Newtonian telescopes, with a large mirror at the bottom of the optical tube, reflect light back up the tube to a small, secondary mirror that reflects the light out the side of the telescope. The secondary mirror is positioned in the centre of the optical tube and held in place by a support called a "spider." The spider must be thin so as not to obstruct light but sturdy enough to support the secondary mirror. A homemade telescope spider can be made and mounted in an hour with simple tools, some inexpensive material and an assistant.
- Skill level:
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Things you need
- Measuring tape
- 2 sheet metal strips, 1/16 inch thick, ¾ inch wide, 1 ½ inches longer than the diameter of the telescope tube
- Electric hand drill
- ¼-inch drill bit
- 4 Roundhead bolts and nuts, ¼ by ½ inch
- ¼-inch box wrench
- Vice grips
- Small metal file
- Bubble level small enough to fit into the telescope tube
- Matt black spray paint
- Silicone glue (optional)
Remove the telescope's primary mirror so it is not damaged during work. Determine the inside diameter of the telescope tube.
Locate the optical opening on the side of the telescope tube. The ocular draw tube, where the eyepiece is held, is located at this point.
Drill a ¼-inch hole centred at the end of each sheet metal strip so that the hole is ½ inch from the end of the metal strip. Make a 90-degree bend on the two ends of both pieces of sheet metal, so that the distance between the outside edges of each bend on the metal strip is exactly the diameter of the telescope tube. The metal strip will be placed in the telescope tube, and the outside edges must contact the telescope tube.
Cut a 1/16-inch slot halfway through each metal strip at the centre. Use a metal file to remove enough metal so that the two metal strips can be connected by sliding the open slot on one strip into the open slot of the second strip. Ensure that the two metal sheets are connected so that all edges are flush. Spray paint the sheets matt black.
Attach the secondary mirror to the spider so that it is angled at 45 degrees. You may attach the mirror using silicone glue so that it can be easily adjusted before the glue dries, but some secondary mirrors come with an attachment. Secondary mirrors come in different sizes, shapes and on many types of mounts, so you will need to make minor mechanical adjustments to obtain the correct orientation.
Have an assistant look through the side hole of the telescope tube as you lower the spider into the telescope. The assistant will help you get the secondary mirror centred on the optical axis of the side hole.
Angle the telescope tube straight up and use a bubble level to ensure that the spider is level within the telescope tube. Secure the spider in place using ¼-inch Roundhead bolts, then apply a small amount of matt black paint to the bolt heads to reduce light reflection.
Tips and warnings
- Attach the secondary mirror using the attachment that comes with the mirror or fabricate a mounting surface using small strip of sheet metal. In most cases, you can attach the mirror to the spider using a large glob of silicone glue.
- Wear protective gloves when cutting sheet metal.
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