Calibration
Calibration aligns your iPhone’s sensors with the actual sky. The more calibration stars you use, the more accurate the navigation becomes.
Why Calibrate?
The iPhone sensors (compass, gyroscope) are not precise enough for astronomy. By aligning on known stars, the app calculates a correction that is then applied to all positions.
- 1 star: Rough alignment (offset correction)
- 2+ stars: Accurate alignment (rotation correction across the whole sky)
- 3+ stars: Best accuracy (compensates for sensor errors)
Calibration Procedure
- Select a star — Tap a star in the sky view on the main screen
- Help banner — Shows “Calibrate with [star name]” with actions: Deselect, Center, Show Details
- Align the telescope — Mount the phone on the telescope and center the star in the eyepiece
- Confirm — Press the Confirm button (✓, floating bottom right)
Calibration Indicator
In the upper right corner of the main screen, a small calibration indicator (pill) shows the current status:
- Colored dot — Green (good), Orange (warning), Red (poor)
- Star count — e.g. “3★” for three active calibration stars
Detail View
Tap the calibration indicator to open the detail view. It contains:
Status Overview
A large icon with title and description of the current calibration state (e.g. “Good”, “Warning”, “Poor”).
Metrics
Four color-coded metrics in a grid:
| Metric | Description |
|---|---|
| Star count | Number of active calibration stars (good: 3+, minimal: 2, too few: <2) |
| Mean error | Average residual error in degrees (very good: ≤1.5°, acceptable: ≤2.5°, high: >2.5°) |
| Age | Time since the most recent measurement in minutes (fresh: <15 min, OK: <30 min, old: >30 min) |
| Sky coverage | Maximum angular distance between calibration stars in degrees (well distributed: ≥15°, too close: <15°) |
Each metric is color-coded: Green = good, Orange = warning, Red = problem.
Altitude Warnings
Icons warn of problematic star altitudes:
- Red: Star near zenith (>80°) — compass data unreliable
- Orange: Star very high (>75°) or very low (<30°)
Calibration List
List of all active calibration stars (most recent first):
- Colored dot — Age of the measurement (green: <2 min, yellow: <5 min, red: >5 min)
- Name and time of measurement
- Age in seconds or minutes
Actions per entry:
- ↻ — Recalibrate on this star (update the measurement)
- 🗑 — Remove the calibration entry
Older entries are displayed more faintly (after 30 minutes).
Issues and Recommendations
If problems are detected, the detail view shows:
- Issues: e.g. “Too few stars”, “Zenith star”, “High residual error”, “Stars too close together”
- Recommendations: e.g. “Calibrate on another star in a different direction”
Legend
Explanation of the color coding:
- Green: Stable — calibration is reliable
- Orange: Warning — calibration could be improved
- Red: Poor — calibration is unreliable, recalibration recommended
Weighting Over Time
Older calibration stars gradually lose influence (half-life: 15 min). They are not deleted, but newer stars dominate the correction. A maximum of 20 calibration stars are stored simultaneously.
Tips
- Use stars that are well distributed across the sky (not all in the same direction)
- Calibration stars should be bright (mag ≤ 2.5) and at medium altitude (25°–75°)
- After repositioning the telescope or tripod: recalibrate
- The calibration indicator always shows you whether your calibration is still good