Telescope Magnification Calculator
Found on the telescope tube label — typically 400–3000mm
Diameter of the primary mirror or lens — used for exit pupil, limits, and capabilities
Affects the maximum useful magnification (Good = 2.5×/mm, Average = 2×/mm, Poor = 1×/mm)
Stamped on the eyepiece barrel — common values: 4, 6, 10, 17, 25, 32mm
Listed in eyepiece specs — Plössl ≈ 50°, wide-angle ≈ 68–82°
Enter Your Telescope Details
Fill in your telescope focal length and eyepiece focal length to calculate magnification, exit pupil, field of view, and more.
So verwenden Sie diesen Rechner
Enter Your Telescope's Focal Length and Aperture
Look at the label on your telescope tube or check the manual for the focal length (e.g., 1000mm) and aperture (e.g., 200mm). Use the preset buttons for common telescope configurations to fill in values instantly. If you use a Barlow lens or focal reducer, select the appropriate multiplier — the calculator automatically adjusts the effective focal length.
Enter Your Eyepiece Focal Length
The eyepiece focal length is stamped on the barrel — common values include 4mm (high power), 10mm (medium), 17mm (medium-wide), and 25mm (low power). Use the eyepiece preset buttons to quickly select standard sizes. Optionally enter the apparent field of view (AFOV) from your eyepiece's specifications to calculate the true field of view.
Select Seeing Conditions and Read Your Results
Choose your expected atmospheric seeing (Good for stable nights, Average for typical conditions, Poor for turbulent skies). The calculator displays magnification, exit pupil, true field of view, resolution limits, and light grasp. The color-coded magnification range bar shows whether your setup is under-powered, optimal, or over-powered for the conditions.
Use Compare Mode for Session Planning
Switch to Compare Eyepieces mode and enter your entire eyepiece collection with focal lengths and apparent field of view values. The calculator generates a side-by-side table showing magnification, exit pupil, true field of view, and quality rating for each eyepiece — helping you choose the best eyepiece for each target before your session begins. Export to CSV for offline reference.
Häufig gestellte Fragen
What magnification is best for viewing planets?
For planets, most observers find 150×–250× optimal under average seeing conditions. Saturn's rings become clearly separated at around 100×, and its Cassini Division is visible around 150×. Jupiter's main cloud belts are clear at 100×, with finer details emerging at 150–200×. Mars shows polar ice caps and surface albedo features around 150×. The key limitation is atmospheric seeing — pushing beyond 200× on a turbulent night produces a bigger but blurrier image, actually revealing less detail. Wait for steady nights (when stars twinkle less) and try incrementally increasing magnification while watching whether the image sharpens or degrades.
What is exit pupil and why does it matter?
Exit pupil is the diameter of the light beam that exits the eyepiece and enters your eye, calculated as aperture divided by magnification. A human dark-adapted eye has a maximum pupil diameter of about 7mm, so exit pupils larger than 7mm waste collected light. For planetary viewing, 1–2mm exit pupils provide maximum contrast. For deep-sky objects under dark skies, 4–7mm exit pupils give bright, comfortable views. Exit pupils below 0.5mm become practically unusable because tiny vibrations, eye floaters, and eyelashes obscure the view. Matching the exit pupil to the target and conditions is often more useful than focusing on magnification numbers alone.
What is the maximum useful magnification of my telescope?
The commonly cited rule is 2× per millimeter of aperture — so a 200mm telescope has a maximum useful magnification of about 400×. However, this theoretical limit is rarely reached in practice. Atmospheric seeing typically limits useful power to 200–300× on average nights, regardless of aperture. Excellent nights with stable air may support 300–350×, while poor nights cap out around 100–150×. Our calculator adjusts the maximum based on your selected seeing condition. Large aperture telescopes are limited by seeing before they run out of resolution; for a 150mm refractor, the theoretical 300× limit is often atmospheric, not optical.
How does a Barlow lens affect magnification and exit pupil?
A Barlow lens is a negative lens inserted between the focuser and eyepiece that increases the effective focal length of the telescope by its multiplier. A 2× Barlow doubles the telescope's effective focal length, doubling magnification for any given eyepiece. A 25mm eyepiece in a 2× Barlow on a 1000mm telescope gives the same 80× magnification as a 12.5mm eyepiece used alone. The exit pupil is halved when using a 2× Barlow (or halved when magnification doubles). Barlows are excellent value: one good Barlow effectively doubles your eyepiece collection. A quality Barlow preserves image sharpness; inexpensive Barlows can introduce aberrations, particularly at the field edge.
What is the Dawes limit and does it affect what I can see?
The Dawes limit (116 ÷ aperture in mm) gives the theoretical minimum angular separation of two equal-brightness stars that the telescope can resolve, measured in arcseconds. A 100mm telescope has a Dawes limit of about 1.16 arcseconds. For visual observation, you need sufficient magnification to see the split — roughly 1.5× per arcsecond of separation per 25mm aperture, or just enough to make the gap apparent. The Dawes limit is most relevant for double star observers. For planetary detail and extended objects, contrast and seeing quality matter more than angular resolution alone. The Rayleigh limit (138 ÷ aperture in mm) is a slightly more conservative criterion based on diffraction theory.
What does light grasp mean and how does it compare to the naked eye?
Light grasp measures how much more light a telescope collects compared to the unaided human eye. The human eye's pupil dilates to about 7mm in the dark, so a telescope with aperture D (in mm) has a light grasp advantage of (D/7)². A 100mm telescope collects (100/7)² ≈ 204× more light than the naked eye, allowing it to see stars about 5.8 magnitudes fainter. A 200mm telescope has about 816× greater light grasp and reveals stars roughly 7.3 magnitudes fainter. This translates directly to limiting magnitude: more aperture reveals fainter stars, more nebula detail, and fainter galaxy structure. Light grasp, not magnification, is why astronomers covet large apertures for deep-sky observing.