telescopecomparison · 2-way
William Optics FLT 91 vs Askar FRA400
William Optics
FLT 91
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
Key differences
+35% focal length
+26% aperture
Slower by 0.3 f-stops
Specifications
| Specification | William Optics FLT 91 | Askar FRA400 |
|---|---|---|
| Optics | ||
| Type | Refractor | Refractor |
| Optical design | Triplet APO | Petzval Quintuplet APO |
| Aperture | 91mm | 72mm |
| Focal length | 540mm | 400mm |
| Native f/ratio | f/5.9 | f/5.6 |
| Image circle | 44mm | 44mm |
| Full-frame support | Yes | Yes |
| Physical | ||
| Weight | 4.5kg | 3.9kg |
Advantages of William Optics FLT 91
- Tighter framing — 540mm vs 400mm focal length
- 26% more light-gathering aperture
Advantages of Askar FRA400
- Wider field — 400mm vs 540mm focal length
- Faster optics at f/5.6 vs f/5.9
Best for — William Optics FLT 91
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
Best for — Askar FRA400
- Large emission nebulae and star clusters
- Full-frame astro cameras and DSLRs
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
Askar FRA400 — typical FOV & scale
- With Full-frame (36×24mm, 3.76µm)FOV: 5.16° × 3.44°Scale: 1.94"/px
- With APS-C (23.5×15.7mm, 3.76µm)FOV: 3.37° × 2.25°Scale: 1.94"/px
- With IMX533 square (11.3×11.3mm, 3.76µm)FOV: 1.62° × 1.62°Scale: 1.94"/px
William Optics FLT 91 — recommended user
Best suited for intermediate imagers building a versatile kit running a portable travel rig.
Askar FRA400 — recommended user
Best suited for beginners and wide-field enthusiasts running a portable travel rig.