Telescope Field of View Calculator

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Calculate how much sky your telescope setup can see for Visual (eyepiece) or Imaging (camera) using focal length, eyepiece or sensor specs, and optional Barlow/reducer.

Choose Visual if you are looking through an eyepiece. Choose Imaging if you are using a camera sensor at the telescope focus.
The telescope focal length, usually printed on the tube or in the specs (in millimeters).
The simple method uses eyepiece apparent field of view (AFOV). The accurate method uses eyepiece field stop diameter (if you know it).
Eyepiece focal length in millimeters (for example, 25 mm). Used to compute magnification.
AFOV is a manufacturer spec (often 40-82 degrees). Used only for the simple TFOV estimate.
Advanced options
Formatting
Auto switches to scientific notation only for very large or very small numbers. Plain always uses normal numbers. Scientific always uses scientific notation.
Tip: The format affects how numbers are shown, not the math.
Optics
Use 2 for a 2x Barlow. Use 0.63 for a 0.63x reducer. This scales the effective focal length.
Optional. If provided, the calculator can compute exit pupil for visual observing.
Optional. If you know the eyepiece field stop diameter, TFOV can be computed more accurately (field stop method). Used only when Visual method is set to Field stop (accurate).
Imaging inputs
Choose Sensor size if you know sensor width and height in mm. Choose Pixel size if you know pixel size and image dimensions in pixels.
Imaging mode only (Sensor size style). Physical sensor width in millimeters.
Imaging mode only (Sensor size style). Physical sensor height in millimeters.
Optional. If provided, the calculator can give a simple sampling note by comparing seeing to image scale (arcsec/pixel).
This changes only how results look. It does not change the underlying values.
Effective focal length (mm)
Use this value for all FOV and magnification calculations when you have a Barlow or reducer.
Magnification (x)
Higher magnification shows a smaller patch of sky.
True field of view (simple AFOV method) (deg)
This is a quick estimate. It can be off if the eyepiece AFOV spec is optimistic or for very wide designs.
True field of view (field stop method) (deg)
This is usually more accurate when field stop diameter is known.
True field of view (deg, arcmin, arcsec)
Example: 1.25 deg = 1 deg 15 arcmin.
Exit pupil (if aperture provided) (mm)
Bigger exit pupil looks brighter but is limited by your eye pupil.
Imaging horizontal field of view (deg)
This is how wide your camera frame is on the sky.
Imaging vertical field of view (deg)
This is how tall your camera frame is on the sky.
Imaging diagonal field of view (deg)
Useful for comparing to lenses that advertise diagonal FOV.
Plate scale (arcsec/mm)
This tells how many arcseconds on the sky fall on 1 mm at the focal plane.
Image scale (if pixel inputs provided) (arcsec/pixel)
Smaller arcsec/pixel means more zoom (and more sensitivity to seeing and tracking).
Sampling note (if seeing provided)
This is a rough guide, because real seeing and tracking change over time.
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How to use our Telescope Field of View Calculator

  1. Choose Mode: Visual (eyepiece) or Imaging (camera).
  2. Enter Telescope focal length (mm). If you use a Barlow or reducer, open Advanced options and set Barlow or reducer factor (x).
  3. Visual mode: enter Eyepiece focal length (mm).
  4. Visual mode: choose a Visual method. For the simple method, enter Eyepiece apparent field of view (degrees). For the accurate method, open Advanced options and enter Eyepiece field stop diameter (mm).
  5. Optional (Visual): enter Telescope aperture (mm) to get exit pupil (a brightness-related number).
  6. Imaging mode: open Advanced options and choose Imaging input style. Enter either sensor width and height (mm), or pixel size (micrometers) plus image width and height (pixels).
  7. Optional (Imaging): enter Typical seeing (arcseconds) to get a short sampling note based on your image scale.
  8. Click Calculate.
  9. Sanity-check: if effective focal length gets larger (bigger Barlow, longer telescope), your FOV should get smaller. In Visual mode, using a shorter eyepiece focal length should increase magnification and shrink TFOV.

Definitions

TFOV (true field of view): The real angle of sky you can fit in the view or the photo. It is shown in degrees (deg) and also as degrees, arcminutes ('), and arcseconds (").

