telescopecomparison · 2-way
ZWO FF90 APO vs William Optics FLT 91
ZWO
FF90 APO
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
William Optics
FLT 91
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
Key differences
+13% focal length
Slower by 0.9 f-stops
Specifications
| Specification | ZWO FF90 APO | William Optics FLT 91 |
|---|---|---|
| Optics | ||
| Type | refractor | Refractor |
| Optical design | Apo quadruplet ED | Triplet APO |
| Aperture | 90mm | 91mm |
| Focal length | 609mm | 540mm |
| Native f/ratio | f/6.8 | f/5.9 |
| Image circle | 44mm | 44mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 3.9kg | 4.5kg |
Advantages of ZWO FF90 APO
- Tighter framing — 609mm vs 540mm focal length
Advantages of William Optics FLT 91
- Wider field — 540mm vs 609mm focal length
- Faster optics at f/5.9 vs f/6.8
Best for — ZWO FF90 APO
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
Best for — William Optics FLT 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
ZWO FF90 APO — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 3.39° × 2.26°Scale: 1.27"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 2.21° × 1.48°Scale: 1.27"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.06° × 1.06°Scale: 1.27"/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
ZWO FF90 APO — recommended user
Best suited for intermediate imagers building a versatile kit 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.
