telescopecomparison · 2-way
William Optics FLT 91 vs William Optics GT 81
William Optics
FLT 91
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
William Optics
GT 81
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
Key differences
+13% focal length
+12% aperture
Specifications
| Specification | William Optics FLT 91 | William Optics GT 81 |
|---|---|---|
| Optics | ||
| Type | Refractor | refractor |
| Optical design | Triplet APO | Apo triplet FPL-53 |
| Aperture | 91mm | 81mm |
| Focal length | 540mm | 478mm |
| Native f/ratio | f/5.9 | f/5.9 |
| Image circle | 44mm | 44mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 4.5kg | 3.8kg |
Advantages of William Optics FLT 91
- Tighter framing — 540mm vs 478mm focal length
- 12% more light-gathering aperture
Advantages of William Optics GT 81
- Wider field — 478mm vs 540mm focal length
- Lighter (3.8kg vs 4.5kg) — easier to travel with
Best for — William Optics FLT 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
Best for — William Optics GT 81
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
William Optics FLT 91 — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 3.82° × 2.55°Scale: 1.44"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 2.49° × 1.67°Scale: 1.44"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.20° × 1.20°Scale: 1.44"/px
William Optics GT 81 — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 4.32° × 2.88°Scale: 1.62"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 2.82° × 1.88°Scale: 1.62"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.35° × 1.35°Scale: 1.62"/px
William Optics FLT 91 — recommended user
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
William Optics GT 81 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.