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
Ten lines lead off the refractor, the upper drawing, and four off the reflector below it: one to each part, ready for a name.
Read the full guide
The fourteen names to write in
Upper drawing, in the order of the numbered copy: eyepiece, objective lens, optical tube, finderscope, star diagonal, focus knob, equatorial mount, tripod, counterweight, tube rings. Lower drawing: primary mirror, secondary mirror, spider vanes, focuser.
Three pairs that get swapped
The objective lens and the primary mirror both gather the light: the lens at the front of the refractor, the mirror at the closed end of the reflector. The star diagonal and the secondary mirror both turn the light toward the eyepiece: the diagonal is the angled block at the back of the refractor, the secondary the small tilted mirror inside the reflector. The focus knob is a round knob under the refractor's tube; the focuser is the short tube holding the reflector's eyepiece.
For a younger class, ask for the first eight names only, eyepiece to tripod.
The lower telescope has an eyepiece, a finderscope and tube rings too, with no line: each is named once, on the upper drawing.
The dashed lines inside the lower tube are the path of the light, not parts.
Questions and answers
What does the finderscope do on a telescope?
It aims the main telescope. It is a small telescope fixed along the same line of sight, with a much lower magnification and so a wider view, which makes a target easier to find.
What are spider vanes on a telescope?
The thin arms that hold the secondary mirror in the middle of a reflector's tube. Two lines leave that spot on the sheet: the one ending lower left is for the spider vanes, the one ending lower right for the mirror.
Where is the eyepiece on a reflecting telescope?
On a Newtonian reflector, the kind drawn here, it is on the side of the tube near the open end. The secondary mirror sends the light out at a right angle, so the focuser and the eyepiece sit there and not at the back.
What is an equatorial mount on a telescope?
The head between the tripod and the tube. One of its axes is set parallel to Earth's axis, so turning the telescope about that axis alone follows a star across the sky. Its line leaves the block above the three legs.
Does the telescope diagram come with an answer key?
Yes. The key is the same sheet with all fourteen names written at the ends of the lines. The numbered version prints a numbered line for each name instead.