When I saw the call for the “Untochable Challenge”, it immediately reminded me about a visit to a museum when I was a kid. I remember that I got really amazed with a device exhibited in the museum… it was a device that created a great optical illusion. I remember seeing an object in front of me, and I was trying to touch it, but there was nothing there. I had that memory in my mind but I did not have a name for that device, so I Googled different terms that would describe the device that I saw in that museum, that Google search took me to a pair of “Holographic Parabolic Mirrors” Also Known As: Mirascope.
This Instructable will lead you step by step on how to make a Mirascope, from the simple math needed to a simple process that you can replicate at home.
Step 1: Mirascope Description
- The Mirascope is composed by two identical parabolic mirrors.
- One of these parabolic mirrors has an opening at the vertex of the parabola.
- The reflective surfaces of these mirrors are on the concave side of the parabolas.
- The reflective sides are set one in front of the other.
- When set one in front of the other, the Focal point of one mirror is located at the vertex of the other mirror and vise versa.
- The light reflection in all directions creates the Optical Illusion of seeing an object at a location where it is not located.
- Nowadays, the Mirascope is sell like a toy, and it can be found at on line stores like this one:
Step 2: Materials and Tools:
- Plaster (about 4 lb for the mold size shown here).
- Wood Glue
- Wood Filler (Automotive Body Filler would work as well).
- 0.75” Thick Particle Board (about 2 pieces of 11" x 11" for the mold size shown here)
- 0.125” Thick MDF (about 3 pieces of 12" x 20" for the mold size shown here)
- 0.125” Thick Acrylic Sheet (0.0625" thick sheet would work better as it will be easier to mold)
- Epoxy Resin
- Self adhering Sealing Tape
- 1 Air Pump Needles
- 1 Medium size balloon
- Mirror Effect Spray (RUST-OLEUM Mirror Effect Spray Paint seems to be the best option available in Market)
- Jig Saw
- Rotary Tool
- Multimeter with Temperature Measuring Function + Thermocouple
- Ratcheting Bar Clamps (4 or more)
- Cooking Oven
- Cookie Tray or any other flat and smooth metallic surface to support the hot acrylic sheet in the oven.
- Air Compressor
- CNC Laser Cutter or CNC Router
Step 3: Define the Parabolic Profile
We need to define the Parabolic profile to be used. Let's think of this Parabola as "Parabola # 1", this parabola has to meet the condition of having its Focal Point at the Vertex of the other Parabola "Parabola # 2". To meet this condition, we have to define the Focal Point " f " as two times the " y " value at the extreme right (or extreme left) of the Parabola, this because the 2 Parabolic Mirrors will be exactly the same.
We will use the Parabola basic formula:
- y = sq(x) / 4*f
- being f = 2*y
We substitute the variables and clear the formula to find the Parabola characteristic value " f ", using the " x " value at the extreme right.
Please watch the Embedded video for a more detailed explanation.
The attached spreadsheet includes the formulas used to generate the Parabolic Profile at " x " increases of 1 / 128".
Step 4: Profile Post Processing in AutoCAD
- Copy the 432 coordinate values from column "D" in the Parabola.xlsx file attached.
- Use the Pline command In Autocad and paste the copied values from the Parabola.xlsx spreadsheet.
- The resulting Polyline will be a half parabola with its origin at the 0,0 coordinates. To get the whole parabola dish shape you can "Mirror" the initial half parabola using "y" axis as the mirror line.
- The profiles shown in the "Profiles 0X-Model.xps" file were defined to build a parabolic dish shaped dome.
- The Assembled profiles structure is intended to get embedded in cast plaster, reason why each of the parabolic shaped profiles has a hole in the inside of the profile, this to allow the cast plaster to flow through all sides of the profiles and to gain structural resistance.
- The Embedded Video shows the basic process to export spreadsheet coordinates to AutoCAD.
- Attached "Profiles 0X-Model.xps" files shows the resulting pieces of the structure needed to make a Parabolic Blow Mold.
Step 5: Profiles Laser Cut & Structure Assembly
- Cut with a CNC Laser Cutter the resulting profiles that will be used to Assemble the Blow Mold Basic Structure. Use the 0.125" thick MDF as the material for the resulting profiles.
- The Embedded Video show the process to assemble the Mold basic structure.
- Place the assembled structure on a flat table, with the structure flat side facing the table. This will allow the profiles to get leveled.
