Monthly Archives: December 2014

Building an observing chair

A telescope‘s eyepiece is never at the right height. Either you have to stand precariously on tiptoe or squat indecorously to reach it. A chair can relieve some backache but it’s usually too tall or too short. Some commercially available observer’s chairs are a little better because they adjust over a range of about six inches in height.

But this range is too small for my Dobsonian. Its eyepiece can vary in height by 30 inches. So I designed my own adjustable chair that uses a tripod base for stability. Its seat slides to a height of 42 inches and down to less than 21 inches. The chair accommodates eyepiece heights of about 40 inches to nearly 70 inches.

You can make a copy of my chair in two or three weekends from five 1x2s eight feet long, a one-foot length of 1×3, a one-foot length of 1×6, a two-foot length of 1×8 or its equivalent in plywood, and a 1′ by 3′ section of 3/4″ exterior-grade (AC) plywood. A one-foot length of oak 1×4 is optional; this piece can be cut instead from 3/4″ plywood. Hardware consists of sixteen 3″-long #8 wood screws and nine sets of 3″-long 1/4-20 bolts with washers, lock washers, and wing nuts. The total cost of parts including the finish should run under $50.

Making the Tripod

Each leg of the tripod uses two 1x2s that are 48″ long. Eighteen 3″-long 1×2 spacers separate the leg pieces. Two of the legs use four spacers glued and nailed 11″ apart. Start with the end of the first spacer flush with the ends of the leg pieces. The third leg uses eight spacers set 3″ apart. This leg will hold the seat. Be sure to use a glue rated for exterior use. Finishing nails (3d) hold the spacers in place while the glue sets.

Round the bottom end of each leg so it contacts the ground properly and the top end so it doesn’t strike the tripod head when the leg pivots. Drill a 1/4″ hole four inches from the bottom of each leg. Drill a second hole in each leg 3/4″ from the top end.

The top ends of the legs attach to the tripod head. The head consists of two parts: a horizontal top piece and three vertical leg supports. The top piece should be round or hexagonal to prevent a sharp corner from digging into your back. Cut it from the 1×6.

The three leg supports are 2 1/2″ lengths of 1×3. Drill a 1/4″ hole i n the center of each support. For added strength, you can cut one support slightly longer, about 3 1/4″. This enables you to bevel the end of the piece, so the support nestles against the ends of the other two.

Set one leg support in place, checking that the wood grain lies parallel with the top piece of the tripod head. Glue and secure this piece in place with two #8 screws. Glue and screw the other two pieces 120 [degrees] apart from the first piece.

Three spreaders keep the tripod legs from moving apart. These are 20″ lengths of 1×2. Round one end and drill a 1/4″ hole 3/4″ from the end. This end connects to the bottom hole of the leg. Drill a second hole 3/4″ from the other end, going through the 1 1/2″ dimension of the piece. This end connects to the spreader plate.

The spreader plate can be round or hexagonal, about 4″ across. Cut it from the remaining 1×6 material. Drill three holes 120 [degrees] apart and 1 1/4″ from the center. Or you can use two sturdy gate hinges mounted on the sides of the spreaders in place of the plate.

Now is the time to sand and finish the tripod parts. This will help keep the parts from swelling and warping. Use an outdoor enamel or an exterior-grade varnish. Be careful that you don’t build up too much paint in the slots of the leg the seat rests on.

The Sliding Seat

The key to the tripod chair is the sliding seat. Hidden in the middle of the seat assembly is a “finger” that engages the slots in the leg. Rocking the seat up disengages the finger and allows you to move the seat up and down.

Start with the sides. Cut these from the 1 x8. The bottom of each piece is 6 1/2″ across, while the tops are 4 3/4″ wide. This trapezoidal shape permits the seat assembly to rock. The back of the seat assembly consists of three 11 1/2″ lengths of 1×2.

The front of the assembly is more complicated (see above). It consists of four pieces: two seat supports, the finger, and a seat cleat. Cut the seat supports from 3/4″ plywood. Each support is basically triangular with a skinny rectangle added to one of the long sides. The base of the triangle forms the top of the seat support. The long sides of the triangle are 11 1/2″, separated by a 21 [degrees] angle, and the triangle’s base is 81/4″. The “rectangle” replicates a 1×2; it’s 1 1/2″ wide and 11 1/2″ long.

The finger is 11 1/2″ long. I made mine from an oak 1×4 since this piece takes the brunt of the weight. The top of the piece is 1 1/2″ wide. About three inches from the bottom it starts to flare out, reaching its full 3″ width 3/4″ to 1″ from the bottom.

Sand the bottom “knuckle” part of the finger. This part must slide easily into the slots of the tripod leg, so test fit it in your assembled leg.

Make the seat cleat from an 81/4″-long piece of 1×2. Cut the ends so they match the angular sides of the top of the seat support.

To assemble the seat support, glue the three back pieces together and the finger and seat cleat between the two seat supports. Then glue these subassemblies to the side pieces using 2″-long #8 screws to hold the pieces while the glue dries. Drill pilot holes for the screws, especially if you use oak for the finger.

The remaining bit of construction is the seat itself. Use 3/4″ plywood for strength. My seat is 9″ long, 4″ wide at the front, and 7 1/2″ wide at the back. Round all the corners and the edges and sand well — you don’t want any splinters. Glue and screw the seat to the seat cleat with two screws.

With the seat in place, you can sand and finish the seat assembly. For additional comfort, you can add some foam and cover the seat with vinyl. Or try the gel seat material they sell in bike shops.

Slide the seat assembly onto its tripod leg and then attach the legs to the tripod head with 1/4-20 bolts. Attach the spreaders first to the plate and then to the legs. Now you have a working tripod chair.

To transport your chair, simply remove the bolts holding two of the spreaders to the legs. Rotate the loose spreaders toward the remaining spreader, fold the spreaders up toward the leg, and fold the legs together.

With your new chair, you’ll no longer perch precariously over your scope, risking life and optics. And as your eyepiece goes up and down, so will you.