Whether you are estimating on the spot or checking a result somebody else gave you, the telescope field of view calculator is the quickest way to get from custom focal length (eyepiece), apparent field of view (fova) and custom focal length (telescope) to area of the field of view, field of view (fov) and magnification (m).
What Is the Telescope Field of View Calculator?
This telescope field of view calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns custom focal length (eyepiece), apparent field of view (fova) and custom focal length (telescope) into area of the field of view, field of view (fov) and magnification (m).
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Telescope Field of View Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with custom focal length (eyepiece) set to 10, apparent field of view (fova) set to 5, custom focal length (telescope) set to 2, the calculator returns magnification (m) of 0.2, and field of view (fov) of 25, and area of the field of view of 490.87385. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out area of the field of view?
Enter custom focal length (eyepiece), apparent field of view (fova) and custom focal length (telescope). The calculator then works through the relationship and reports area of the field of view, field of view (fov) and magnification (m).
What does the telescope field of view actually calculate?
It takes custom focal length (eyepiece), apparent field of view (fova) and custom focal length (telescope) and converts them into area of the field of view, field of view (fov) and magnification (m) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter custom focal length (eyepiece)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.