EyepieceMagnifies the image; changing eyepieces changes the magnification.
Objective lensGathers the light and forms the image the eyepiece magnifies.
Optical tubeAlso called the telescope tube or the OTA.
FinderscopeLow power and a wide view, for aiming the main telescope.
Star diagonalA mirror or prism turns the light 90 degrees. That makes looking up easy.
Focus knobTurn it until stars are sharp points. It slides the eyepiece in or out.
Equatorial mountOne axis points at the celestial pole. One knob then follows a star.
TripodThree legs stand steady even on uneven ground.
CounterweightBalances the tube on the other side of the mount.
Tube ringsLoosen them to slide the tube and balance it.
The reflector, cut open 4
Primary mirrorGathers the light and bounces it back up the tube.
Secondary mirrorSends the light out the side to the eyepiece. It is small and flat.
Spider vanesThe spider holds the secondary mirror in the light path.
FocuserOn a reflector it sits at the side, near the open end.
How this list was compiled: Part names checked on 2026-09-29, a name kept only when two sources use it. Celestron PowerSeeker series instruction manual, https://s3.amazonaws.com/celestron-site-support-files/support_files/1245780194_powerseekerinst.pdf (telescope optical tube, tube rings, finderscope, eyepiece and diagonal, focus knob, tripod, counterweight bar and counterweight, equatorial mount, objective lens; for the Newtonian: primary mirror at the back of the tube, secondary mirror under the focuser). Sky-Watcher Esprit ED and Quattro Newtonian instruction manuals, https://inter-static.skywatcher.com/upfiles/en_download_caty01390352363.pdf and https://inter-static.skywatcher.com/upfiles/en_download_caty01440182865.pdf (tube rings, diagonal, finderscope, focuser; primary mirror, secondary mirror, spider vanes). Wikipedia, text only: Refracting telescope (a lens as its objective forms the image), Newtonian telescope (concave primary and flat diagonal secondary mirror; the spider holds the secondary), Finderscope, Star diagonal (turns the view perpendicular to the eyepiece axis), Equatorial mount (one axis parallel to Earth's axis; counterweight). Licence: own work
The drawing: Drawn for this site on 2026-09-29 from plain geometry; no maker's product is copied. Licence: own work
How it works
A refractor gathers light with a lens at the front; a reflector gathers it with a mirror at the back. This page draws both, so the eyepiece, the finderscope and the two mirrors each have a place to point to.
Read the full guide
Two telescopes, one set of rings
The top drawing is a refractor on a tripod and equatorial mount. The bottom one is a Newtonian reflector with the near side of its tube cut away; the dashed lines are the light, in at the open end, off the big mirror and out through the focuser.
Worksheet words first
Numbers 1 to 8 are the parts a school worksheet asks for, from the eyepiece to the tripod. Numbers 9 to 14 add the counterweight, the tube rings and the reflector's mirrors.
A star diagonal flips the view left to right, so a map of the Moon looks mirrored through it.
The finderscope must point where the main tube points: line them up on a far tree in daylight.
Questions and answers
What are the main parts of a telescope?
The optical tube, the objective lens or primary mirror that gathers light, the eyepiece that magnifies the image, the focus knob, the finderscope for aiming, and the mount and tripod that hold it steady.
What is the difference between a refracting and a reflecting telescope?
A refractor uses a lens at the front of the tube to gather light. A reflector uses a curved mirror at the back, and a small flat mirror sends the light out the side to the eyepiece.
What does the counterweight do on a telescope?
On an equatorial mount the tube sits on one side of the mount's axis. The counterweight on the other side balances it, so the telescope stays where it is pointed and turns smoothly.