telescopecomparison · 2-way
William Optics RedCat 51 vs William Optics FLT 91
William Optics
RedCat 51
Best suited for beginners and wide-field enthusiasts 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
-54% focal length
-44% aperture
Faster by 1.0 f-stops
Specifications
| Specification | William Optics RedCat 51 | William Optics FLT 91 |
|---|---|---|
| Optics | ||
| Type | Refractor | Refractor |
| Optical design | Petzval APO | Triplet APO |
| Aperture | 51mm | 91mm |
| Focal length | 250mm | 540mm |
| Native f/ratio | f/4.9 | f/5.9 |
| Image circle | 45mm | 44mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 1.6kg | 4.5kg |
Advantages of William Optics RedCat 51
- Wider field — 250mm vs 540mm focal length
- Faster optics at f/4.9 vs f/5.9
- Lighter (1.6kg vs 4.5kg) — easier to travel with
Advantages of William Optics FLT 91
- Tighter framing — 540mm vs 250mm focal length
- 78% more light-gathering aperture
Best for — William Optics RedCat 51
- Wide-field nebulae and Milky Way regions
- Fast imaging — short exposures per sub
- Full-frame astro cameras and DSLRs
Best for — William Optics FLT 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
William Optics RedCat 51 — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 8.25° × 5.50°Scale: 3.10"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 5.39° × 3.60°Scale: 3.10"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 2.59° × 2.59°Scale: 3.10"/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
William Optics RedCat 51 — recommended user
Best suited for beginners and wide-field enthusiasts 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.