Backfocus Calculator
Calculate total backfocus distance from your imaging train components. Match your field flattener or reducer's required backfocus for sharp stars across the field.
mm
Check your corrector/flattener manual for this value (typically 55mm, 85mm, etc.)
Imaging Train Stack
Add components from corrector to camera sensor (in order)
mm
mm
mm
Quick Add Common Components
Filter Wheels & Accessories
Spacers
Camera Sensor Depths
Total Backfocus
51.0
mm
Required
55mm
Difference
+4.0mm
Need More Spacers
Add 4.0mm of spacing to reach the target backfocus. Stars at the field edges may be elongated until corrected.
Stack Breakdown
Filter Wheel21.0mm
OAG12.5mm
Camera Sensor Depth17.5mm
Total51.0mm
What is Backfocus?
Backfocus is the distance from the last optical element (field flattener, reducer, or corrector) to your camera sensor. Each corrector is designed for a specific backfocus distance.
Incorrect backfocus causes elongated stars at the corners (field curvature) or across the entire field. Aim for ±1mm of the specified value.