TFOV is the actual angular width of sky visible through your telescope-eyepiece combination. It's calculated as: TFOV = AFOV ÷ Magnification. For example, a 60° AFOV eyepiece at 100× magnification gives a 0.6° TFOV — just slightly wider than the full Moon.

AFOV (Apparent Field of View) is the angular width your eye perceives when looking into the eyepiece — typically 50° to 100°. TFOV (True Field of View) is the actual slice of sky you see. TFOV = AFOV / (Telescope FL ÷ Eyepiece FL). Wide AFOV eyepieces (82°–100°) provide immersive views and larger true fields at the same magnification.

Exit pupil = Aperture ÷ Magnification (or Eyepiece FL ÷ Focal Ratio). It's the diameter of the light beam exiting the eyepiece. The human eye pupil dilates to ~7mm maximum (young adults). If exit pupil exceeds 7mm, light is wasted. For older observers, practical maximum is ~5–6mm. An exit pupil of 2–3mm is ideal for sharp, bright views.

Planets require high magnification. Choose eyepieces yielding 150×–300× magnification (depending on telescope aperture and seeing conditions). Exit pupil should stay above 0.5mm. A 5mm–10mm eyepiece on a typical SCT or Dobsonian works well. The maximum useful magnification is roughly 2× per mm of aperture (e.g., 400× for a 200mm scope under perfect conditions).

Large objects like M31 (3° wide) or the Pleiades (2°) need a TFOV of at least 1.5°–3°. Use a long focal length eyepiece (25mm–40mm) with a wide AFOV (68°–100°). A 31mm Nagler (82°) in an f/6 Dobsonian gives ~1.9° TFOV — enough to frame most large targets beautifully. Short focal ratio telescopes (f/4–f/5) paired with wide-field eyepieces excel at this.

Dawes' Limit = 116 / Aperture(mm) arcseconds. It's the theoretical resolution limit for splitting double stars. A 200mm telescope resolves ~0.58 arcseconds. Beyond this, atmosphere ("seeing") usually limits practical resolution to 1–2 arcseconds. This tool helps you see if your target's angular size is large enough to resolve detail with your setup.