telescopecomparison · 2-way
William Optics RedCat 91 vs William Optics GT 81
William Optics
RedCat 91
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics
GT 81
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
Key differences
+12% aperture
Faster by 0.5 f-stops
Specifications
| Specification | William Optics RedCat 91 | William Optics GT 81 |
|---|---|---|
| Optics | ||
| Type | Refractor | refractor |
| Optical design | Petzval APO | Apo triplet FPL-53 |
| Aperture | 91mm | 81mm |
| Focal length | 496mm | 478mm |
| Native f/ratio | f/5.5 | f/5.9 |
| Image circle | 45mm | 44mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 4.3kg | 3.8kg |
Advantages of William Optics RedCat 91
- 12% more light-gathering aperture
- Faster optics at f/5.45 vs f/5.9
Advantages of William Optics GT 81
Closely matched on the compared specs.
Best for — William Optics RedCat 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
Best for — William Optics GT 81
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
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 GT 81 — 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 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
William Optics GT 81 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.