Image scale links sky angle to a pixel
For small angles, image scale in arcseconds per pixel is approximately 206.265 multiplied by pixel size in micrometres and divided by focal length in millimetres. Longer focal length reduces the sky angle per pixel; larger pixels increase it.
Use effective focal length after any reducer or barlow. Manufacturer nominal values and actual optical spacing can differ, so treat the calculation according to the precision of the inputs.
Sensor dimensions determine field of view
Horizontal and vertical field use the corresponding active sensor dimension: two times the arctangent of sensor dimension divided by twice the focal length. The diagonal uses the sensor diagonal.
Compare the field with the target's angular size and allow composition margin. A target fitting mathematically does not guarantee that the corrected image circle, filters or mechanical train support the full sensor.
- Effective focal length
- Active pixel size and sensor dimensions
- Target angular size
- Corrected image circle and back focus
Sampling is not an image-quality promise
Seeing, tracking, focus, optical quality, signal, exposure and processing all affect the final image. A numerically fine sampling rate may add data volume without recoverable detail under a particular sky.
Use the calculator to explore geometry, then design the full optical, mechanical and control chain around actual targets and site conditions.