Monocular

I was asked by the Exploratorium to design a minimalist telescope that might be used in the new building (the Exploratorium is moving from the Palace of Fine Arts to Pier 15) . This was my first attempt, an open framed 3.5x Galilean telescope. I attached the focusing screw to the objective lens not the eye piece (which is typical) to further simplify the design. Galilean telescopes have the advantage of producing an upright image with only 2 lenses but suffer from high distortion above 3x.

Looking through the scope, the spherical distortion and chromatic aberrations are notable, especially if the scope is poorly focused. I ended up using 1/4''-20 threaded rod for the focus adjuster, this was way too fine for accurate focus control as it was hard to tell if you were getting more or less focused in less then 5 or 10 turns of the knob.

Given the image quality problems with the Galilean scope I made this "looks like" model for a monocular using Keplerean optics using 8020 aluminum stock and laser cut cardboard. A Kepler style scope produces a clear but inverted image, so the model included a pentagonal case that was scaled to hold erecting prisms to un-invert the image. To solve the slow focus problem I modeled a transverse focus knob like a microscope capable of moving the objective lens quickly but precisely.

Tracing Camera Prototype

The Exploratorium asked me to prototype a portable camera obscura for the new building to be used as a tool to make observations and draw the local landscape.  It is a very optically simple device consisting of a box with a lens mounted in front of a 45 degree mirror inside to flip the image right side up and a piece of glass on top onto which tracing paper can be placed to receive the image.     

Originally the tracing camera was mounted on a sturdy aluminum camera tripod, this proved usable but not stable enough to make complicated drawings easily. In trying to design the tripod to replace the aluminum one, I looked at a lot of heavy designs used for survey equipment.  The common thread between all these designs was the triangular multi-element legs.  As I wanted to avoid cutting 2 or 3 pieces precisely for each leg I decided to simply bend them together instead. To simulate how the plywood might behave I made the preliminary model with popsicle sticks to find out how much more rigid it was than a single element.  These legs afforded me a very stable platform to draw on which resisted shaking and twisting very well.

    

The Tracing camera in use at Pier 3 in San Francisco.

Here you can see the camera's three controls, the lens barrel is coarsely threaded and can be turned to focus the camera. The upper knob is a tilt lock and the lower is a pan lock. The lens barrel assembly is machined pvc pipe fitting and delrin.   

Paper holder tripod is a reduction of the original camera tripod, to make it lighter and more easily collapsable.

The legs, joint plate and desk top (the indentations for the pencils and sharpener) are made on a cnc router and the rest of the wooden parts were made by hand.

The cardboard shroud blocks ambient light from the front of the camera casting a shadow on the tracing paper making the image appear bright.

photo by Gayle Laird, Exploratorium

Drawing detail looking at downtown from Pier 3 in San Francisco.

Another drawing detail looking out at docked boats at Pier 3. 

Alidade prototype

An Alidade is a device to locate a distant object on a map by line of sight using the 2 sights on the rotating bezel.  This working prototype for the Exploratorium made of laser cut cardboard is based loosely on the construction of the Osborne Fire Finder (a type of alidade) invented around the turn of the last century to aid in locating forest fires.  As the operation of the my model is essentially the same as an Osborne Fire Finder I suggest you watch this video on fire finder operation to learn more about how it's used.  To make it work you need a map with the alidade's location at its center, and to align the map and the compass dial (not seen here) to reality.  


Here you can see the Alidade with the map platen removed, the map platform in the middle where the map platen will be velcroed on and the rotating bezel is on the outside.

With the bezel removed you can see the graduated compass dial.  This was originally etched right into the corrugated cardboard (as seen above) but was replaced with a layer of chip board (think cereal box material) as it etches much more legibly.  

Here you can see the bezel removed from the base of the alidade. In front you can see the bearing window and then the two sights with the range rule between them.  The far sight has as single vertical thread to place over  the object you're sighting like a cross hair.  

Here's the range rule, in this case its calibrated for the particular Bay map seen above, but using a simple ruler and posting the scale as was done with the Osborne Fire Finder would be a good strategy if you're likely to use maps of different scales.  With a permanent range rule like this all maps would have to conform to this scale to be useful. Which direction I'll go with this is not worked out yet given the museum has yet to choose a map or maps for the eventual exhibit.



