Pixel Scale Calculator
Enter your camera's pixel size and your telescope's focal length to see how much sky each pixel covers and whether that suits the seeing where you image.
Pixel scale
Arcseconds per pixel and whether your camera suits your seeing.
On the camera's spec sheet. ASI2600 and ASI533: 3.76 µm.
Include any reducer or Barlow.
Star size (FWHM) on a normal night. 2 to 3″ is typical from the plains; 1.5″ is a good night.
Pixel scale
1.29″ per pixel
Well sampled. Stars land on enough pixels to look round and sharp without wasting signal.
- Ideal for this seeing
- 1.00 to 1.50″/px
- Focal length for the ideal centre
- 620 mm
- Well sampled between
- 517 and 776 mm
The seeing ÷ 3 to seeing ÷ 2 range comes from Nyquist sampling. It is a guide, not a hard rule.
How this is calculated
- 1
Pixel scale
Pixel scale in arcseconds per pixel = 206.265 × pixel size in µm ÷ focal length in mm. A 3.76 µm pixel at 600 mm gives 206.265 × 3.76 ÷ 600 = 1.29″ per pixel.
- 2
Binning
Binning joins neighbouring pixels into one, so 2x2 binning doubles the effective pixel size and the pixel scale. It trades resolution for signal per pixel.
- 3
Ideal range
Following the Nyquist idea, a star's blur (its FWHM, set by seeing) should span two to three pixels. So the ideal scale is between seeing ÷ 3 and seeing ÷ 2. For 2.5″ seeing that is 0.83 to 1.25″ per pixel.
- 4
Best focal length
Rearranging the first formula, focal length = 206.265 × pixel size ÷ target scale. Aiming for the middle of the ideal range with 3.76 µm pixels and 2.5″ seeing gives about 745 mm.
Frequently asked questions
What is a good pixel scale for astrophotography?
For most deep sky imaging from typical sites, 1 to 2 arcseconds per pixel works well. Under very steady skies you can go below 1″. Wide field imaging of large nebulae often uses 2 to 4″ per pixel.
How do I calculate arcseconds per pixel?
Multiply the pixel size in micrometres by 206.265 and divide by the focal length in millimetres. Include any reducer or Barlow in the focal length, and multiply the pixel size by the binning factor.
What does oversampled mean?
Oversampled means each star is spread over more pixels than needed to record its detail. The light is shared between more pixels, so each one gets less signal and the image is noisier for the same exposure.
Is undersampling bad?
Heavy undersampling makes stars look square and loses fine detail, but mild undersampling is common and acceptable for wide fields. It also gives a larger field and a stronger signal per pixel.