telescopecomparison · 2-way
Borg 90FL vs William Optics Zenithstar 81 II
William Optics
Zenithstar 81 II
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
Key differences
+11% aperture
Faster by 0.3 f-stops
Specifications
| Specification | Borg 90FL | William Optics Zenithstar 81 II |
|---|---|---|
| Optics | ||
| Type | refractor | refractor |
| Optical design | Apo doublet fluorite | Apo doublet FPL-53 |
| Aperture | 90mm | 81mm |
| Focal length | 500mm | 478mm |
| Native f/ratio | f/5.6 | f/5.9 |
| Image circle | — | 44mm |
| Full-frame support | No | Yes |
| Physical | ||
| Weight | 2.5kg | 3.6kg |
Advantages of Borg 90FL
- 11% more light-gathering aperture
- Faster optics at f/5.6 vs f/5.9
- Lighter (2.5kg vs 3.6kg) — easier to travel with
Advantages of William Optics Zenithstar 81 II
- Larger image circle (44mm) — supports bigger sensors
Best for — Borg 90FL
- Large emission nebulae and star clusters
Best for — William Optics Zenithstar 81 II
- 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 Zenithstar 81 II — 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
Borg 90FL — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics Zenithstar 81 II — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
