Observatory Platform: Telescope Aperture, Exposure & Limiting Magnitude

Slew

Right ascension 6.00h

Declination 10°

Optics

Aperture D 150 mm

Focal ratio f/6.0

Seeing (turbulence) 1.5″

Exposure

Exposure time 10 s

Live Readouts

Limiting magnitude

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Resolution

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Focal length

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Field of view

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Target magnitude

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Signal / noise

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CCD Readout

How it works

A telescope's limiting magnitude — the faintest star it can detect — grows with aperture because a bigger mirror collects more photons: m_lim ≈ 2 + 5·log₁₀(D_mm). Its angular resolution is set by diffraction (Dawes' limit ≈ 116″/D_mm) combined in quadrature with atmospheric seeing, since the two blurring effects are independent and add like uncorrelated errors.

m_lim  = 2 + 5·log10(D_mm)
Dawes  = 116 / D_mm  (arcsec)
res    = sqrt(seeing² + Dawes²)
FOV    = sensor_width / focal_length  (radians)
SNR    = signal / sqrt(signal + sky·N_pix + read²·N_pix)

Field of view narrows as focal length (aperture × focal ratio) grows — the classic zoom-vs-framing trade-off. Signal-to-noise grows with aperture² and with the square root of exposure time, since collected light scales with mirror area and total time while shot noise only grows with its square root — the reason long exposures reveal faint detail. A narrowband Hα filter throws away most starlight but blocks almost all sky glow, so it dramatically improves contrast on emission nebulae even though it dims ordinary stars — the real reason astrophotographers reach for it under light-polluted skies.

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Drag to slew · scroll to adjust exposure

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