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Building a 10" Reflector

Construction Pictures

Laying out parts on plywood

Draw parts with pencil and compass. Tools needed are an inexpensive compass, a carpenter's square, a 12 to 15 inch steel ruler and a yard stick.

 

I used good quality 2 x 4 foot birch plywood. It is very important to measure with great care and keep things square. Keep the pencil sharp. Note how layout of side panels made use of one 12" piece and one 11 9/16" piece side by side to keep length under 2 feet. Take your time, think a lot and don't rush or do this in a distracting environment. Label parts clearly on the plywood before cutting them out to avoid confusion. I mark the parts with dimensions at times to show long and short sides when the difference is close and confusion can arise.

 

Layout for bottom cell plate and adjustable cell plate. Use the compass to find exact 120 degree positions for location of adjusting screws and triangular shape.

 

Cutting and gluing parts

Parts must be carefully cut with attention given to cutting along the outside of the line and not on the line. Practice with scrap if you are not a skilled table saw user until you can cut to about 1/32" accuracy. Observe all common safety precautions advised when using power tools of this type and keep children away from work area. I always unplug my table saw when I walk away from it.

 

Assembly of parts can be difficult and frustrating if they have not been accurately cut. Start with a bottom square part (if there is a thick square start with that) and apply a little glue to two sides and position them as a two panel corner. Finish with two more panels to form a complete box. Use paper masking tape to hold parts together as shown. Mark the position of intermediate interior square parts before assembly and position them after sides have been taped. This is where accuracy is important or the interior squares or opposite end squares may not fit.

 

Only a small amount of glue is required. The polyester resin will later completely fill the voids at the joints and create a virtually one-piece highly rigid structure.

 

Assembly of base ring for rotating upper tube. Use the flat surface of your table saw to ensure a proper and square assembly. Make sure the blade is retracted below the surface and the saw is unplugged.

 

Use weights to insure that parts dry in their proper position.

 

Table saws make excellent sanders and shapers. This is the bottom cell plate. Note how the base saw cut made to cut out the center hole was filled in with paper card material and glued into position. Later, a coat of polyester resin with solidify everything.

 

Rotating head

A typical section of builder's tube and rotation rings. These tubes come in four foot sections and you should get the uncoated variety. Even then, there may be some sort of coating on the inner and outer walls. The interior wall should be thoroughly wiped down with acetone or some similar solvent to cut any release agent or protective coating that might be there. The diameters are nominal, often not quite 12" but maybe a little larger or smaller. They use this devise to allow for telescoping for more efficient shipping. Measure the diameter across several positions and try and find one in the pile that's the closest to being round.

 

Measure off about 12" in about eight spots around the tube.

 

Use a steel ruler to connect the measurement points with a pencil line around the exterior circumference of the tube. Tube end should be sanded flat and true by rubbing it with circular motions on a level concrete garage or basement floor.

 

Cut out the upper rings as shown and explained on the plan and glue the inner one to the end of the tube. Glue another similar ring on the other outer or front end as a stiffener. Mark the ring that is the upper rotating ring with a nail punch to distinguish it from the end stiffening ring.

 

Ring is held tightly to the tube by masking tape until glue is dry.

 

Tube top is set into top box and tied. It should rotate freely with a little slop between the two rings. You should be able to rotate the tube top 360 degrees and the tube top should not rock but lie flat and even.

 

Coating with resin

NOTE: Resin and acetone should only be used in an open and well ventilated area.  Read the instructions on the resin and acetone containers and use good sense and take precautions in their use.

I use Evercoat polyester resin. It dries fairly quickly. Ace Hardware can get it for you by the gallon or quart. Two quarts ought to be enough. Get some extra resin hardener, perhaps two of the larger 1.35 fluid ounce tubes. I use about three to four times as much hardener as suggested. You will use acetone to clean things up. Acetone is a flammable liquid and gives off potentially harmful fumes in a small poorly ventilated area. Do not use around an open flame and allow no smoking in the area you are working. Small and unsupervised children should not be in the area when you work with resins and acetone. You will possibly have these liquids in wax coated paper cups and similar containers and they may be mistaken for soft drinks.

Working with polyester boat resin is not quite the same as painting. Wear goggles and don't wipe your eyes. It will ruin any clothes you wear. This stuff acts as much like a glue as a protective coating. It also has a short working life and can be a bit nasty to clean up. All cleanup is done with acetone. I use about three or four times as much hardener as recommended. This speeds up drying but shortens the working life. Plan on using a batch for no more than 10 minutes. When it starts to gell up, stop at once and clean the brush in a container of acetone. It should dry enough to handle in about three hours in a warm and dry environment. On humid days and in colder temperatures it will take longer. All parts should be laid out on ordinary wax paper. It will not stick to wax paper but will stick to about anything else and it will have an extreme reaction with and destroy Styrofoam. Use cheap brushes of 1" to 1.5" width. Application should be on flat level surfaces. Paint one inside and one outside surface. Let dry and turn over and do another two surfaces. You'll try to be neat but things will get a bit messy. Just have plenty of acetone ready to clean up things.

 

Use boxes and cans to raise parts up above the floor level. Cover these with wax paper. Note box to left. I have painted two surfaces, an inner and outer, and am waiting for them to dry before turning the box over 90 degrees and doing two more. More complex shapes and boxes must just be painted as best as you can. Watch out for runs. Take your time and expect to take a week of evenings to get the job done. Don't rush and keep your head clear.

