• John with daughter checking the 18" The Following are some images and drawings of some of my telescope making projects including design and construction of an 18" Dobsonian (Newtonian) telescope. Be sure and read the article on telescope making. Sometimes telescope construction keeps me so involved I forget to observe! Almost. It is always a great pleasure to use an instrument you have made yourself. Also check out my new page on Newtonian Collimation! TMy most recent project has beed to redesign my 10" Binoculars

    The Photos

      Two 10" Dobsonians I have built: I have built several dobsonians of this type (three 10" and one 18") these are like the 18" in that they are made of 3/4" oak plywood and have solid oak trusses. They were a good learning experience that prepared me for the problems I would run into building the 18".
      The 18" Before Finish was applied: I usually use the instrument for several months before applying the finish so that I can make any modifications that may become apparent with use. Once I have a tried and true design, then I apply the finish coat.
      The Finished 18": Applying the finish to all the parts takes almost as long as building the scope. It has two coats of stain and four coats of polyurethane applied with an airless sprayer. 
      The 18" With Shroud: My wife made a black shroud out of a black slightly elastic cloth. 
      The 18" and its Mirror Cover (Before Finish): The hinged mirror cover shows better before it was painted black. The hinges are standard spring loaded cabinet type.
      The Finished 18" Mirror Box: Now the flat black paint has been applied. I use Krylon Ultra Flat Black. Its odd stuff and if it is too hot when spraying it doesn't stick, but simply forms a black powder on the wood! Putting the can in the frige for thirty minutes does the trick. 
      The 18" Wood Finder: Years ago I purchased a pair of Celestron 11x80 binoculars that were badly out of alignment. Since they were useless as binoculars I salvaged the optics and built the finder you now see with wood to match the main scope. There is a front surface flat inside the box which allows the light to enter a conventional 1 1/4" eyepiece rather than a small 1" eyepiece. 
      The Secondary/ Spider Cage: This assembly is simply made from plastic laminate wrapped and glued around two wood rings. There is no other structure inside, the laminate is its own structure. It is very light yet strong. 
      A sequence showing the assembly of 18": As can be seen the upper cage stores inside the mirror box and a custom hand cart attaches to the rocker box and mirror box for easy transport. 
      The rear of the Mirror Box and Fan: The three knobs are for primary collimation. The knobs are from Reed Tool Supply (no relation). The mirror cell floats on three valve springs (from a car engine) which are the only springs I could find strong enough to support the 50 pound mirror. The cell is a 9 point flotation type with the three triangles made of 1/2" aluminum. The three eye bolts are for locking the cell once collimation is achieved. There is a hole in the center of the cell which houses a muffin fan for cooling the mirror. 
      The Teflon Bearings: I use 1/4" thick Teflon sheets which I cut into squares using a radial arm saw. Holes are then counter sunk so that screw heads will not protrude and scratch the laminate bearings. The side bearings are disks of plywood cut in a circle and edged with plastic laminate (Formica). In this way I can have a bearing of any diameter.
      The 18" Mirror: comes from Pegasus Optics formally of Huntsville, AR. The mirror is high quality and even gives good planetary images. 

    The Drawings

      Drawing of 18": I draw using Autocad 14. Each major system is on a different layer such as wood, glass, hardware, etc. The light path of a center star along with the edge of 100% illumination are shown to check for vignetting. 
      Drawing of 18" Mirror Box - Side View: This view shows the side bearing, mirror box, mirror and mirror cell. Also note the dashed arc which checks for clearance from the rocker box.
      Drawing of 18" Mirror Box - Front View: This view primarily shows the mirror cell inside the mirror box. Also visible are the three triangles which support the mirror with a 9 point system. Since the mirror is a full two inches thick this is sufficient to support the mirror and give diffraction limited images.
      Drawing of 18" Spider - Side View: Here can be seen the spider which holds a 3.1" secondary diagonal mirror. Also notice the recessed position of the focuser which helps to keep the diagonal mirror as small as possible by keeping the distance from the focal plane to the center of the tube as short as possible. 
      Drawing of 18" Spider - Front View: This shows the same area as the drawing above, but from the front. Again note the position of the focuser due to the focuser platform cutting a chord across the circle of the secondary cage. 

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  • This page was created by John W. Reed on December 6 2000. Contact: jreed007 at centurytel dot net.