What it is
A field‑of‑view calculator tells astrophotographers how large an area of the sky will be recorded on a given sensor for a specific focal length. The horizontal and vertical angles are derived from θ=−1²…atan( s/(2f) ), where s is the sensor dimension in the same unit as the focal length f. These angles, expressed in degrees, define the rectangle of sky captured by the image. The total sky coverage is simply the product of the two angles written as a square‑degree figure; multiplying by 3 600 converts it to arcminutes squared, allowing comparison with catalogued fields of view or mosaic requirements. Knowing these values helps photographers plan field coverage, exposure settings and stacking strategies, ensuring that their target patch of sky will fit within the sensor limits.
How to use it
Enter the physical width and height of your camera’s CCD/CMOS sensor in millimeters (zero dimensions are permitted only for edge testing). Input the effective focal length of your lens assembly in the same units. The calculator instantly shows you the horizontal, vertical and diagonal coverage in degrees as well as total sky area in both deg² and arcmin². Use the numbers to assess whether a particular field, such as a deep‑sky target or a wide‑field mosaic, can be captured with your gear.
Worked example
For a 2‑inch square sensor of 200 mm on each side and a 100 mm focal‑length lens, 1. Compute the half‑angle: ratio = width/(2·f) = 200/(200)=1. 2. atan(1)=π/4 rad ≈0.785398; double for full angle → 1.570796 rad. 3. Convert to degrees: 1.570796 × 180/π = 90°. The same calculation applies to the vertical dimension, giving a square coverage of 90° × 90° = 8 100 deg². Multiplying by 3 600 yields 29 160 000 arcmin². This example demonstrates how a simple arithmetic sequence reveals that the sensor can capture an entire hemisphere (a flat‑sided 90° field) of sky with this focal length.
Inputs
- Sensor Width (mm): 200
- Sensor Height (mm): 200
- Focal Length (mm): 100
Result
- Horizontal Field of View (°): 90
- Vertical Field of View (°): 90
- Sky Coverage (deg²): 8100
- Sky Coverage (arcmin²): 29160000
Frequently asked questions
How accurate are the results for sensors that have significant distortion or non‑rectangular geometry?
The calculator assumes a perfectly rectilinear, rectangular sensor. For lenses that introduce strong radial distortion or sensors that are markedly oblong, the resulting field of view will be slightly over‑ or under‑estimated; use camera calibration data if high precision is required.
Can I calculate coverage for a mosaic of multiple exposures?
Yes – once you have the FOV per exposure, simply multiply by the number of tiles in your mosaic grid to estimate total area and plan overlap margins.
What happens if I set the sensor width or height to zero?
Zero dimensions force the corresponding FOV component to zero, yielding a sky coverage of 0 deg². This is useful for testing the algorithm’s robustness but doesn’t reflect any real camera configuration.