telescopecomparison · 2-way
William Optics RedCat 91 vs William Optics Zenithstar 73 II
William Optics
RedCat 91
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics
Zenithstar 73 II
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
Key differences
+15% focal length
+25% aperture
Faster by 0.5 f-stops
Specifications
| Specification | William Optics RedCat 91 | William Optics Zenithstar 73 II |
|---|---|---|
| Optics | ||
| Type | Refractor | refractor |
| Optical design | Petzval APO | Apo doublet FPL-53 |
| Aperture | 91mm | 73mm |
| Focal length | 496mm | 430mm |
| Native f/ratio | f/5.5 | f/5.9 |
| Image circle | 45mm | 44mm |
| Full-frame support | Yes | No |
| Physical | ||
| Weight | 4.3kg | 2.4kg |
Advantages of William Optics RedCat 91
- Tighter framing — 496mm vs 430mm focal length
- 25% more light-gathering aperture
- Faster optics at f/5.45 vs f/5.9
Advantages of William Optics Zenithstar 73 II
- Wider field — 430mm vs 496mm focal length
- Lighter (2.4kg vs 4.3kg) — easier to travel with
Best for — William Optics RedCat 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
Best for — William Optics Zenithstar 73 II
- Large emission nebulae and star clusters
William Optics RedCat 91 — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 4.16° × 2.77°Scale: 1.56"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 2.71° × 1.81°Scale: 1.56"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.31° × 1.31°Scale: 1.56"/px
William Optics Zenithstar 73 II — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 4.80° × 3.20°Scale: 1.80"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 3.13° × 2.09°Scale: 1.80"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.51° × 1.51°Scale: 1.80"/px
William Optics RedCat 91 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics Zenithstar 73 II — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.