One formula · your inputs
Telescope true field of view calculator
The common eyepiece estimate divides apparent field by magnification.
It is useful for planning, but an eyepiece field-stop calculation is more exact and distortion can change the observed edge.
Formula
How it is calculated
true field ≈ apparent field ÷ (telescope focal length ÷ eyepiece focal length)- Enter the manufacturer’s apparent field.
- Calculate magnification from the two focal lengths.
- Divide apparent field by magnification.
What the result represents
The standard apparent-field-over-magnification approximation.
The page keeps every decision-bearing quantity visible as an input. It does not look up a product catalogue, current market value or hidden recommended preset.
Limits of the model
The result is an idealised optical or geometric estimate from user-entered values. It does not model seeing, optical quality, central obstruction, transmission, distortion, focus error, tracking or sensor calibration.
Use the number to compare transparent scenarios. Verify critical observations, equipment combinations and coordinates independently.
Questions about this calculation
Does this true-field calculator choose equipment for me?
No. It evaluates only the displayed formula with the numbers you enter and does not make a purchase, compatibility or safety decision.
Why can the observed result differ?
Real optics and observing conditions add obstructions, transmission losses, aberrations, seeing, focus and measurement uncertainty that are outside the arithmetic model.
Can I use the result for navigation or safety-critical work?
No. SkyClock tools are transparent planning estimates, not certified navigation, engineering or safety instruments.
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