of components on the optical table. I've used many other types of mounting systems in the beginning years of making holograms such as screw-down mounts, magnetic mounts, and sand table PVC tube mounts, but the gravity/mass system is the best, easiest, and most versatile to use in my experience.

Table mounts are needed not only to hold, support, and stabilize various components but also to make sure the laser beam runs parallel to the table surface at a height of 9 inches. I have found this height to be optimal in all of the optical setups on this web site, both in terms of component stability and in successfully positioning certain components above & below this height. This means that the output aperture of the laser is positioned at 9 inches above the table surface. More on this later.

Each table mount is made of a 5 pound lead diving weight (the lead provides the necessary mass stability) and a 13 inch long, 1/2 inch diameter, solid aluminum pole (Figure 18, parts A & B). To construct the mount, first take the lead weight and using a 9/16 inch bit, drill a hole in the center of the weight but not all the way through to the bottom. I use a piece of duct tape or masking tape around the bit shaft to show where I need to stop. Make sure you drill as perpendicular to the weight as possible. If your drill has a level bubble, that would be most helpful. If you have a drill press, that's even better.

Caution: diving weights are made of lead, so be sure to clean up all loose lead and working surfaces, and wash your hands afterwards.

Note about drilling holes: start the hole with a small bit, such as a 1/8 inch bit, and work your way up to the required bit size using the bits sizes in between. By starting with a small bit, you can more accurately start the hole where it should be located and obtain a vertical hole. I mark the hole's position to be drilled with a permanent black marker pen and then make an indention at that mark using a small nail (or awl) and hammer. This helps to keep the bit in the proper location when I start the drill. I also start the drill at a low RPM and then increase the RPM once the hole is started. Use this method for all drilling requirements that follow.

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Figure 18: Table mount showing 5 pound (2.27 kg) lead diving weight (A),
solid aluminum pole (B), Plexiglas base (C), connector (D), short rod (E),
and optic mount (F).

Solid aluminum, 1/2 inch diameter poles are commercially available in 3 foot lengths at most hardware stores. I prefer aluminum over zinc because zinc is a softer metal. Each mount pole should be cut to a 13 inch length and the edges filed smoothly on the ends and beveled. Before you attach this pole to the lead weight, spray paint the pole with flat black paint. Rust-Oleum fast drying enamel spray paint works well. By painting the poles black, stray laser light will not reflect off the poles and, therefore, the poles will not be seen in your hologram. Unless you want to, you do not need to paint the lead weight because of its dark gray color. You can find lead weights shown in Figure 18 at diving shops. Make sure the weights are not covered with plastic and that the lead is solid, not hollow in any way except for the belt slots. Many mounting parts that follow will need to be painted flat black and you should allow the paint to dry 24 hours before using that painted item in the mount building steps. You should use more than one thick coat to prevent the thick coat from running and allow about an hour of drying between coats.

Insert the aluminum pole into the drilled hole in the weight to make sure it will rest on the bottom of the weight's hole, then remove it. Mix up a small amount of 5 minute epoxy glue and insert about a teaspoon into the lead weight hole. Next, insert the pole into the hole. It will be a tight fit with the epoxy now in the hole and you may find that a pocket of air develops in the hole under the pole. Place the weight on the floor with a paper towel between them and hit the top end of the pole gently with a rubber mallet. The glue will gradually seep out around the pole and the pole will finally come to rest on the floor of the hole. If the pole has some play in the hole, you will need to hold the pole vertically until the glue starts to cure and harden. You can use a small level to help you determine how vertical the pole is. Wipe away any excess seepage glue between the weight and the pole with rubbing alcohol and a paper towel before it starts to cure or leave it there for extra strength. The small amount of seepage will not interfere with the connector movement up and down the pole.

Next, using 1/4 inch thick acrylic plastic, cut a rectangle slightly smaller than the size of the lead weight's bottom. Make sure you remove any protective paper or plastic covering on both sides of the acrylic plastic before you do the next step. Cement the acrylic plastic with 5 minute epoxy to the base of the lead weight opposite the pole (Figure 18, part C). The acrylic plastic provides an absolutely flat contact surface with the polished granite surface of the optical table, thus eliminating any rocking. The lead weights are not smooth on the bottom, and without the acrylic plastic , they will not lay flat on the table and rocking may occur. This reduces the stability of your system.

Short Rod

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      Figure 19: Short rod for attaching optic mount to table mount.

