ForHosting KIT · Developer Utilities

True field of view calculator

A telescope's true field of view tells you how much sky you can see through a particular eyepiece and telescope combination.

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This calculator estimates that angular width by dividing the eyepiece's apparent field of view by the resulting magnification. Enter the apparent field in degrees and the magnification ratio to receive the true field in degrees. It is a practical planning estimate for comparing eyepieces, framing extended targets, and deciding whether an object or star pattern is likely to fit in one view.

Understand apparent field and true field

An eyepiece's apparent field of view describes how wide the circular view seems while your eye is at the eyepiece. It is commonly printed in the eyepiece specifications and expressed in degrees. True field of view describes the actual angular span of sky visible through the complete telescope and eyepiece combination. These are related but not interchangeable: an eyepiece can present a broad apparent window while high magnification restricts the amount of sky inside that window. This calculator connects the two values using the standard approximation true field equals apparent field divided by magnification. For example, a 50-degree apparent field at 100x produces an estimated true field of 0.5 degrees. That is approximately the apparent diameter of the full Moon, so the result has an immediate observational meaning. Use the apparent field stated for the exact eyepiece model, and use the magnification produced by that eyepiece in the telescope you intend to observe with.

Enter magnification for the complete optical setup

Magnification is usually found by dividing the telescope focal length by the eyepiece focal length. If a Barlow lens, focal extender, reducer, or other optical accessory changes the effective focal length, calculate the resulting magnification first and enter that final value here. The calculator expects a ratio: enter 80 for 80x, not the text “80x” or a percentage. Magnification must be greater than zero because division by zero is undefined and a negative observing power has no physical meaning in this estimate. Apparent field must also be a positive angle in degrees. Once both values are valid, the algorithm performs one division and returns the true field in degrees, rounded only to keep the JSON output stable and readable. The same inputs always produce the same result. No telescope catalog, location, time, camera sensor, or internet connection is involved, so the result depends entirely on the two optical values you provide.

Use the estimate for observing and eyepiece comparisons

True field is useful when planning views of large targets such as the Pleiades, the Andromeda Galaxy, open clusters, nebula complexes, lunar features, and star-hopping patterns. Compare the calculated field with a target's published angular size, while leaving some margin around the object if comfortable framing matters. You can also calculate several eyepiece combinations to see the tradeoff between image scale and sky coverage: increasing magnification makes details appear larger but reduces the true field, while a wider apparent field can preserve more context at the same power. The apparent-field-over-magnification rule is an estimate rather than a measurement of the telescope's field stop geometry. For the most precise value, especially with eyepieces whose stated apparent field is approximate or whose distortion is significant, use the eyepiece field-stop diameter and telescope focal length when those specifications are available. For ordinary selection and session planning, however, this simple calculation is fast, transparent, and widely useful.

What you can do with it

Check whether a target fits

Compare an extended object's angular size with the calculated true field before choosing an eyepiece.

Compare eyepieces at the same telescope

See how apparent field and the resulting magnification change the amount of visible sky.

Plan a star-hopping route

Estimate the sky width in one view so chart steps and recognizable star patterns are easier to plan.

FAQ

What formula does the calculator use?

It uses true field of view in degrees = apparent field of view in degrees / magnification.

What should I enter for 100x magnification?

Enter 100. Magnification is a ratio, so do not add the letter x or enter a percentage.

Why must magnification be positive?

Zero would make the division undefined, and negative magnification is not physically meaningful for this field-of-view estimate.

Is this the most precise way to calculate true field?

It is a useful approximation. A field-stop calculation can be more precise when the eyepiece field-stop diameter is known.

How much does the API request cost?

Each API request costs $0.002. The calculator can also run free in the browser.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/astro/true-field-of-view

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/astro/true-field-of-view \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"apparent_field_deg":50,"magnification":100}'

const res = await fetch("https://api.kit.forhosting.com/astro/true-field-of-view", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "apparent_field_deg": 50,
    "magnification": 100
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/astro/true-field-of-view",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "apparent_field_deg": 50,
    "magnification": 100
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/astro/true-field-of-view", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"apparent_field_deg":50,"magnification":100}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"apparent_field_deg":50,"magnification":100}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/astro/true-field-of-view", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "apparent_field_deg": 50,
  "magnification": 100
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "astro.true_field_of_view",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →