---
title: Focal Length Calculator
description: "Your telescope’s *real* focal length is rarely the number on the box – and on mirror-spacing designs (RC, SCT, Cassegrain) it drifts with collimation and focus position. This calculator measures the truth with a plate solve and, for adjustable designs,…"
url: "https://astro-pm.com/docs/focal-length-calculator/"
updated: "2026-07-12"
category: "Astro PM App Guide / Analysis Tools & Calculators"
---

# Focal Length Calculator

Your telescope’s *real* focal length is rarely the number on the box – and on mirror-spacing designs (RC, SCT, Cassegrain) it drifts with collimation and focus position. This calculator measures the truth with a plate solve and, for adjustable designs, tells you **which mirror to move, which direction, and how far**.

### Step 1: Manufacturer Optics

Select the telescope and the spec-sheet numbers populate: native and reduced focal length, focal ratios, Dawes and Rayleigh limits. These are the *targets* the measurement will be compared against. Backfocus and focuser travel fields round out the mechanical picture.

### Step 2: Measure With a Plate Solve

Open any recent sub (FITS, XISF, TIFF – XISF is converted automatically) and the calculator runs **ASTAP** on it. The solved plate scale plus the pixel size from the image header yield the **measured effective focal length**:

`EFL = 206.265 × pixel size (µm) / plate scale (arcsec/px)`

This is ground truth – the focal length your optical train actually delivered on that frame, including reducer, spacing, and temperature effects.

[![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-132324.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-132324.png)
14" RC with platesolved image. Shows direction to move mirrors.

### Step 3: The Delta & Mirror Adjustment

For adjustable designs the calculator turns the target-vs-measured difference into mechanical guidance. The key concept is **secondary magnification (m)** – how much the secondary mirror amplifies the primary’s focal length. Presets cover the common designs (e.g. an f/8 GSO-style RC uses an f/2 primary, m = 4; an EdgeHD runs m ≈ 5.26), or enter your own.

Why m matters: mirror-spacing changes are amplified by **m²** at the secondary – on an m = 4 RC, moving the secondary 0.1mm changes focal length roughly 16× more than the same move of the primary. The calculator computes both options with their sensitivities, and an **adjustment split slider** lets you distribute the correction between mirrors. Direction is spelled out in plain words (“0.05 mm toward sky”), along with the focuser backfocus shift that compensates.

[![](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-132411.png)](https://astro-pm.com/wp-content/uploads/2026/07/Screenshot-2026-07-12-132411.png)
Use slider to choose secondary or primary or both to adjust

### The Accuracy Bar & Tolerance

A gradient bar visualizes where the measured focal length sits: green zone = within your chosen tolerance (default 98%, i.e. ±2%), amber and red beyond. The **Percent Change** slider scales the recommended correction if you deliberately want a partial move – useful when chasing a target in small, safe steps.

   **Tip:** Iterate: adjust, refocus, shoot one frame, re-solve. Two or three rounds typically walks an RC to within a few millimeters of spec. Never move the secondary more than a fraction of a turn at a time – m² is not your friend in reverse.

### The Optical Diagram

The right-hand panel draws your telescope’s optical layout – primary, secondary, and focal plane – annotated with the ideal and measured light paths, plus a live summary of every figure: native and reduced focal length and ratio, measured EFL, backfocus shift, and the per-mirror moves with their sensitivities. A screenshot of this panel is the perfect note to keep with your maintenance log.