Here's a detail of the bearing window.  When you set the alidade up you align the map, reality and the compass dial, a thread stretched over window will indicate the exact direction you looking in as long as the alidade is not moved.

I've sighted where the Bay Bridge goes into Yerba Buena Island, from Pier 3 in San Francisco.

Looking down at the map you can see the range rule, which represents the line of sight, passes right over the mouth of the Yerba Buena Island tunnel.

Rangefinder #2

photo by Gayle Laird, Exploratorium

Rangefinder #2 is based off an interwar British model by Barr and Stroud. The main difference between this model and the first one is that it does not use mirrors only prism wedges and penta prisms to manipulate the lines of sight. I was lucky enough to find and download a training manual for this particular device and reverse engineer it. Unfortunately, in the interim I seem to have lost the particular web address it came from. Searching for "Barr and Stroud rangefinder" images will find several diagrams outlining its function, but I will try to post a link to the original documents I used once I find them. Generally speaking you look into the center eyepiece via an integrator prism (which I got from a old 3 lcd projection television) you see out through 2 objective lenses and then 2 penta prisms at the ends of the rangefinder, which direct your vision forward. There are 2 prism wedges between the objective lenses and the integrator in the prism. The one on the right of the photo is for calibration and the one on the left is for range finding. Other than the above mentioned optical components the range finder consists of 8020 aluminum stock, machined delrin and laser cut acrylic.

photo by Gayle Laird, Exploratorium

Here you can see the range rule on the top of the sliding prism. When used the two channels of the Rangefinder appear red (the adjustable side) and blue (the fixed side) which makes its somewhat easier to orient yourself when looking through the Rangefinder. This is because the integrator prism had filters for red, green and blue for the corresponding lcds. I only used the 2 side channels, the green channel faces forward and is covered by cardboard. In general this set up was way more accurate than the first version. It was able to get the same reading over and over on objects under 1000 yards, but I found that it is very sensitive to vibration so it might not give the same readings after being moved. I continue to try and improve the design.

A detail of the eye piece and range rule

What you see looking through the eyepiece with coincidence not yet been achieved

Rangefinder #1

This is a Coincidence Range Finder prototype I built for the Exploratorium late last year. Its essentially an optical trigonometry tool which allows you to measure the length to a distant object based on the distance between the pods and the angle of deflection needed to see the same thing from both pods. When the range finder is dialed in you will see 2 perfectly superimposed images of the same object, meaning you set the angle of the mirror in the control pod so it sees what is directly in front of the telescope pod. For a general overview of the theory behind Coincidence Range Rinders work take a look at this wikipedia article and the diagrams at the bottom of this page.

This is the inside of the telescope beam splitter pod. I mounted an inexpensive 10x monocular in some pvc pipe fittings. In front of the monocular I mounted a half height adjustable first surface mirror to receive light from the control pod window and the window directly in front of the monocular simultaneously.

This is the inside of the control pod, a larger first surface mirror collects light from the closed window and sends it out the removed one to the telescope pod. The mirror is adjustable via a knob driven screw which also drives a gear train to the range gauge.

Here is the back of the control pod. Note that the range gauge window is made to open as the ranges were written on by hand using a laser range finder for this first prototype.

Looking into the control pod mirror from the front you can see the light path back to the beam splitter and into the telescope. I used speed rail to connect the 2 pods for easy adjustment and disassembly.

While you could achieve coincidence (see 2 super imposed images) this range finder only proved vaguely accurate, mainly because the range gauge was driven by the control knob and not directly on the position of mirror. The gear train tended to slip disconnecting the mirror position thus the angle of coincidence from the read out on the gauge. In response to the problem of inconsistent accuracy I decided to look at some military range finder designs, which lead to the creation of Rangefinder #2.

Cardboard Scooter for the Exploratorium

The Exploratorium was having monthly material themed events earlier in the year, and when cardboard came around I decided I should try to push the limits of this material. So inspired by Phil Bridge's cardboard bicycle, I decided a kick scooter would be an interesting follow up on the idea corrugated transportation. After a lot of experimentation with laminating cardboard with different corrugation paterns to form beams, I took what I thought was half again as much cardboard to hold my weight (175lb) the scooter was born:

While its far from elegant, it was surprisingly durable surviving for about 3 months of on and off use by myself, other staff and museum visitors. Composed of entirely laminated laser cut cardboard with the exception of the wheels, axles and the steering hinge and hinge pins, the scooter only weighed about 6 pounds, and could carry people up to 200lbs.

