Week-11 :- Machine design

Realising the telescope

Dobson's telescopes is a simplified mechanical design that was easy to manufacture from readily available components to create a large, portable telescope. This telescope can capture faint, deep-sky objects such as nebulae and galaxies. However Dobson's telescopes require two concave mirrors. Inorder to make a lens of size 4" to 8" size require high temperature furnace and special abrasive powders with different grith sizes. We dont have furnace which can handle such high temperatures, also the lens cost is too much to bear. Hence we implemented Keplerian refracting telescope in lieu of Dobson's telescope.

Construction of Keplerian refracting telescope

Keplerian telescopeis simple in construction having one convex lens and eye piece. The advantage of this arrangement is that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted. Simple objective lens needs to have a very high f-ratio for higher magnification. The convex lens with maximum focal length available in our local market is upto 1 meter. The specification of telescope and its parts are given below.

  • Convex lens :- Diameter 5cm, Focal Length 100cm.
  • Eyepiece :- Diameter 4cm, Focal length 5cm.
  • Optical zoom :- 20x.
  • Pipe reducer 3" to 2.5"
  • Pipe reducer 2.5" to 2"
  • 3" PVC pipe of 50cm length.
  • 2.5" PVC pipe of 22cm length.


    The assembly of this telescope is simple, connect the 3" pipe with 3"-2.5" reducer, the other end can be connected with 2.5"-2" reducer. The 2.5" pipe can be connected with 2.5"-2" coupling. The bigger end of the telescope is equiped with 5cm convex lens and the smaller end is connected with eyepiece. The eyepiece can be mounted in another sliding tube, such that the focusing can be easily adjusted. I have used an old cardboard casing of local soldering iron, which exactely slides through the 2.5" pipe. Both ends have been masked with black color to create a realastic appearance.



    The lens can be glued to the end face or else you can make a small holder which slides into the pcv and keep the lens in its position. The design of lens holder and its stl files can be downloaded here. The same model is shown below.



    Realising the lens holder

    The lens holder part is designed in NX software, these parts have been realised. Both lens were inserted from slits provided onto the parts. The picture of the same is mentioned here.



    Assembly of objective

    The objective of the telescope having a diameter of 50mm and focal length of 100cm. The assembly of the objective 3" to 2.5" reducer is shown below.

    Assembly of eyepiece

    The eyepiece of the telescope having a diameter of 50mm and focal length of 5cm. The assembly of the objective to the holder is shown below.

    The telescope will be provided with rotating mechanism in both horizontal and vertical direction. The vertical motion is controlled by stepper motor through spur gears (gear ratio 1:10). The whole telescope weight is shared by two roller bearings at each arms. The telescopic arms are realised from plywood using shopbot CNC machine. The whole supporting structures have been milled out from CNC.



    All the cutting features are realised using tight fit. The telescope can rotate 360 deg in the vertical plane. The design files for the same can be downloaded here. Three different arms have been realised so far and we have selected the third design because of additional degree of freedom on vertical plane.




    The rest of the designing are covered under group project page, Click here.