telescopecomparison · 2-way
William Optics FLT 91 vs Borg 90FL
William Optics
FLT 91
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
Key differences
+8% focal length
Slower by 0.3 f-stops
Specifications
| Specification | William Optics FLT 91 | Borg 90FL |
|---|---|---|
| Optics | ||
| Type | Refractor | refractor |
| Optical design | Triplet APO | Apo doublet fluorite |
| Aperture | 91mm | 90mm |
| Focal length | 540mm | 500mm |
| Native f/ratio | f/5.9 | f/5.6 |
| Image circle | 44mm | — |
| Full-frame support | Yes | No |
| Physical | ||
| Weight | 4.5kg | 2.5kg |
Advantages of William Optics FLT 91
- Larger image circle (44mm) — supports bigger sensors
Advantages of Borg 90FL
- Faster optics at f/5.6 vs f/5.9
- Lighter (2.5kg 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 — Borg 90FL
- Large emission nebulae and star clusters
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
Borg 90FL — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 4.13° × 2.75°Scale: 1.55"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 2.69° × 1.80°Scale: 1.55"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.29° × 1.29°Scale: 1.55"/px
William Optics FLT 91 — recommended user
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
Borg 90FL — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.