Gil's Soap Scope
5/31/2004
By Dr. Gil McFarlane
copyright 2004 Gil McFarlane |
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I call this the "Soap Scope", not because it is unexpectedly sudsy, but because it is made with recycled 5 gallon laundry soap buckets (credit for naming it goes to an annonymous ATM at this years RTMC). Any sturdy 5 gallon buckets will work, but recycling is fun, cheap, and good for what ails you. This scope won the Warren Estes merit award at the 2004 RTMC Astronomy Expo. The tube assembly was made with my son Ian, and uses a 3" f10, slightly flexed mirror in a 4" drain pipe.
The mount is designed to be made with a minimum of tools, time, and money. The goal was to require nothing more than hand tools, about two hours of labor, and under $20. The only power tool needed is a hand drill with a 1/4 inch drill bit. The rest can be fashioned with hand saws, sandpaper, and a sturdy pair of scissors.
Plastic buckets are made from HTPE plastic, a substance that is so chemically inert that it is simply impossible to glue anything to it. You can bolt things to it, and some say that you can melt/weld things to it, but you can not glue it. That is the bad thing about this plastic, what is good is that it is very tough and is very slippery. Telescope mounts seek to avoid "stiction ". When you start to move a telescope you do not want to have to push hard just to get it going, and then have it move too quickly. The thing you wanted to center in your eyepiece would suddenly jump out of your field of view. This mount is remarkable in its avoidance of stiction and this unglueable plastic is responsible.
Special qualities of this mount
- The telescope tube is rotatable.
- Small dobsonian telescopes often have trouble moving correctly because they weigh too little. Springs are often used to "add virtual weight" to the scope. Small dobsonians are often loose when looking near the horizon and stiff near the zenith. This telescope does not have this problem - it seems that the taper of the small buckets and the flexibility of the top bucket join forces to cradle the scope like a pair of hands.
- It is cheap.
- It is cool.
- It takes advantage of the remarkable qualities of expandable foam. (I love this stuff!)
Material List
Lets start with the buckets and work around clockwise.
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1. |
Two 5 gallon plastic buckets and one lid. Recycle or buy new ones at a paint store for about $5 each. |
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2. |
One gallon jug of water, punch, or other emergency provision to give ballast to the bottom of the mount. |
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3. |
Four small plastic buckets (two on each side). (Mine are 7" in diameter and 6" deep, they are from Lowe's, $1.25 each). These are used as the axis for the declination bearing. I call this the Declination Axis. |
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4. |
Expandable Foam to hold the four small buckets together. (I suggest you avoid the "water clean-up" type.) I like the HILTI CF116 "Grip Filler Foam", sold at Home Depot because it is dense and the can be resealed for later use. |
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5. |
One 11 1/2 inch pre-cut, pre-surfaced, circle of particle board that is sold at Home Depots in the Lumber section. These are typically used as small table tops. These are very convenient. If you wish, you can always cut your own circle and put Formica or similar plastic laminate on each side, but these save a lot of time and only cost about $7. I will refer to this as the Center Bearing. |
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6. |
One 1/2 inch plywood circle that fits inside the bottom of your bucket. Cut this with a coping saw, scroll saw, keyhole saw, jig saw, router, or whatever you want. It need not be exact to work, and it does not need to be beautiful to work. I will refer to this as the Wood Circle. |
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Hardware: One 1/4 inch by 3 1/2 inch bolt. Two Large washers for the 1/4 inch bolt. One Spring that is large enough to fit over the bolt, and is fairly stiff. One 1/4 inch nut. |
Construction
We will work out way up from the bottom.
1. Bottom bucket. Drill a 1/4 inch hole in the middle of the bottom.
2. Center Bearing. Drill a 1/4 inch hole in the middle. If you are off center a little it will not really hurt. If you do not have a way to make a good perpendicular hole, you may want to make it a little bigger (3/8") so that the bolt will go through easily.
3. Wood Circle. Drill a 1/4 inch hole very close to the center of this piece of wood. Try to be within an 1/8th of an inch.
4. Top bucket.
A. Put the wood circle in the bottom of the top bucket. You will use the hole used in step 3 to make a matching 1/4" hole in the top bucket with a hand drill.
B. Cut away a U shaped space on the side for the telescope to swing into. Leave an inch or two at the bottom of the bucket to give it stiffness. My cutout measures 6" across and 12" deep. I cut through the thick areas with a hack saw and the thin with strong scissors. A jig saw can also work but be careful.
C. The declination cutouts. These need to match your buckets, but it is not terribly technical. Mine are 5 1/2 inch wide and 2 1/2 inch deep. The idea is for the buckets to rest on the upper inch or so of the cutouts. Cut them in the same way as in step 4. B. above.
5. Declination Axis
A. Cut out a semicircle of plastic on a pair of the small buckets so that they will meet all of the way around, but do not have too much of a gap between them and the telescope tube (less than 1/4"). Sand the inside of the buckets with a rough sandpaper, then spray them with water (it helps the foam cure.)
B. Clamp them together with anything handy. I used the small black paper clips that are used to clamp 25 or so pages together. Put about four per side.
C. Tape wax paper all the way around your telescope tube. For this to rotate later, you will need to use a smooth tube like plastic or metal. A sonotube may not work (but I have not tried it.)
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D. Drill a hole in each of the buckets, in the middle, and slip them onto the tube over the was paper. Lay the tube over a trash can, wedging it so it stays still, with a bucket pointed up.
E. Wear Gloves when using the foam! Put about 20 seconds worth of foam into one bucket - letting it fall down onto the tube. Wait for an hour or so for it to expand, then roll it over and do the same to the other side. Repeat this until it is filled with foam. It is important for the foam to meet in the middle to hold the thing together.
F. Now put on the other two buckets by sanding them, spraying them, and cutting them a little if necessary. I drilled holes in their centers as well and sprayed foam in them as well.
G. Do not worry if foam gets on the plastic, once it has cured it can be snapped off. Don't be surprised if foam erupts unexpectedly even days later - sometimes foam that was not fully expanded gets access to the air and expands much later - like magma finding its way up to the surface!
6. Assembly.
A. The telescope slips into the Declination Axis after you remove the wax paper.
B. Sit it on the matching cutouts of the top bucket and place sandpaper face down between them. Now swing the telescope back and forth in such a way that the sandpaper shapes the cutouts to match its shape.
C. Place the bolt through the Wood Circle, then the top bucket then the Center Bearing then the bottom bucket. Add a washer, spring, washer and nut. Tighten the nut to give appropriate tension as you use the telescope. You should again use sandpaper to smouth the surfaces of the bucket bottoms so they smoothly slide against the Center Bearing disk.
D. Place the water on the lid, and place the finished mount on the lid. Step around the edge to clamp it down, and you are ready.
Other pictures
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Plans for the telescope tube will be here later, after I rework the design to be easier to build. It consists of a 3" f10 mirror lightly pulled to a parabola, in a 4" ABS drain pipe tube. You may use any typical configuration for this basic telescope tube and use with the soap scope mount.
Any questions? E-mail me - ( I must admit I don't check my e-mail real often, so please excuse delays.)