William Optics Flat68III Field Flattener A 1.0× field flattener that preserves the telescope's native focal length for the manufacturer's listed refractor combinations. Its optical system uses 2 elements in 2 groups.
Manufacturer model: William Optics Flat68III Field Flattener · Specifications checked against manufacturer data on 2026-11-03
A 1.0× field flattener that preserves the telescope's native focal length for the manufacturer's listed refractor combinations. Its optical system uses 2 elements in 2 groups.
The telescope connection is M92x1; the camera connection is M48x0.75. Weight is 718.2 g. Optical length: 64.5 mm.
Backfocus varies by telescope: GT81 with back adapter: 57 mm; GT102: 68.6 mm; GT153: 57.8 mm; ZS126: 58 mm; FLT91 with back adapter: 60 mm; FLT120: 65.7 mm; FLT132: 62.7 mm; FLT156: 57.6 mm. Use the listed front or rear adapter where specified; backfocus and mechanical adjustment are different measurements.
William Optics manufacturer specification and compatibility chart
Use telescope-specific spacing; GT81 WIFD and FLT91 require the documented adapter. Optical body length is not backfocus.
| Optical groups | 2 |
|---|---|
| Backfocus gt81 with back adapter (mm) | 57 |
| Backfocus gt102 (mm) | 68.6 |
| Backfocus gt153 (mm) | 57.8 |
| Backfocus zs126 (mm) | 58 |
| Backfocus flt91 with back adapter (mm) | 60 |
| Backfocus flt120 (mm) | 65.7 |
| Backfocus flt132 (mm) | 62.7 |
| Backfocus flt156 (mm) | 57.6 |
| Camera thread engagement (mm) | 4 |
| Telescope thread engagement (mm) | 6.5 |
| Adjustment gt81 with back adapter (mm) | 2 |
| Image circle gt81 with back adapter (mm) | 44.4 |
| Adjustment gt102 (mm) | 13.6 |
| Image circle gt102 (mm) | 44.4 |
| Adjustment gt153 (mm) | 2.8 |
| Image circle gt153 (mm) | 44.4 |
| Adjustment zs126 (mm) | 3 |
| Image circle zs126 (mm) | 43.4 |
| Adjustment flt91 with back adapter (mm) | 5 |
| Image circle flt91 with back adapter (mm) | 43.4 |
| Adjustment flt120 (mm) | 10.7 |
| Image circle flt120 (mm) | 44.4 |
| Adjustment flt132 (mm) | 7.7 |
| Image circle flt132 (mm) | 44.4 |
| Adjustment flt156 (mm) | 2.6 |
| Image circle flt156 (mm) | 44.4 |
| Spacing reference | Use the backfocus and adjustment setting for the named telescope and required adapter. Manufacturer model-specific values; not a universal camera spacing. |
Data sheet
William Optics Flat68III Field Flattener A 1.0× field flattener that preserves the telescope's native focal length for the manufacturer's listed refractor combinations. Its optical system uses 2 elements in 2 groups.
Manufacturer model: William Optics Flat68III Field Flattener · Specifications checked against manufacturer data on 2026-11-03
