Observed arrival · 2026-09-10

The Back-Focus Problem, Measured in Millimetres

backfocus.tools Observed source

Editorial interest 84/100 Selection signal · not a rating of the site

An astrophotography calculator that builds spacer stacks between a telescope corrector and camera sensor.

Landing page captured for the 2026-09-10 issue.

Field notes

The calculator combines telescope, camera, and filter selections to derive the distance from corrector to sensor, then decomposes that distance into named spacer components. Its example reaches 26.4 mm using 21 mm, 5 mm, 0.4 mm, and 16.5 mm parts, while reporting 60.4 mm total back focus, 432 mm focal length, and 1.80 arcseconds per pixel. The page also preserves shareable configuration URLs and lists explanatory notes about measuring planes and filter glass.

Observed signals

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Editorial observations of this landing page, not a rating.

○OpenPublic substance visible

✦PrettyNotable craft visible

●ProPolished or operationally mature

◎NicheUnusually specific use

ƒJavaScriptBrowser-side code central

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