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William Optics AP 91/540 Red OTA Specs
| Brand | William Optics |
| Full Model Code | William Optics Fluorostar 91 Apochromatic Refractor Optical Tube Assembly - Red Edition |
| Country of Origin | Taiwan (Design, Precision Engineering, Structural Fabrication, and Optical Assembly) |
| Optical Design | Apochromatic Refractor (3-Element Air-Spaced Triplet Lens Layout with fully baffled tube matrix) |
| Aperture | 91 mm (3.58 inches) |
| Focal Length | 540 mm |
| Focal Ratio | f/5.93 |
| Optical Coatings | Ultra-Pure Multi-Layer Anti-Reflective coatings (STM Coating) applied across all structural air-to-glass lens borders |
| Glass Material | Premium FPL-53 Synthetic Fluorite Extra-Low Dispersion (ED) Japanese Glass substrate paired with high-index matching lenses |
| Secondary Mirror Obstruction | 0 mm (Unobstructed Optical Path Setup) |
| Secondary Mirror Obstruction by Diameter | 0% |
| Secondary Mirror Obstruction by Area | 0% |
| Resolving Power | 1.27 Arcseconds (Dawes), 1.52 Arcseconds (Rayleigh) |
| Limiting Visual Magnitude | 11.5 |
| Highest Useful Magnification | 182x |
| Lowest Useful Magnification | 13x |
| Optical Tube Material | CNC-Machined High-Grade Aluminum Alloy Tube Body finished in powder-white with premium hard-anodized red accent panels |
| Focuser Style & Mechanism | 3.3-inch Extra-Rigid Heavy-Duty Rack and Pinion Focuser featuring a 10:1 dual-speed micro-adjustment drive wheel |
| Mirror Lock Mechanism | No (Fixed structural lens elements prevent manual cell shifting or focus mechanics tilt) |
| Optical Tube Dimensions | 103 mm x 441 mm (Tube Outer Diameter x Minimum Collapsed Structural Length with retracted dew shield) |
| Optical Tube Weight | 4.18 kg (9.22 lbs) (Total weight rating inclusive of standard dual CNC tube rings and Vixen-style dovetail bar assembly) |
| Mount Attachment Type | Dual CNC Mounting Rings linked to a standard combined Vixen-style and Arca-Swiss compatible Dovetail Mounting Plate |
| Finder Scope Specification | Integrated Patented Bahtinov Focus Mask configured inside the structural aluminum slide-out dew shield cap assembly |
| Included Accessories | 91mm Triplet Optical Tube, Dual CNC Tube Rings, Saddle Handle Bar, Arca-Swiss/Vixen Dovetail Rail, Integrated Bahtinov Mask Cap |
| Standard Warranty | 2-Year Limited Warranty |
| Application | High-performance wide-field astrophotography and pristine field viewing, providing a flat color-free focal layer optimized for modern medium-to-large imaging sensors. |
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