Telescope Field of View (FOV) Calculation

Nov 17, 2024 by WO Team

The Field of View (FOV) refers to the extent of the observable sky that you can see through your telescope at any given moment. Think of it as looking through a window— the wider the FOV, the more of the sky you can observe.

FOV in Visual Observation

\text{FOV}=\frac{\text{Apparent FOV}}{\text{Magnification}}=\frac{\text{Apparent FOV}\times\text{Eyepiece Focal Length}}{\text{Telescope Focal Length}}

Apparent FOV - Typically provided by the eyepiece manufacturer in degrees.

FOV in Astrophotography

\text{FOV (degree)} = 2tan^{-1} \left( \ \frac{\text{Sensor Size (mm)}}{\text{Focal Length (mm)}} \ \right)

Or you can easily estimate by:

\text{FOV (arcminute)} \approx \frac{\text{Sensor Size (mm)} \times 3460}{\text{Focal Length (mm)}}

Unit conversion

1° (degree) = 60’ (arcminute) = 3600″ (arcsecond)

Sensor Size - Typically refers to the diagonal measurement of a rectangular sensor, but you can also use the horizontal or vertical measurement to calculate the FOV. Below are common sensor sizes:

	Horizontal	Vertical	Diagonal
Medium Format	44mm	33mm	55mm
Full Frame	36mm	24mm	43.3mm
APS-C	23.5mm	15.6mm	28.2mm
Micro 4/3	17.3mm	13mm	21.6mm
Planetary Camera sensor	6~8mm	4~6mm	7~10mm

Using FOV to Determine Focal Length

On the contrary, you can also use the desired FOV (based on the celestial object you want to observe) to determine the focal length of the telescope you want to use. (Refers to Apparent Size of Common Celestial Objects)

\text{Focal Length (mm)}= \frac { \text {Sensor Size (mm)}}{2tan \left( \ \frac{1}{2} \ {\text{FOV (degree)}} \right)}

Or you can easily estimate by:

\text{Focal Length (mm)} \approx \frac{\text{Sensor Size (mm)} \times 3460}{\text{FOV (arcminute)}}