telescopecomparison · 2-way
Askar 120APO vs William Optics RedCat 91
Askar
120APO
Best suited for intermediate imagers building a versatile kit running a backyard imaging setup.
William Optics
RedCat 91
Best suited for beginners and wide-field enthusiasts running a portable travel rig.
Key differences
+69% focal length
+32% aperture
Slower by 1.5 f-stops
Specifications
| Specification | Askar 120APO | William Optics RedCat 91 |
|---|---|---|
| Optics | ||
| Type | Refractor | Refractor |
| Optical design | Triplet APO | Petzval APO |
| Aperture | 120mm | 91mm |
| Focal length | 840mm | 496mm |
| Native f/ratio | f/7.0 | f/5.5 |
| Image circle | 44mm | 45mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 8.5kg | 4.3kg |
Advantages of Askar 120APO
- Tighter framing — 840mm vs 496mm focal length
- 32% more light-gathering aperture
Advantages of William Optics RedCat 91
- Wider field — 496mm vs 840mm focal length
- Faster optics at f/5.45 vs f/7
- Lighter (4.3kg vs 8.5kg) — easier to travel with
Best for — Askar 120APO
- Medium-sized galaxies and planetary nebulae
- Full-frame astro cameras and DSLRs
Best for — William Optics RedCat 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
Askar 120APO — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 2.46° × 1.64°Scale: 0.92"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 1.60° × 1.07°Scale: 0.92"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 0.77° × 0.77°Scale: 0.92"/px
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
Askar 120APO — recommended user
Best suited for intermediate imagers building a versatile kit running a backyard imaging setup.
William Optics RedCat 91 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.