Telescope LabOPTICS / SC217
Two converging lenses · Keplerian telescope
INTERACTIVE RAY DIAGRAM

See how a telescope works

Adjust the lenses and object distance to see how the intermediate image becomes the object for the eyepiece.

Telescope ray diagram

Ideal thin lenses
Horizontal distances share a physical scale; the vertical scale is enlarged for clarity. The objective is at x = 0.
Parallel incident rayRay through objective centerRay through front focusBackward virtual extensions
Measured from the objective
For a distant object with parallel output

Follow the rays from objective to eyepiece

1 / fₒ = 1 / uₒ + 1 / vₒ　　uₑ = L − vₒ
1 / fₑ = 1 / uₑ + 1 / vₑ

For a closer object, vₒ increases. To keep the outgoing rays parallel, increase the tube length: L = vₒ + fₑ. For a star at infinity, L = fₒ + fₑ.

Explore the assignment with presets

1. Distant star: parallel incoming rays form an inverted real image at the back focal plane of the objective.
2. Finite distance: bring the object closer and watch the intermediate image and focused tube length change.
3. Magnifier: place the intermediate image between the eyepiece and its front focal point to form a final virtual image.

Model assumptions and terminology in the assignment

The assignment specifies two converging lenses, so this simulation uses a Keplerian telescope. The terms “Galilean” and “both ... concave” in the Theory section conflict with that requirement: a Galilean telescope uses a converging objective and a diverging eyepiece. This model uses ideal thin lenses and the paraxial approximation; it excludes chromatic aberration, diffraction and aperture clipping. Finite object distances stay above fₒ so the objective forms a real intermediate image. Infinity mode uses a parallel bundle at an off-axis angle of 1°. At finite distances, the object height is approximately 0.01745uₒ to keep its angular size nearly constant.