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Optics

Telescope Magnification Calculator

Calculate magnification, exit pupil, and useful range for your telescope setup.

Interactive calculator

Telescope Magnification Calculator

Calculate magnification, exit pupil, and useful range for any telescope and eyepiece combination.

Try an example

Your result will appear here.

Choose a calculation mode, fill in the known values, and click Calculate.

Quick Guide

  • Enter telescope and eyepiece focal lengths.
  • Include aperture to see exit pupil assessment.
  • Stay within the useful magnification range.

Table of Contents

Key Takeaways

  • Magnification = telescope focal length / eyepiece focal length.
  • Exit pupil = aperture / magnification — must be ≤ 7 mm to avoid wasting light.
  • Maximum useful mag ≈ 2× aperture (mm); minimum ≈ aperture/7.
  • Higher magnification means dimmer images — aperture determines brightness.
  • A Barlow lens doubles magnification without changing the eyepiece.

Telescope Magnification

Telescope magnification tells you how much larger an object appears compared to the naked eye. It depends on the ratio of the telescope’s focal length to the eyepiece’s focal length. More magnification isn’t always better — it comes at the cost of a dimmer image and narrower field of view.

Formulas

M=FtelFepM = \frac{F_{tel}}{F_{ep}}
EP=DM(exit pupil in mm)EP = \frac{D}{M} \quad \text{(exit pupil in mm)}
Mmin=D7,Mmax=2D(D in mm)M_{min} = \frac{D}{7}, \quad M_{max} = 2D \quad (D \text{ in mm})

Exit Pupil Guide

Exit PupilUseQuality
> 7 mmToo largeLight wasted (pupil too small)
5–7 mmDeep-sky widefieldMaximum brightness for faint objects
2–5 mmGeneral observingOptimal — comfortable and detailed
1–2 mmPlanetsGood detail, dimmer image
< 1 mmVery high powerOnly for Moon/bright planets

How to Use

  1. Choose a calculation mode.
  2. Enter telescope and eyepiece specs.
  3. Click Calculate for magnification with exit pupil assessment.

Examples

8" Dob + 10mm eyepiece

M = 1200/10 = 120×; EP = 200/120 = 1.67 mm (good for planets)

60mm refractor range

M_min = 60/7 ≈ 9×; M_max = 2 × 60 = 120×

FAQ

Why can't I just use maximum magnification?›

Higher magnification spreads light over a larger image, making it dimmer. Beyond about 2× the aperture in mm, images become too faint and blurry (limited by diffraction and atmospheric seeing). A 200mm telescope maxes out around 400× under ideal conditions.

What is exit pupil and why does it matter?›

Exit pupil is the disc of light exiting the eyepiece into your eye. If it's larger than your pupil (5-7mm dark adapted), some light is wasted. If too small (<0.5mm), floaters become visible and the image is hard to use. Optimal: 2-5mm for most observing.

How does a Barlow lens affect magnification?›

A Barlow lens is a diverging lens placed before the eyepiece that multiplies the telescope's focal length (typically 2× or 3×). A 2× Barlow with a 25mm eyepiece gives the same magnification as a 12.5mm eyepiece alone, but with better eye relief.

What magnification do I need for planets?›

Planets benefit from moderate-to-high magnification: Jupiter and Saturn look best at 100-250×, Mars at 150-300×. The Moon is detailed at 50-200×. Start lower and increase until the image degrades.

Sources

  1. Tele Vue – Understanding Magnification
  2. Sky & Telescope – Choosing Eyepieces
  3. Wikipedia – Telescope Magnification
Manish Kumar

Author & technical reviewer

Manish Kumar

PhysicsCalcs tools are reviewed with an educational focus: clear formulas, transparent assumptions, and practical context for students and science learners.

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