 

Jig made from a stick for resining exterior of tube.

 

I mount my focusers on a flat base made of plywood with curved sides to fit the tube.

 

Base is resined on and held down to fit with a weight. Don't use too much weight or the tube will distort.

 

Finished rotating head piece.

 

Rotating retainers cut from plywood are covered with resin. The back is painted first and then squished down on the wax paper and the top and sides painted. When dry, the excess is cut off.

 

Spider and Secondary

The spider and secondary should be considered at this point, though not necessarily constructed. This information will be found here

 

Mirror cell

 

Front of lower box showing front of mirror. This is just an unfinished mirror blank used to determine weight and position.

 

Rear of lower box showing lower mirror plate and three adjustment screws. This is the simplest cell imaginable, two pieces of plywood and three adjusting bolts. The central nut holds the conical mirror in place. The four screws at the corners hold the lower mirror plate in place.

 

Lower box with mirror and cell removed.

 

Side view of mirror and cell. Note adjusting screws and compression springs in between the upper and lower mirror plates.

 

Another side view of mirror and cell. Note adjusting screws and compression springs in between the upper and lower mirror plates. The order of assembly is, from upper mirror plate downward; is 3/8" carriage bolt (tapped in with hammer to set head), upper plate, washer, spring, washer, lower plate, washer, wing nut. Compression springs can be gotten at hardware store and cut to length (+/- 3/4") with hacksaw.

 

Strut holes

Holes at corners were finally placed at 3/4" from each side on the center boxes and 3/4" from the center point on the upper and lower boxes. This creates the classic Surrier truss configuration and appearance.

 

Painting

 

Painting the parts. All parts must be sanded smooth starting with #100 and finishing with #220 and water. Any defects will only be amplified by the paint. Try and not sand through the polyester resin and into the wood. Note use of jig made from a stick for painting interior and exterior of tube and holding other parts as well. I used Rustoleum Painter's Touch spray paint. Apply moderately and evenly putting on one coat on a leveled flat surface followed by another in a few minutes. Be careful not to put on excessively thick coats and putting on additional coats after drying may result in crinkling if additional coats are too thick. Flat black is applied with a brush. Spraying both green and black will cause problems with over-spraying. I sprayed on the green first and then wiped out the inside of dust with a damp paper towel and brushed in the black.

 

Final assembly and placing of struts

 

The boxes and rotating head completed and ready for final finish and attachment of struts.

 

Screws and locating holes that hold the two center boxes together. Bolts come up through the inside of the lower center box and screw into tee nuts. Note two cross mark alignment marks (see red arrows) driven into the wood at the top left of upper center and lower center boxes. These are made by tapping a Phillips head screwdriver with a hammer. 

Tee nuts are simply torqued in taking care to position bolt perpendicularly.

 

Both center boxes assembled. Note 3/4" reinforcing piece at bottom. This is resined in place when resining boxes. Note red arrow denoting alignment marks. These are drilled with holes to take 3/8" bolts for mounting to telescope mount.

 

Rear of bottom box showing lower mirror base plate held to upper base plate with four 3/8" bolts at nuts at corners. Upper mirror plate is triangular piece attached with three equi-spaced 3/8" bolts and compression springs. Note alignment marks to insure proper installation upon re-assembly.

 

Bottom box looking downward from top.

 

Begin attaching the struts by arranging the bottom and center boxes on small pieces of plywood and making sure they are square and in line and separated by the correct distance. Distances between boxes and the correct position of the center box for correct balance is determined by using the spreadsheet program provided on this web page. Once the distance between the boxes is established the distance between the holes is measured and marked off on the strut tubes, leaving an excess length of 3/4" beyond the holes. 1.5" of the end of the strut is squashed in a vise and the holes drilled. I used 1/4-20 round head bolts 3/4" in length and one washer and a hex nut on the inside. One must take care to make the distances of the holes the same on all struts. The holes should be oversized a bit so as the not be tight or bind and allow for slight differences in hole position and to assist in final alignment. If you are careful, the boxes will come out straight and true. Do one side and the turn over 90 degrees and do another, keeping careful check on alignment of the two boxes as you proceed.

 

The finished boxes with the upper center box now attached.

 

Attaching the upper struts is begun by standing the telescope on end on a level floor and attaching the struts form the bottom trying to align as best as one can and tightening the nuts only a little bit.

 

The top is then put on and the nuts tightened by hand only. (NOTE: The strut mounting holes in the top section should be elongated with a rattail file so that there is sufficient longitudinal adjustment available.) A tape measure is used to make sure the top has the same distance from the floor on all sides. This might take some adjusting time but it's important to making the rotating head work without error. This will only be a preliminary adjustment but getting it pretty close is a good idea. See Adjusting the Rotating Head for final adjustment instructions.

Once the top box is set the felt is attached to the inside track with double sided Scotch tape. 

 

Care must be taken to have four of the felt pieces line the sides as well to form a buffer bearing against the head ring.

 

Three 1/4-20 bolts and tee nuts are set in and then the rotating head placed on top.

 

Retainers and knobs. Note felt stuck to bottom with double sided tape.

 

Retainers are now put on. 

 

And then the washer and knob. Knobs can be gotten from McMaster-Carr

 

Finished top section

 

Completed telescope