Tools

Image Scale Calculator

What This Calculator Tells You

Image scale (also called plate scale) tells you how many arcseconds of sky each pixel in your camera captures. This is fundamental to understanding your imaging resolution and whether your setup is well-matched to your seeing conditions.

Calculator

Effective focal length including any reducers or barlows.

Found in your camera's specifications.

0.78"
per pixel
Optimal (Well Sampled)

Ideal range for most deep-sky imaging. Good balance of resolution and SNR under average seeing (1.5–2.5").

Object Sizes at This Scale

ObjectSizePixels Across
Jupiter (max)50"64
M57 (Ring Nebula)86"111
M13 (Hercules Cluster)1200"1547
M31 (Andromeda full disk)10800"13925
Full Moon1860"2398

Common Camera Pixel Sizes

Camera TypeTypical Pixel Size
Full-frame DSLR4-6 μm
APS-C DSLR3.5-5 μm
Dedicated astro camera (cooled)2.4-9 μm
Planetary camera (small pixels)1.2-3 μm

Sampling Guidelines

  • Nyquist sampling: For optimal resolution, your image scale should be about half your seeing FWHM. With 2" seeing, aim for ~1"/pixel.
  • Oversampling (scale < seeing/3): More pixels than needed; wastes download time and storage without adding detail.
  • Undersampling (scale > seeing): Missing resolution your optics could provide; stars may look blocky.
  • Typical deep sky imaging: 1-2"/pixel works well for most amateur setups.

Rules of Thumb

  • Double focal length = half the image scale: Objects appear twice as large in pixels
  • Double pixel size = double image scale: Each pixel covers more sky
  • Planetary imaging: 0.1-0.3"/pixel: Need to oversample to capture detail
  • Wide-field: 2-4"/pixel: Captures large areas but loses fine detail

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