- Add wood glue at all the loose joints that you are able to identify and allow it to dry.
Step 6: Embed Assembled and Glued Structure Into Cast Plaster
- Trap the semi inflated balloon between the concave side of the Assembled and Glued Structure and a MDF sheet. The purpose of trapping the balloon there is to avoid the concave cavity to get filled with the cast plaster, avoiding this way an even bigger rework (removing plaster from the whole cavity)
- Mix the plaster powder with water, be sure to give the mixture good fluidity, as the mixture has to reach every single corner.
- Cast the plaster and allow it to dry at least for 24 hours.
- Remove any excess or un-desired plaster from the dry plaster structure using the Dremel and the Palm Sander.
- Add the wood filler on the concave cavity and allow it to dry.
- Sand the concave cavity of the mold, using coarse sandpaper first and fine sandpaper at the end. Do this in circular or tangential movements. Do not do this with radial movements. Stop sanding until the profile ribs becomes visible out of the filler.
- Flatten both flat faces of the mold using the Palm Sander. You can use a piece of particle board to check the mold flatness.
Step 7: Blow Mold and Paint With a Reflective Spray
- Besides the mold cast plaster part, you will need 2 squares of particle board of about 10.5" x 10.5".
- One of these particle board squares will be the lid of the blow mold and the entrance of the air to deform the softened Acrylic sheet.
- For this lid, cut a MDF square, same size as the lid particle board square. Glue them together using the wood glue and allow to dry. The MDF layer will help to have a quite flat and smooth surface that will help to avoid air leakages.
- Make a centered hole thru the whole lid (particle board + MDF).
- Insert the Air Pump Needle thru the hole, leaving the tip of the needle pointing to the MDF side.
- Use the Epoxy Resin to fix the Air Pump Needle in place and allow to dry.
- Cut the excess tip of the needle that may stick out of the lid being sure to not obstruct the air passing thru the needle (lid surface should be flat at the MDF side).
- Use sealing tape strips to seal between the MDF side of the lid and the softened Acrylic sheet. (a square at the outside edges will suffice)
- Connect the Air Pump Needle to the Air Compressor with valve.
- Make a small hole (No more than 1/8" diameter) at the exact center of the Parabolic dome, that pass thru the mold from the parabolic cavity to the mold flat surface.
- The 2nd particle board square will be used as reinforcement support for the flat side of the cast plaster mold. This hole will allow the air trapped in the parabolic cavity to escape to the outside.
- Make a centered hole thru the reinforcement support for the flat side of the cast plaster mold, as well to avoid obstructing the air escape.
- Heat the oven to 338 degrees Fahrenheit (170 degrees Celsius). Please note that most of the time the Acrylic Glass Transition Temperature is quite variable between different batches or manufacturers. I strongly recommend to start with the over at 212 degrees F, and then increasing the temperature by a few degrees until reaching the appropriate set point - That is when the Acrylic sheet bends easily when you take it using a heat protective glove.
- Put the Acrylic sheet inside the oven (over a flat and smooth metallic surface - I used a cookie tray).
- Allow the Acrylic to reach the temperature - I recommend to not leave the Acrylic sheet in the oven for too long (not more than 4 minutes). Doing so would generate small bubbles inside the Acrylic sheet.
Quickly assemble sandwich layers in the following order:
- Softened Acrylic Sheet
- Cast Plaster Mold
- Reinforcement support for the flat side of the cast plaster mold
- Close the sandwich using the ratcheting clamps.
- Inject compressed air to allow the softened Acrylic sheet to take the shape of the parabolic dome. Allow 1.5 minutes for the Acrylic to cool down and recover rigidity.
Warning: Blown up parts, during the blow molding process, can cause serious injury or death. The dimensions of the the device and the air pressure used in this process were small and low enough to avoid this risk.
- Cut the Upper Dome to leave an opening. Being the opening Diameter of about 1/3rd of the dish diameter,
- Re-place the cut portion using adhesive tape by the concave side of the dome, this to avoid getting paint inside the parabolic mirror.
- Paint the convex (outside) of the dome with the reflective spray.
Step 8: Have Fun!
- Insert a small object inside the Mirascope, at the vertex of the bottom parabolic mirror (e.g. a Penny).
- When you see thru the opening of the mirror above, you will see the optical illusion of the object being at the level of the above opening, but the object is not there, it is at the bottom of the Mirascope.
- You Can See it but you Can't Touch it!
- Untouchable it is!