Each table mount will have a short rod (Figure 19 and Figure 18, part E) attached to it with a connector (Figure 18, part D). Some of these rods will have optical holders (Figure 18, part F) attached to them for the mirrors, diverging lenses, and the fixed beam ratio beamsplitter (if you're using one). Others will be used with the table mounts to mount the laser, parabolic mirror, plate holder, and object scene. You will need two different types of short rods. One will need a 8-32 bolt and the other a 4-20 bolt. Each type of short rod will be used in different situations depending on what you are mounting to them. If you are using an optic mount for a small mirror, you will use the 8-32 rod. The telescope mounts for the parabolic mirror will use a 4-20 rod because the mount and mirror are heavier.

The short rods need only be 3 inches long and are cut from the 1/2 inch diameter solid aluminum poles. After the rod is cut to length and the end edges filed smooth and beveled, place it vertically in a vise (use a level to make sure the rod is vertical). On the top center end of the rod, mark the center of the end surface, then use an awl to indent a small hole in the aluminum to help position and guide the starting bit. Start with a 1/16 inch bit and work up to a 1/8 inch bit. The starting bit should be drilled to a depth of 1 inch as well as the following bits. Between changing bits, clean out the hole of any drilled debris by removing the rod from the vise, turn the hole downward and lightly tap the rod against a table's edge to dislodge any debris from the hole, then replace the rod back in the vise vertically with the hole upward for the next bit size.

Once you have completed drilling with the 1/8 inch bit and cleaned the hole, use an 8-32 tap to make threads in the drilled hole with clockwise and counterclockwise turns. Once you have got the tap started in the hole, use one clockwise turn, or less, at a time to minimize breaking the tap. Then reverse the direction about one turn, then continue forward with one turn again. Keep going forward and reversing until the threads are about 3/4 inch deep. As the threads are created, bits of aluminum will fall to the bottom of the hole and will need to be dumped out as you proceed and when you finish tapping the hole. Tapping a hole with threads takes some practice. I've had my share of broken taps in the hole and have had to start again using the other end of the rod. 8-32 taps are especially prone to breaking. So when you are creating the thread clockwise and you feel strong resistance, stop, back-off, and reverse direction (counterclockwise) about a full turn. Then continue clockwise again. This helps dislodge small pieces of aluminum that get stuck in the teeth of the tap.

Note: Always make sure the drill bit diameter is slightly smaller than the thread tap size diameter you're using. If the drill bit is too small, you'll have an impossible time creating the threads. If the drill bit is too big, you won't get any threads. For a 8-32 tap, the final bit size is 1/8 inch. For a 4-20 tap, the final bit size is 3/16 inch.

After the hole is drilled and tapped, screw a 1 inch long, 8-32 bolt into the hole until it is tight. Take the rod and place it horizontally in a vise, and with a hacksaw, cut off the bolt head, leaving a 1/4 inch length of threads. With a file, smooth the end. With a fine file, clear the threads of any metal on the end. Test the thread functionality by screwing on a nut. You are now ready to add an optic mount (to be discussed in a moment) to the end of the rod . Spray paint the rod and bolt with enamel flat black paint.

Use the above method for creating a short rod having a 4-20 bolt. The bolt length for this size bolt should be 1/2 inch long instead of the 1/4 inch inch length because the 1/4-20 bolt will be used for heavier components.


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   Figure 20: The top connector was purchased from a supply house.
               The bottom connector was made in a machine shop.

The connectors shown in Figure 20 used with all the table mounts and short rods can be purchased from supply houses or can be made using your own drill press or a machine shop can do them for you. Whichever approach you use, the connector holes should be able to accommodate the 1/2 inch diameter solid aluminum poles, painted. If you plan to make them yourself, it is important that the holes be drilled exactly perpendicular to help maintain a right-angle alignment between the table mounts and rods. This requires a drill press.

If a machine shop makes them or you make them yourself, here is the method you should use. A solid piece of aluminum was used with the dimensions of 1.75 inches x 1 inch x 1 inch. Two 5/8 inch diameter holes were drilled in the aluminum at right angles to one another, with the center of each hole 1/2 inch from each end. Poles and rods are clamped into the holes with 1/4-20 thumb screws on each end screwed into tapped 1/4-20 threads.

I prefer the commercially available clamps because there are multiple holes for the table mounts and short rods which provide more versatile ways to mount the rods. The solid aluminum poles fit very well in these connectors. Also the cost is the same for either type of connector. Whatever connector you use, spray paint the connectors and tightening screws with enamel flat black paint. A supply house for these commercial connectors is covered under Resources.