telescopecomparison · 2-way
William Optics RedCat 91 WIFD vs William Optics Zenithstar 81 II
William Optics
RedCat 91 WIFD
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics
Zenithstar 81 II
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
Key differences
-6% focal length
+12% aperture
Faster by 1.0 f-stops
Specifications
| Specification | William Optics RedCat 91 WIFD | William Optics Zenithstar 81 II |
|---|---|---|
| Optics | ||
| Type | refractor | refractor |
| Optical design | Petzval (5-element advanced) | Apo doublet FPL-53 |
| Aperture | 91mm | 81mm |
| Focal length | 448mm | 478mm |
| Native f/ratio | f/4.9 | f/5.9 |
| Image circle | 55mm | 44mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 4.7kg | 3.6kg |
Advantages of William Optics RedCat 91 WIFD
- 12% more light-gathering aperture
- Faster optics at f/4.9 vs f/5.9
- Larger image circle (55mm) — supports bigger sensors
Advantages of William Optics Zenithstar 81 II
- Lighter (3.6kg vs 4.72kg) — easier to travel with
Best for — William Optics RedCat 91 WIFD
- Large emission nebulae and star clusters
- Fast imaging — short exposures per sub
- Full-frame astro cameras and DSLRs
Best for — William Optics Zenithstar 81 II
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
William Optics RedCat 91 WIFD — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 4.60° × 3.07°Scale: 1.73"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 3.01° × 2.01°Scale: 1.73"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.45° × 1.45°Scale: 1.73"/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
William Optics RedCat 91 WIFD — 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.
