Pixel Scale in Astrophotography: The Complete Guide

Pixel scale is the angle on the sky that each pixel of your camera covers. It is usually expressed in arcseconds per pixel (″/px). A pixel scale of 1″/px means each pixel spans one arcsecond of sky; a scale of 3″/px means each pixel spans three arcseconds.

Think of it as the resolution of your digital image. It tells you how finely the sensor slices up the view coming through the telescope. Pixel scale is determined by two things:

Pixel scale decides how much detail you can record and how much signal each pixel collects. The choice affects almost every part of your imaging chain:

• Detail: a finer pixel scale can resolve smaller features, but only if the atmosphere, optics and guiding let you.
• Signal-to-noise: coarser pixels collect more photons per pixel, which can produce cleaner images in less time.
• Guiding demands: fine pixel scales reveal guiding errors more readily.
• File size and processing: fine scales with large sensors create bigger images and longer processing runs.

The goal is not the smallest possible pixel scale. The goal is a pixel scale that suits your equipment, your site and the targets you want to shoot.

The standard formula is simple, but the inputs matter:

The constant 206.265 is the number of arcseconds in one radian, expressed with the micron-to-millimetre conversion built in. If you prefer to work from first principles, you can also calculate the angular size of one pixel as the arctangent of its physical size divided by the focal length; the formula above is the small-angle approximation, which is accurate for any practical astrophotography setup.

All values below are illustrative examples. Use your own equipment specifications.

Any optical accessory that changes the focal length also changes your pixel scale. A 0.8× reducer shortens the effective focal length, which makes the pixel scale coarser. A 2× Barlow doubles the effective focal length, which makes the pixel scale finer.

Always use the effective focal length in the pixel-scale formula. Many calculators ask for the reducer/Barlow factor separately so you do not forget it.

Pixel scale alone does not tell you whether your system is appropriately matched to the sky. You also need to know the size of a star in your images. A convenient way to express that is the full width at half maximum (FWHM) of a star profile, measured in arcseconds.

FWHM is not the same as pure atmospheric seeing. A measured FWHM includes seeing, guiding errors, focus quality, optical aberrations and even the demosaicing or debayering choices you make during processing. It is the number that matters for sampling.

Common rules of thumb describe the result as coarse, moderate or fine sampling. These are guidance ranges, not hard rules:

• Less than ~1.5 pixels across FWHM: coarse sampling. Stars may look blocky, but each pixel gathers more signal.
• ~1.5 to ~2.5 pixels across FWHM: moderate sampling. A practical balance for many amateur setups.
• More than ~2.5 pixels across FWHM: fine sampling. More detail is possible, but guiding, seeing and total exposure time become more demanding.

Oversampling means your pixels are small enough to record detail finer than the typical star size. Undersampling means your pixels are relatively large compared with the star size. Neither is inherently bad.

The right choice depends on your priorities. A wide-field imager chasing large nebulae may happily run undersampled to capture more sky in less time. A planetary or small-galaxy imager may deliberately oversample to preserve tiny details.

Beginners often confuse pixel scale with field of view. They are related, but they measure different things:

A camera with tiny pixels on a small sensor can have a very fine pixel scale but a narrow field of view. A camera with large pixels on a big sensor can have a coarse pixel scale but a wide field of view. You need both numbers to understand what your final image will look like.

Use the calculators below to plan your system. Remember that these are planning tools: real optics, filters, sensor windows and atmospheric conditions will change the exact numbers.

Binning groups adjacent pixels into one super-pixel. A 2×2 bin turns four native pixels into one output pixel, doubling the effective pixel size and doubling the pixel scale. This can be useful when you want coarser sampling, smaller files or faster downloads.

On CCD cameras, hardware binning usually improves read noise because the charge from multiple pixels is read once. On most CMOS cameras, software binning simply resamples the pixels and does not provide the same read-noise benefit. Know which kind of binning your camera uses before relying on it for faint targets.

Your guiding performance must be matched to your imaging pixel scale. A guiding RMS of 1 arcsecond is irrelevant on a 4″/px wide-field system, but it is a disaster on a 0.5″/px planetary system.

A useful rule of thumb is to aim for a guiding RMS below roughly 0.3–0.5 of your imaging pixel scale. That gives you some headroom for occasional worse-than-average moments. The converter above turns guiding RMS directly into imaging pixels so you can judge for yourself.

The best way to choose a camera-telescope combination is to work backwards from your sky and your targets:

1. Measure your typical FWHM from real images on your site.
2. Pick a target sampling range. Around 1.5–2.5 pixels across FWHM is a safe starting point for many deep-sky imagers.
3. Calculate the pixel scale that gives that sampling: pixel scale ≈ FWHM ÷ target pixels-across-FWHM.
4. Find a camera-telescope pair that produces that pixel scale, accounting for any reducer or Barlow.
5. Check field of view to make sure the target fits and the framing is attractive.
6. Check guiding demands to make sure your mount can support the chosen scale.

If you image from multiple sites or under widely varying seeing, you may end up with different ideal sampling for different nights. That is normal.

Print this worksheet and fill it in at the bench. It is faster than switching between browser tabs when you are planning a new imaging train.

• Find your camera pixel size from the manufacturer specs
• Note the native telescope focal length
• Apply any reducer or Barlow multiplier
• Calculate pixel scale with the formula
• Measure your typical FWHM from real images
• Work out pixels across FWHM
• Decide whether your current sampling suits your goals
• Check field of view for target framing
• Check guiding RMS in imaging pixels
• Consider binning if you need coarser sampling
• Test under real skies before committing to a purchase

• Manufacturer sensor specifications for pixel size and active sensor dimensions.
• Telescope and reducer/Barlow specifications for effective focal length and backfocus.
• Small-angle approximation: 206.265 × pixel_size_µm ÷ focal_length_mm for pixel scale in arcseconds per pixel.
• Tangent projection formula for field of view: 2 × atan(sensor_size / (2 × focal_length)).
• General astrophotography literature on sampling, Nyquist criterion, and the trade-offs of over- and under-sampling.