To use this tool effectively, you need to know four specifications about your telescope and eyepiece setup. These are typically printed on the telescope tube and eyepiece barrel:
Enter your telescope's focal length and aperture, then your eyepiece's focal length and AFOV. Click "Calculate" to see all derived optical parameters. You can also use the preset buttons in the sidebar to quickly load common telescope configurations and see how they perform.
Magnification is calculated by dividing the telescope's focal length by the eyepiece's focal length: Magnification = Telescope FL / Eyepiece FL. A 1000mm telescope with a 25mm eyepiece gives 40x magnification. With a 10mm eyepiece, the same telescope gives 100x.
Higher magnification is not always better. As magnification increases, the image becomes dimmer (the same amount of light is spread over a larger area), the field of view narrows, and atmospheric turbulence (seeing) has a greater effect. Most experienced observers use moderate magnification for most targets and reserve high power for planets and double stars on nights of excellent seeing.
The True Field of View is the actual angular diameter of sky visible through the eyepiece. It is calculated as: TFOV = AFOV / Magnification. A 52° AFOV eyepiece at 40x magnification gives a TFOV of 1.3°. The full Moon is about 0.5° in diameter, so this setup would show the Moon with comfortable space around it.
TFOV is critical for choosing the right eyepiece for a target. Large objects like the Andromeda Galaxy (3° × 1°) require a wide TFOV. Planetary detail requires high magnification and a narrow TFOV. Knowing your TFOV helps you select the appropriate eyepiece before going outside.
The exit pupil is the diameter of the beam of light leaving the eyepiece, calculated as: Exit Pupil = Aperture / Magnification. It represents the size of the light cone that enters your eye. For the image to appear at full brightness, the exit pupil must be smaller than your eye's pupil diameter.
The dark-adapted human eye has a pupil diameter of about 5–7mm (decreasing with age). An exit pupil larger than your eye's pupil means some light is wasted — the image will not appear brighter than with a smaller exit pupil. An exit pupil smaller than about 0.5mm produces a very dim image and makes the effects of eye floaters more noticeable.
Optimal exit pupil depends on the target: 4–7mm for wide-field views of large nebulae and star fields; 2–4mm for general deep-sky work; 1–2mm for globular clusters and galaxies; 0.5–1mm for planetary and lunar detail.
The focal ratio (f/number) is the telescope's focal length divided by its aperture. A 1000mm focal length telescope with a 100mm aperture is f/10. Focal ratio affects the brightness of extended objects (nebulae, galaxies) in the eyepiece — lower f/numbers (faster telescopes) produce brighter images of extended objects at a given magnification. It also affects the performance of eyepieces: some designs work better at fast focal ratios than others.
The limiting magnitude is the faintest star your telescope can detect under ideal conditions. It depends primarily on aperture: larger apertures collect more light and can detect fainter objects. The formula used is approximately: Limiting Magnitude = 2.1 + 5 × log₁₀(aperture in mm).
Resolving power is the smallest angular separation between two stars that the telescope can distinguish as separate objects. It is given by the Dawes limit: 116 / aperture (in mm), in arcseconds. A 100mm telescope can theoretically resolve double stars separated by 1.16 arcseconds.
| Target Type | Recommended Magnification | Exit Pupil |
|---|---|---|
| Wide star fields | 20–40x | 4–7mm |
| Open clusters | 30–80x | 3–5mm |
| Nebulae (large) | 20–60x | 3–6mm |
| Galaxies | 50–150x | 1–3mm |
| Globular clusters | 100–200x | 1–2mm |
| Planets | 150–300x | 0.5–1.5mm |
| Double stars | 150–400x | 0.3–1mm |