What Magnification Will I Get?
What This Calculator Tells You
This calculator determines the magnification (power) you'll achieve when using a specific eyepiece with your telescope. Add your telescope's aperture to see a power level classification that accounts for your specific telescope's capabilities.
Calculator
Magnification
Add your telescope's aperture to see power level analysis:
Diameter of your primary mirror or lens.
1 = no Barlow, 2 = 2x Barlow, etc.
Rich-field viewing. Great for open clusters, large nebulae, and framed views.
Your telescope: 200mm aperture at f/5.0
Power Levels by Exit Pupil
| Level | Exit Pupil | Best For |
|---|---|---|
| Very Low | > 5.0 mm | Milky Way sweeping, large nebulae, finding objects |
| Low | 3.0 – 5.0 mm | Open clusters, large nebulae, rich-field views |
| Medium | 2.0 – 3.0 mm | General deep-sky, lunar observing |
| High | 1.0 – 2.0 mm | Planets, lunar detail, globular clusters, doubles |
| Very High | 0.5 – 1.0 mm | Planetary detail, tight doubles (needs steady seeing) |
| Excessive | < 0.5 mm | Usually beyond useful power |
Why Exit Pupil Matters
Using fixed magnification thresholds (like "50x is low power") doesn't account for different telescope apertures. A 50x magnification means very different things on a 50mm refractor versus a 200mm reflector.
Exit pupil (aperture ÷ magnification) gives a consistent measure across all telescopes:
- Same exit pupil = same image brightness regardless of telescope size
- Larger exit pupil = brighter image but lower magnification
- Your eye's pupil limits useful exit pupil to about 5-7mm (dark-adapted)
Rules of Thumb
- Maximum useful magnification: About 2× per mm of aperture (50× per inch). Beyond this, you're usually just magnifying blur.
- Minimum useful exit pupil: Around 0.5mm. Smaller exit pupils give dim images and are very sensitive to seeing conditions.
- Atmospheric limit: Even with perfect optics, turbulent air rarely allows more than 250-300× on most nights.
- Start low, go high: Always begin at low power to find and center your target, then increase magnification as needed.
Related Calculators
- Exit Pupil Calculator - Explore exit pupil in more detail
- Maximum Useful Magnification - Find your telescope's magnification limit
- True Field of View Calculator - See how much sky you can view at this magnification