Formula Used
- From magnification and true field (linear): AFOV ≈ M × TFOV
- From magnification and true field (trigonometric): AFOV = 2 · arctan(M · tan(TFOV/2))
- From eyepiece field stop: AFOV ≈ 57.2958 · (Field Stop / Eyepiece Focal Length)
How to Use This Calculator
- Select a method based on the information you have.
- Enter your values. Use telescope and eyepiece focal lengths to auto-compute magnification.
- Press Calculate. The result appears above the form.
- Use Download CSV or Download PDF to save the output.
- Compare linear vs trigonometric when your measured true field is wide.
Example Data Table
| Case | Telescope FL (mm) | Eyepiece FL (mm) | True Field (deg) | Field Stop (mm) | Computed AFOV (deg) |
|---|---|---|---|---|---|
| Measured TFOV | 1000 | 13 | 1.30 | — | 100.0 (linear) |
| Field stop spec | — | 24 | — | 27.0 | 64.5 |
| Known AFOV | 1200 | 20 | — | — | 68.0 (direct) |
Apparent Field of View Guide
1) What AFOV means at the eyepiece
Apparent field of view (AFOV) describes how wide the circular “window” looks when you place your eye at the eyepiece. It is an eyepiece property, not a telescope property. Common categories are narrow (around 40–50°), medium (about 60–70°), and wide (80° or more). Wider AFOV can feel more immersive, especially at higher magnifications.
2) AFOV versus true field of view
True field of view (TFOV) is the patch of sky you actually see, measured on the sky in degrees. TFOV depends on magnification, while AFOV describes the apparent angular size of the field stop at your eye. A simple relationship is TFOV ≈ AFOV ÷ magnification, which is useful for planning whether an object fits.
3) Using magnification and TFOV
If you can measure TFOV (for example with star-drift timing or plate-solving), you can estimate AFOV with AFOV ≈ M × TFOV. This linear approach is quick and works best when TFOV is small. With very wide angles, it can slightly overestimate because it ignores angular geometry and distortion.
4) Trigonometric method for wider fields
The trigonometric option uses AFOV = 2·arctan(M·tan(TFOV/2)). It better respects the way angles combine and can be more stable when TFOV is large. Use it when you have wide-field eyepieces, fast optical systems, or when your TFOV measurement is already precise.
5) Field stop method and why it is practical
Many observers trust field-stop based estimation because the field stop is a physical aperture inside the eyepiece. The approximation AFOV ≈ 57.2958 × (field stop / eyepiece focal length) gives a consistent comparison across designs. If you also know telescope focal length, you can cross-check TFOV from the same field stop.
6) Typical observing numbers
At 50×, an AFOV of 60° implies a TFOV near 1.2°, similar to the Moon’s diameter times two. At 100×, the same eyepiece feel yields about 0.6°. Wide 82° eyepieces at 100× show about 0.82°, which helps frame large clusters while keeping magnification useful.
7) Distortion and why two eyepieces can “feel” different
Eyepiece designs may trade distortion for sharpness or edge correction. Some eyepieces show pincushion distortion that changes how stars drift, affecting TFOV measurements. That is why comparing linear and trigonometric results is helpful, and why a published AFOV can differ slightly from an observed value.
8) Choosing the best method in this tool
Use the direct method when the eyepiece AFOV is known from specs. Use field stop when the manufacturer lists it, or when you want a design-neutral estimate. Use magnification plus TFOV when you have a measured TFOV. Save your result via CSV or PDF for repeatable observing logs.
FAQs
1) Is AFOV the same as true field of view?
No. AFOV describes how wide the view appears at your eye. True field is the actual angular size on the sky. True field changes with magnification, while AFOV is tied to the eyepiece design.
2) Which method is most accurate?
Field stop is often the most consistent when the field stop is known. If you measure TFOV precisely, the trigonometric mode can be very accurate. Linear is fine for small TFOV and quick estimates.
3) Why does the linear method sometimes differ?
Linear AFOV ≈ M × TFOV assumes small-angle behavior and ignores geometric effects and distortion. With wider fields, the approximation can slightly overestimate. Trigonometric mode reduces this problem.
4) What is a field stop?
The field stop is a physical ring or aperture inside an eyepiece that limits the illuminated field. Its diameter strongly influences the maximum true field a given eyepiece can show in a telescope.
5) Can I compute magnification automatically?
Yes. Enter telescope focal length and eyepiece focal length and the tool computes magnification as telescope FL ÷ eyepiece FL. You can also enter magnification directly if you already know it.
6) What units should I use?
Enter angles in degrees. Enter focal lengths and field stop in millimeters. The tool also reports arcminutes and arcseconds for convenience when comparing narrow fields or doing detailed planning.
7) How should I use the exports?
Use CSV to paste results into a spreadsheet or observing log. Use PDF for a clean printable record. Both exports pull directly from the results table so your saved values match what you see.