telescopecomparison · 2-way
Borg 90FL vs William Optics FLT 91
William Optics
FLT 91
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
Key differences
-7% focal length
Faster by 0.3 f-stops
Specifications
| Specification | Borg 90FL | William Optics FLT 91 |
|---|---|---|
| Optics | ||
| Type | refractor | Refractor |
| Optical design | Apo doublet fluorite | Triplet APO |
| Aperture | 90mm | 91mm |
| Focal length | 500mm | 540mm |
| Native f/ratio | f/5.6 | f/5.9 |
| Image circle | — | 44mm |
| Full-frame support | No | Yes |
| Physical | ||
| Weight | 2.5kg | 4.5kg |
Advantages of Borg 90FL
- Faster optics at f/5.6 vs f/5.9
- Lighter (2.5kg vs 4.5kg) — easier to travel with
Advantages of William Optics FLT 91
- Larger image circle (44mm) — supports bigger sensors
Best for — Borg 90FL
- Large emission nebulae and star clusters
Best for — William Optics FLT 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
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 — 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 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics FLT 91 — recommended user
Best suited for intermediate imagers building a versatile kit running a portable travel rig.