Knackpad

Astrophotography Session Planner

FOV, pixel scale, altitude windows, moon phase & lunar separation for 150+ objects — all computed in your browser, nothing uploaded.

Session Setup

Positive = North/East · Negative = South/West

Bortle class affects the imaging score — faint objects score lower under bright skies.

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Fill in your setup above and tap Calculate to plan your session.
All computation happens locally — no data leaves your device.

How It Works

All astronomy math runs in JavaScript inside your browser — no server, no API key, no sign-up. Calculations are accurate to better than 1° for dates 2000–2040.

Julian Date & LST

Your local datetime converts to UTC then Julian Date (JD). Local Sidereal Time (LST) uses GMST from the J2000 formula (280.46061837 + 360.98564736629 × (JD − 2451545)) adjusted for longitude.

Altitude & Azimuth

RA/Dec → Hour Angle (HA = LST − RA). Spherical trig: sin(alt) = sin(φ)sin(δ) + cos(φ)cos(δ)cos(H). Azimuth from the full four-quadrant formula. Applied to every object + the Moon every 10 min over 24 h.

FOV & Pixel Scale

FOV (arcmin) = 3438 × sensor_mm ÷ focal_mm. Pixel scale (arcsec/px) = 206.265 × pixel_µm ÷ focal_mm. Sampling ratio = pixel_scale ÷ (FWHM_seeing / 2). Airy disk diameter = 2.44 × λ × f-ratio.

Moon Phase

Illumination from the Moon–Sun ecliptic elongation (Meeus Ch. 48). Phase age uses a known new moon epoch (JD 2459198.177) and the 29.53059-day synodic period. Moon position uses the low-precision Meeus simplified series (8 longitude + 5 latitude terms).

Imaging Score

Score 0–100 per object. Altitude contribution: below horizon = 0, <20° = 20, 20–45° = 60, >45° = 100. Moon penalty: illum × proximity factor (0 if >45° away). Bortle penalty: −5 per class above 4, applied to objects dimmer than mag 8.

Best Windows

For each top-scored object the tool scans tonight (dusk +30 min to dawn −30 min) in 10-min steps and finds the longest continuous window where altitude ≥ 30°. Displayed with start/end time and peak altitude.

Object catalog: J2000 epoch coordinates for all 110 Messier + ~50 selected Caldwell objects. No precession correction applied (error < 1° for 2000–2040).

Frequently Asked Questions
What is FOV in astrophotography and why does it matter?
Field of View (FOV) is the patch of sky your sensor captures at a given focal length. A 600 mm telescope with an APS-C sensor (22.3 × 14.9 mm) yields a FOV of about 128 × 86 arcminutes — wide enough for Andromeda (190 × 60 arcmin). Knowing your FOV before going outside prevents the frustration of a target that does not fit the frame or is too small to resolve. The formula is: FOV (arcmin) = 3438 × sensor_mm ÷ focal_mm.
What pixel scale and sampling ratio should I aim for?
Pixel scale (arcsec/pixel) = 206.265 × pixel_size_µm ÷ focal_length_mm. For typical seeing of 2–3 arcsec FWHM, Nyquist sampling means you want 2 pixels per FWHM, so target 1–1.5 arcsec/pixel. Under 0.5 arcsec/pixel you are oversampling (dim stars, long subs needed); over 3 arcsec/pixel you are undersampling and losing resolution. Binning (2×2) effectively halves pixel scale and quadruples sensitivity per pixel.
How does moon phase affect astrophotography?
Moonlight is broadband sky glow. A full moon (100% illumination) raises sky background by 2–3 magnitudes near the moon, washing out faint galaxies and nebulae. A moon below 25% illumination (crescent) or below the horizon is ideal for broadband imaging. Narrowband filters (Ha, OIII, SII) are far less affected because they reject the continuous moon spectrum. This planner shows illumination, altitude and angular separation from your target — the three numbers that determine moon impact.
What is a good altitude for imaging?
Objects below 20° suffer from heavy atmospheric refraction, differential refraction (star elongation) and large airmass. Airmass at 20° is about 2.9×, versus 1.0× at zenith. For best results image above 30°, ideally above 45°. The altitude timeline shows when each object crosses those thresholds across the night.
Does this tool account for atmospheric refraction or precession?
Atmospheric refraction shifts apparent altitude by up to ~0.5° near the horizon (34 arcmin at 0°). This tool does not apply refraction — add roughly +0.5° for objects below 5°. Precession (the slow drift of equatorial coordinates over centuries) is not applied; J2000 coordinates are used as-is, giving errors under 1° for dates between 2000 and 2040, which is well within planning accuracy.
Which Messier objects are best for beginners?
Large, bright objects with forgiving FOV requirements: M42 (Orion Nebula, 65×60 arcmin), M45 (Pleiades, 110 arcmin), M31 (Andromeda, 190×60 arcmin), M13 (Hercules Cluster, 20 arcmin), M57 (Ring Nebula, 1.4 arcmin — needs long focal length). Filter this table to Messier and sort by Score to find tonight's best targets at your location.