Moon diameter in pixels calculator

Enter the actual focal length of the lens or telescope and the sensor pixel pitch, and the calculator projects the Moon onto the focal plane and converts that physical image size into pixels. A representative apparent diameter is supplied by default, while an observation-specific angular diameter can be entered when precise framing matters. The result helps compare cameras, focal lengths, mosaics, and cropping plans without confusing sensor crop factor with genuine optical magnification.

Enter optical focal length and true pixel pitch

Use the actual focal length delivered by the optical system in millimetres. If a teleconverter, Barlow lens, focal extender, or reducer is installed, include its effective multiplication in the focal length you enter. Do not enter the marketing-style full-frame-equivalent focal length caused by a crop sensor: crop factor changes the field captured around the Moon, but it does not change the physical lunar image formed at a given focal plane. Pixel pitch is the centre-to-centre spacing of sensor pixels in micrometres. Camera specifications sometimes publish it directly. If they do not, estimate horizontal pitch by dividing sensor width in millimetres by the number of horizontal pixels, then multiplying by 1,000. Use the native still-image dimensions rather than a resampled video mode. Binned capture should use the effective binned pitch; for example, two-by-two binning doubles the pitch along each output dimension and approximately halves the lunar diameter measured in output pixels. Accurate inputs make comparisons between bodies and cameras meaningful.

Understand the projection and the angular-size setting

The calculator treats the camera or telescope as a rectilinear optical system. It converts the Moon's angular diameter to radians, calculates the focal-plane diameter as two times focal length times the tangent of half that angle, and divides the answer by pixel pitch expressed in millimetres. This exact projection is preferable to silently assuming that angle and tangent are identical, even though the difference is tiny for the Moon. The default apparent diameter of 0.518 degrees is a useful representative value, not a promise for every night. The Moon's apparent size varies because its distance from Earth changes, and an ephemeris can provide a better angular diameter for a planned time and observing location. Enter that value when testing tight framing, comparing perigee and apogee, or preparing a mosaic. The result includes both a fractional calculation and a nearest-whole-pixel planning value. Fractional pixels are mathematically useful, while an actual raster measurement will depend on how the limb falls across the pixel grid.

Use the estimate for framing, sampling, and equipment choices

Compare the computed diameter with the sensor's pixel width and height to estimate the space left around the lunar disc. A Moon spanning 2,000 pixels fits comfortably across a 6,000-pixel frame, but composition, tracking error, and later alignment may justify a generous margin. The value also makes equipment comparisons concrete: doubling actual focal length doubles the projected diameter, while halving pixel pitch doubles the number of samples across it. More pixels across the Moon do not automatically produce more resolved detail. Atmospheric seeing, focus, vibration, diffraction, optical quality, exposure, and stacking technique can limit usable resolution long before the nominal pixel count does. Likewise, this calculator does not predict field of view, orientation, phase illumination, or whether the bright limb will clip. Treat it as a geometric sampling and framing estimate. For critical sessions, verify the complete frame in planetarium software or with a test capture, and leave room for drift, stabilization, stacking alignment, and the changing position of the Moon during a sequence.