The scooter is supported by two beams 3'' tall and eight layers thick that runs from the steering hinge to the rear axle with the corrugation running long ways. The deck is five layers thick, with the corrugation running at alternating 90 degree angles to prevent denting. The front wheel supports were again eight layers thick with ten layers in between to hold the hinge.

Here's a detail of the front end: ten aluminum hinge pins were used to distribute the weight of the rider to the cardboard and nylon bushings were added around the axles to spread out the weight of the rider to a larger amount of cardboard. The use of a steel steering hinge ultimately proved to be the scooter's downfall. After the first few weeks of heavy use the hinge plate started to slide around on the pins compressing the cardboard next to it and eventually making the front end very wobbly. The structural integrity of the board itself was actually very good, as it became clear that it was becoming overly wobbly I actually tried jumping on it lightly and did not break the cardboard beams. As the over all design seems to have worked I plan to make a sleeker second attempt abandoning the hinge for a caster based steering method.

Plastic Manipulation Tools for The Exploratorium

Vacuum forming activity at The Exploratorium

I built a simple vacuum forming rig for The Exploratorium consisting of a sealed wooden box with a peg board top and gasket, and a laser cut frame to hold the vacuum forming stock (PETG) while heating and forming it.

After the stock was heated over the hot plate for three or four minutes it was ready to form, a shop vac drew the air out of the box pushing the plastic down over the objects as a result of the pressure differential. I found that with my fairly low powered vacuum hot plate only provided enough heat to form the major shapes, but that a great deal of detail could be captured by going over the plastic with a heat gun.

Below are several examples of vacuum formed sheets visitors made while I had machine out on the museum floor.

A heat gun was used in this sheet to clearly capture the writing on the lower gear.

This is a crude water lens made by vacuum forming a glass lens with a small dowel to allow a filling port and hot gluing it to a flat sheet of stock to make a vessel. When filled with water it make a surprisingly low distortion magnifying glass.

Acrylic molding "press"

Preparation to cast the negative of the bowl: I made studded the top plat with screws hang down into the paster securing it to wood, and sprayed to the bowl with mold release.

Pouring the plaster positive through the top plate into the bowl.

The resulting casting.

Felt was stretched over the positive and adhered with a tacky non-permanent glue to prevent the acrylic from sticking to the plaster. After the felt was firmly in place I was able to easily trim off the excess felt using a razor blade, making the mold ready to use.

After about 15 minutes at 315f in a convection toaster oven the acrylic is ready to press. It has the consistency of probably most similar to slight dried out Provolone cheese, it sags and bends very easily but does not stretch.

Pressing the acrylic.

The first acrylic bowl came out a little small given I underestimated the amount of material necessary to cover the curve, but I have since dialed in the disk diameter and can produce bowls which are the same size as the original. I tried this activity a few time on the museum with some success though it is a time consuming process. In an effort to deal with the long plastic heating time for the bowls I laser cut a bunch of acrylic fork blanks and made a fork press from laser cure plywood again covered in felt to produce a nice s curve. The forks proved a much more realistic activity for the museum floor as it still displayed the behavior of thermo plastic only took about 8 minutes per fork . I'll try to post pictures of this some time in the future.

Simple blow molding flask

The mold flask was made by casting plaster around an acrylic tube with a plastic ball stuck it, in some old shipping tubes capped with plywood.

I used some cheap 1/4'' ply to make a linkage to bring the halves of the mold together and lock them shut. To turn a tube in to a ball I heated the acrylic tube for about 15 minutes in a toaster oven (which happened to have a convenient rotisserie function) at around 315f. When the tube achieved the proper provolone texture it was closed in the flask, one end was crimped off with players and a stopper with a schrader valve was inserted into the other side allowing it to be inflated by a bike pump. Inflating the bulbs is correctly surprisingly hard, the stopper always leaked a little so you had to keep pumping constantly. If you pumped to much the vessel could explode out of the seam in the flask. If you pumped to little you would never get the tube to expand to the sides of the mold as the acrylic is very elastic and would try to return to its tube form as soon as the pressure was released.

Here is mold opened just after the acrylic tube as been inflated and allowed to cool.

The resulting blow molded vessel is fairly strong but will actually tear not crack if you squeeze the bulb too much. Its not surprising the acrylic is not used to make thin blow molded objects given this odd behavior.