Featuring an image circle big enough for medium-format photography, the RedCat 91 WIFD 91mm f/4.9 Apochromatic Refractor Telescope from William Optics is a photo-only optical tube assembly built for making gorgeous deep-sky images. The RedCat 91 features a fully multicoated five-element lens with super ED, ED, and Lanthanum elements in a field-flattening Petzval design that delivers edge-to-edge sharpness and eliminates complex back-focus calculations. The William Optics Internal Focus Design does away with rear focusers that sag under heavy cameras and accessories; instead, a rigid and balanced 1:10 rack-and-pinion focuser in the middle of the OTA prevents tube tilt and resulting vignetting.
Full-Frame & Medium-Format Capabilities
With a large 55mm image circle, the RedCat 91 is perfect for both full-frame and medium-format cameras. Its advanced optics achieve flat-field images without requiring an additional flattener, giving astrophotographers the freedom to capture expansive views with unparalleled clarity.
Extra-Low Dispersion Glass
The five-element lens includes two super ED, one ED glass, and one lanthanum elements with advanced full multicoating, delivering sharp, vibrant images with true-to-life colors and significantly reducing chromatic aberration.
William Optics Internal Focus Design
The precision-engineered rack-and-pinion system in WIFD Focuser ensures smooth and accurate focusing for both visual observation and astrophotography. Its robust design minimizes backlash and delivers reliable performance, even when supporting heavy imaging equipment. The WIFD is compatible with the optional ZWO Electronic Automatic Focuser.
Integrated Bahtinov Mask
Laser engraved on high-definition and stable optical PMMA plastic, the focusing aid enhances your astrophotography experience with quick and accurate adjustments.
Dual Mounting Ring Assembly
Designed for seamless functionality without compromising stability, the dual ring and handle assembly features wide surface contacts, ensuring a secure fit while enabling fast, easy detachment.
Precision Imaging Adjustment
The camera angle rotator corrects angular discrepancies for the perfect angle on your images, while the built-in tilt adjuster makes it easy to correct camera sensor tilt issues.