By Tom King – Updated Aug 30, 2022
Galileo’s first homemade telescope consisted of two lenses and a leather tube, proving that powerful optics can be crafted at home. Today, amateur astronomers build sophisticated reflectors using readily available parts and careful assembly. This guide walks you through creating a simple reflector telescope that balances performance with practicality.
Sketch the telescope to determine the spacing of the primary mirror, diagonal, and eyepiece. For this example, use a 42‑inch focal‑length mirror with a 6‑inch diameter. The outer tube measures 8 inches in diameter (radius 4 inches). Subtract 4 inches from the focal length to locate the primary mirror 38 inches from the diagonal’s center, ensuring the total distance from the mirror surface to the eyepiece opening is 42 inches.
Cut a sonotube to the length needed: base‑to‑mirror edge + mirror‑to‑diagonal distance + spider and diagonal mount depth + a couple of extra inches. Coat the interior with flat, non‑reflective black paint to reduce stray light.
Secure the mirror in its cell following the manufacturer’s instructions. Then attach the cell to the sonotube base by drilling holes that match the mounting screws. Tighten the screws until the cell is perfectly centered.
Mount the spider opposite the viewing side of the scope, with the diagonal attached to it. To position the spider, measure 38 inches from the mirror to the diagonal’s center. Add the distance from the mirror to the sonotube base to find the exact screw‑hole spacing. Cut slot‑shaped holes around the tube’s top and insert the spider legs; adjust them to center the diagonal.
Perpendicular to the diagonal’s center, drill a hole for the focuser. Calculate its position by adding the mirror‑to‑diagonal distance to the cell depth, then drill to the width of the eyepiece mounting tube.
Look through the empty focuser hole and align the diagonal with the primary mirror so the sonotube opening appears centered in the eyepiece. Secure the focuser on the tube’s exterior and insert your chosen eyepiece.
Screw the finder scope mount to the tube so it sits parallel to the main tube, easily reachable, and unobstructed by the focuser or eyepiece.
Measure twice, cut once.
Handle the mirror with care—fingerprints can severely degrade image quality. Use only lens cleaner and dedicated lens cloths; avoid frequent cleaning to prevent scratches, which are far more detrimental than dust. Follow manufacturer guidelines when cleaning optical elements.
