Astrophotography Pixel Scale & Sampling Calculator
Enter your telescope, camera, and seeing conditions to see the image scale, sampling status, field of view, and focal-length range that puts the setup near 2 to 3 samples per limiting detail.
Image scale (arcsec per binned pixel)0.97
- Rayleigh resolution (arcsec)
- 1.38
- Planning resolution (arcsec)
- 2
- Resolution-limiting factor
- Seeing-limited
- Samples across limiting detail
- 2.06
- Sampling assessment
- Matched
- Recommended effective focal-length range
- 775.56-1,163.33 mm
- Required reducer or Barlow factor range
- 0.97x-1.45x; 1.00x is inside this range, so no reducer or Barlow is required for the 2-3 pixel target.
- Field of view
- 0.81° x 0.81°
Rayleigh resolution assumes 550 nm light and a circular unobstructed aperture.
Field of view uses native sensor dimensions, so changing binning does not change the physical field size.
How to use this calculator
- Enter the effective focal length after any reducer, flattener, or Barlow.
- Enter the aperture, native pixel size, binning mode, and typical seeing FWHM.
- Enter the native sensor width and height before binning or cropping.
- Read the sampling assessment, recommended focal-length range, and field of view.
How the pixel scale is calculated
Astrophotography sampling starts with the plate scale of the telescope-camera pair. The calculator uses the standard small-angle relation for a binned camera pixel:
S = 206.265 × p × b ÷ F
- S is image scale in arcseconds per binned pixel.
- p is the native pixel size in micrometers.
- b is the numeric binning factor.
- F is the effective focal length in millimeters.
A smaller value means each pixel covers less sky, so the image is sampled more finely. A longer focal length, smaller native pixel, or lower binning factor all make the scale smaller.
Seeing, diffraction, and useful sampling
The useful detail in an exposure is limited by both the atmosphere and the telescope aperture. This calculator estimates the diffraction limit with the Rayleigh criterion at 550 nm:
R = 1.22 × 0.00055 × 206265 ÷ D
That is about 138.4 ÷ D arcseconds when aperture D is in millimeters. The planning resolution is the larger of your seeing FWHM and this Rayleigh value. The calculator then divides that planning resolution by the image scale to get samples across the limiting detail.
Fewer than 2 samples is labeled undersampled because stars and fine features are spread over too few pixels. From 2 through 3 samples is labeled matched, a common planning range for deep-sky imaging. More than 3 samples is labeled oversampled; that can still be useful for bright targets, lucky imaging, drizzle, or very steady skies, but it asks more from guiding, focus, and signal-to-noise.
Recommended focal length and field of view
The focal-length range reverses the same sampling equation. It solves for the focal lengths that would put 2 to 3 pixels across the planning resolution:
F = samples × 206.265 × p × b ÷ L
The reducer or Barlow range is that recommended focal-length range divided by the focal length you entered. If 1.00x falls inside the interval, your current effective focal length already lands inside the 2 to 3 sample target.
Field of view is calculated from the physical sensor size, not the binned output dimensions. Native sensor width is p × W ÷ 1000 millimeters, native sensor height is p × H ÷ 1000 millimeters, and each angular field uses 2 × atan(sensor size ÷ (2F)). That is why binning changes image scale but does not change the field of view.
What this calculator leaves out
The result is a planning estimate, not a complete image-quality model. It does not include guiding error, focus error, tracking, wind, atmospheric dispersion, optical aberrations, obstruction, deconvolution, drizzle, Bayer-matrix sampling, distortion, rotation, cropping, or unusable edge pixels. Software binning on many CMOS cameras also does not provide the same read-noise behavior as hardware binning.
Worked example
Suppose a 100 mm aperture telescope is used at 800 mm focal length with a camera that has 3.76 µm pixels, 1x1 binning, 2 arcsecond seeing, and a 3008 by 3008 native sensor. The image scale is 206.265 × 3.76 ÷ 800 = 0.97 arcseconds per binned pixel.
The Rayleigh estimate is 138.4 ÷ 100 = 1.38 arcseconds, so 2 arcsecond seeing is the planning limit. The setup places about 2.06 pixels across that limit, which is matched. The 2 to 3 sample focal-length range is about 776 to 1,163 mm, so the current 800 mm setup already falls in range. The field of view is about 0.81° by 0.81°.
Common questions
What is a good pixel scale for deep-sky astrophotography?
A useful pixel scale depends on seeing and aperture, not just the camera. For many deep-sky setups, a scale that gives about 2 to 3 pixels across the seeing-limited star size is a practical planning target. Under typical 2 arcsecond seeing, that often means roughly 0.7 to 1.0 arcseconds per pixel.
Am I oversampled or undersampled?
This calculator calls the setup undersampled below 2 samples across the planning resolution, matched from 2 through 3 samples, and oversampled above 3 samples. Undersampling can make stars blocky or hide small detail. Oversampling spreads the same light over more pixels and can demand better tracking and focus.
How does camera binning change pixel scale?
Binning multiplies the effective pixel size used for sampling, so 2x2 binning doubles the arcseconds per binned pixel. That makes the setup less finely sampled. It does not change the physical sensor size, so the field of view stays the same unless you crop the image.
Should I use a focal reducer or Barlow with this camera?
Compare the required optical-factor range with 1.00x. A range below 1.00x points toward a reducer, while a range above 1.00x points toward a Barlow or extender. If 1.00x is inside the range, the current effective focal length is already inside the 2 to 3 sample target.
Why does telescope aperture affect useful sampling?
A larger aperture has a smaller diffraction-limited angular resolution, so it can support finer sampling when the atmosphere and optics allow it. If seeing is worse than the Rayleigh limit, seeing is the planning limit instead. If the two are equal, the calculator reports the setup as jointly seeing- and diffraction-limited.
Should I use average seeing or measured star FWHM?
Use measured star FWHM from representative subs when you have it, because it includes your local atmosphere and some setup behavior. For planning before a purchase, typical site seeing is enough to compare camera and telescope combinations. The calculator uses the value you enter as a transparent planning input.