TECHVEINNATARAJAMerchant Navy LMS
OP-09 practice — not an approved assessment

Sextant optics and index error

Learning outcomes & permitted supports
  • Explain double reflection and why the arc reads twice the mirror's turn.
  • Determine index error from a horizon check and apply the index correction with the correct (opposite) sign.
  • Distinguish adjustable from non-adjustable errors and defend leaving a small, known IE.

Supports in this lab: interactive ray-path schematic with telescope view and contact detection; two instrument tasks.

Glossary: index error · Hs · Ha · Ho · visible horizon

Two mirrors, one principle — and the error that lives in the frame

graduated arc — reads 2× the mirror turn (that's why a 60° sector measures 120°)index mirror (turned 0.25°)horizon (via mirrors)horizon mirrorhorizon (direct)telescope27.5′ apartreading: 000° 30.0′
Bring the reflected horizon down onto the direct horizon. The reading at contact IS the index error.

Double reflection: turning the index mirror by θ swings the reflected image by 2θ — which is why a 60° arc measures 120° and why the arc is graduated at twice the mirror angle. Task 1: with both images on the horizon the reading SHOULD be zero; whatever it reads instead is the index error. Task 2: a real sight — and the first correction you will apply in OP-10 is the IC you just found.

My notebook — OP-09 (0)

All notes & standing →

Every interaction here is recorded as an ordered evidence trail — 0 events this attempt.