AFOV (apparent field of view): An eyepiece spec for how wide the view looks when you look into the eyepiece. The simple TFOV method uses AFOV, but it is not the same thing as TFOV. [1]

Magnification (x): How much larger objects look in Visual mode. Here it is effective telescope focal length divided by eyepiece focal length. [1]

Field stop (mm): A physical ring inside many eyepieces that limits the light path. Its diameter can be used to estimate TFOV more accurately than AFOV for that eyepiece. [2]

Effective focal length (mm): Telescope focal length after multiplying by a Barlow (greater than 1x) or reducer (less than 1x).

Plate scale (arcsec/mm): How many arcseconds on the sky fall on 1 mm at the telescope focus.

Image scale (arcsec/pixel): How many arcseconds each camera pixel covers. Smaller arcsec/pixel means more zoom and more sensitivity to seeing and tracking.

Exit pupil (mm): The width of the light beam leaving the eyepiece. Bigger exit pupil usually looks brighter (up to your eye pupil), smaller can look dimmer. [4]

Methodology

What the calculator does: It first adjusts your telescope focal length for any Barlow/reducer, then computes Visual or Imaging results from that effective focal length.

1) Effective focal length

F_eff_mm = F_scope_mm * factor_x

Use factor_x = 2 for a 2x Barlow, or factor_x = 0.63 for a 0.63x reducer.

2) Visual (eyepiece) mode

Magnification

M_x = F_eff_mm / F_eyepiece_mm

TFOV (simple AFOV method)

TFOV_deg = AFOV_deg / M_x

This is a popular shortcut, but it is an estimate because AFOV is a spec and can be defined or measured in different ways. [1]

TFOV (field stop method, usually more accurate if you know the field stop)

TFOV_deg = (field_stop_mm / F_eff_mm) * (180/pi)

The field stop is a real physical diameter inside the eyepiece, so this method is often closer to the true sky angle when the field stop value is correct. [2]

Exit pupil (optional)

exit_pupil_mm = aperture_mm / M_x

If you enter aperture, exit pupil helps you compare how bright the view may look at different magnifications. [4]

TFOV breakdown into deg, arcmin, arcsec

deg_int = floor(abs(TFOV_deg)); arcmin_total = (abs(TFOV_deg)-deg_int)*60; arcmin_int = floor(arcmin_total); arcsec = (arcmin_total-arcmin_int)*60; sign follows TFOV_deg

Mini-example:

Telescope 1000 mm, eyepiece 25 mm, factor 1x gives M = 40x. With AFOV = 50 deg, TFOV = 50/40 = 1.25 deg, which is 1 deg 15 arcmin 0 arcsec.

3) Imaging (camera) mode

Field of view from sensor size

FOV_deg = 2 * atan(sensor_size_mm / (2 * F_eff_mm)) * (180/pi)

Compute this using sensor width for horizontal FOV, sensor height for vertical FOV, and the sensor diagonal for diagonal FOV. [3]

Plate scale

plate_scale_arcsec_per_mm = 206265 / F_eff_mm

Image scale (optional, needs pixel size)

image_scale_arcsec_per_px = 206.265 * pixel_size_um / F_eff_mm

206265 is arcseconds per radian, and 206.265 is the same constant after converting micrometers to millimeters (1 um = 0.001 mm). [3]

Sampling note (optional, needs seeing and image scale)

pixels_per_seeing = seeing_arcsec / image_scale_arcsec_per_px

Interpretation: pixels_per_seeing near 1 means the atmosphere blur is about 1 pixel wide (undersampled). Much larger means the blur covers many pixels (oversampled). This is only a rough guide because seeing changes and your processing can change results.

Assumptions and limits

AFOV-based TFOV is an approximation, so it can disagree with field stop TFOV or with real-sky drift timing. [1][2]

Imaging FOV and scales assume ideal geometry and an accurate effective focal length. Reducer spacing, cropping, and binning can change what you actually record.

All required inputs must be numbers greater than 0. Optional inputs may be blank; if blank, related outputs show N/A.

Sources

  1. Determining Field of View - Astronomics
  2. eyepiece field stop | Nikon's MicroscopyU - Microscopyu
  3. Telescope Field of View (FOV) Calculation - Telescope Nerd - Telescopenerd
  4. How to Choose Eyepieces for Your Astronomy Telescope - Agenaastro