What this calculator works out
This calculator divides the telescope's focal length by the eyepiece's, multiplied by any Barlow lens, giving the magnification.
Magnification is the most quoted and least important specification in astronomy. Aperture determines what you can see; magnification only determines how large it appears.
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Before you choose an eyepiece
- Both focal lengths are in millimetres, and both are marked on the equipment.
- There is a practical maximum useful magnification of roughly twice the aperture in millimetres — beyond that the image grows without gaining detail.
- Atmospheric seeing usually limits useful magnification well below the theoretical maximum, frequently to 150× or less.
- Low magnification with a wide field is the more useful setting for most objects, and most nights.
A telescope's aperture determines how much light it gathers and how fine a detail it can resolve. Magnification simply spreads that light over a larger apparent area, so pushing past what the aperture supports gives a bigger, dimmer, blurrier image. The '675× magnification!' printed on a cheap telescope box is a marketing figure, not a capability.
How this calculator works
Focal lengths divided, times any Barlow:
Magnification = telescope focal length ÷ eyepiece focal length × Barlow multiplier
Maximum useful magnification ≈ 2 × aperture in mmA shorter eyepiece gives more magnification, which is why eyepiece sets run from around 4 mm to 40 mm. A Barlow lens multiplies whatever eyepiece it is used with, effectively doubling the useful range of a set.
Worked example: a 900 mm telescope
Using the default figures — a 900 mm focal length telescope with a 25 mm eyepiece and no Barlow:
- Magnification: 36×
36× sounds low and is a genuinely useful magnification — wide enough to find objects, bright enough to see them, and tolerant of imperfect tracking. Adding a 2× Barlow gives 72×; swapping to a 6 mm eyepiece gives 150×. On a 150 mm aperture instrument the theoretical maximum is around 300×, and the atmosphere will rarely permit even half of that.
Common mistakes
- Buying a telescope on its quoted magnification. Aperture is the specification that matters.
- Starting at high magnification. Find the object at low power first, then magnify.
- Exceeding twice the aperture in millimetres. The image grows without gaining detail.
- Ignoring seeing conditions. Atmosphere limits magnification more than optics do.
- Forgetting the mount. High magnification amplifies every vibration.
Frequently asked questions
What magnification do I need?
Less than most beginners expect. Wide views at 30× to 60× suit star clusters, nebulae and finding things. 100× to 200× suits planets and the Moon on a steady night. Above that, conditions rarely cooperate. A good low-power eyepiece is used far more than a high-power one.
What is a Barlow lens?
An optical element that increases the effective focal length of the telescope, multiplying the magnification of any eyepiece used with it — typically by 2× or 3×. A 2× Barlow effectively doubles an eyepiece collection, which makes it one of the better-value accessories.
Why does the image get dimmer at higher magnification?
Because the same amount of gathered light is spread over a larger apparent image. Aperture fixes the light collected; magnification only distributes it. This is why faint objects are best viewed at low power and why a larger aperture, not a shorter eyepiece, is the answer to a dim image.
What is 'seeing'?
The steadiness of the atmosphere, which varies nightly and through the night. Poor seeing makes stars boil and blurs planetary detail regardless of the optics. It commonly limits useful magnification to 150× or less in the UK, and it is why patient observers wait for moments of steadiness rather than pushing the power up.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — consumer rights and product information for optical equipment
- Met Office — cloud and visibility forecasts relevant to observing
Sources are checked at publication and can change — how I choose and check references.