Telescope Field of View Calculator
Calculate telescope field of view from focal length, eyepiece focal length, and sensor or apparent field of view.
What Is Telescope Field of View?
Field of view (FOV) is the angular width of sky visible through a telescope. A wider FOV makes it easier to frame large objects like the Moon or star clusters. A narrow FOV gives higher magnification but shows a smaller patch of sky.
Field of View Formulas
Eyepiece method (true FOV from apparent eyepiece field and magnification):
$$\text{FOV} = \frac{\text{AFOV}}{M}, \quad M = \frac{f_{\text{telescope}}}{f_{\text{eyepiece}}}$$Sensor or imaging method (linear sensor dimension):
$$\text{FOV} = \frac{\text{sensor size}}{f_{\text{focal}}} \times \frac{180}{\pi} \text{ degrees}$$Example: a 50° apparent field eyepiece on a 40× setup (1000 mm scope, 25 mm eyepiece) gives true FOV \(= 50/40 = 1.25°\).
Choosing the Right Method
Visual observers typically use the eyepiece apparent field divided by magnification. Astrophotographers often use sensor size and telescope focal length to estimate how much sky fits on the camera chip.
Related tools: Telescope Magnification Calculator and Camera Field of View Calculator.
Frequently Asked Questions
What is apparent field of view (AFOV)?
AFOV is the angular width you see through the eyepiece alone, printed on many eyepiece specs. True FOV at the telescope is AFOV divided by magnification.
Why are there two FOV calculation methods?
Visual astronomy uses eyepiece specs and magnification. Imaging uses sensor geometry and focal length because the camera chip defines the field.
Does a barlow lens change FOV?
Yes. A barlow increases magnification, which narrows true field of view proportionally when using the eyepiece method.
What sensor size should I enter?
Use the dimension along the axis you care about, such as the horizontal width of the sensor in millimeters for horizontal FOV.
Is higher magnification always better?
Not for wide targets. High magnification shrinks FOV and can dim the image. Match magnification to the object size and sky conditions.
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