The Secondary Spider
Making a 4.25 Inch Dobsonian Reflector Telescope
The Spider is a contraption to hold the secondary mirror in place, and allow its adjustment. Like other components such as the focuser and the mirror cell, you can purchase a ready made spider on-line. The version I made is nicely adjustable with thumb screws, but because it is relatively small to fit in the 5 inch tube, in order to fabricate it I needed to use a milling machine. This is not a machine for the faint of heart. I built a previous version using just wood. This one was made from plastic. The design is almost identical to the mirror cell except for size. There are two cylindrical parts that are connected by three bolts. The mirror side needs to be cut at a 45 degree angle for the mirror. The mirror is attached with silicon caulk. The connection to the tube is made with long bolt threads, cut with a hack saw.