













Cat 91 WIFD
Step up your astrophotography game with the William Optics RedCat 91, an advanced apochromatic refractor tailored for full-frame and medium-format cameras. This latest addition to the RedCat series combines portability with superior optical performance, making it a versatile tool for capturing breathtaking detail in deep-sky objects.
Highlights:
Petzval Design, No Backfocus Hassles
The 5-element Petzval design eliminates the need for backfocus adjustments, providing sharp, edge-to-edge images with pinpoint stars—ideal for wide-field and detailed astrophotography alike.
Full-Frame & Medium-Format Ready, No Extra Flattener Needed
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.
Fast f/4.9 Focal Ratio
The RedCat 91's fast f/4.9 focal ratio is designed for bright, vivid captures of galaxies, nebulae, and beyond, ensuring excellent light-gathering power and sharp detail even on faint celestial objects.
Seamless WIFD Focusing
Equipped with William Optics' WIFD focuser, the RedCat 91 provides precise, stable focusing without additional adjustments, enhancing the ease of use for both seasoned astrophotographers and beginners.
The RedCat 91 blends the trusted RedCat heritage with William Optics’ innovative design, making it the ultimate choice for those ready to elevate their night-sky explorations.
Accessory Add-ons
Guider










UniGuide 32mm f/3.75
- Lens Type
- Doublet Lens
- Focal Length
- 120 mm
- Diameter
- 32 mm
- Focal Ratio
- f/3.75
- Tube Length
- 147 mm
- Total Weight
- 0.23 kg / 0.51 lbs





Guiding Scope 50mm f/4.0
- Lens Type
- Doublet Lens
- Focal Length
- 200 mm
- Diameter
- 50 mm
- Focal Ratio
- f/4.0
- Tube Length
- 252 mm
- Total Weight
- 0.42 kg / 0.93 lbs










UniGuide 50mm f/4.0
- Lens Type
- Doublet Lens
- Focal Length
- 200 mm
- Diameter
- 50 mm
- Focal Ratio
- f/4.0
- Tube Length
- 252 mm
- Total Weight
- 0.5 kg / 1.1 lbs








GuideStar 61
- Lens Type
- Doublet APO Prime Lens
- Focal Length
- 360 mm
- Diameter
- 61 mm
- Focal Ratio
- f/5.9
- Image Circle
- 42 mm
- Focuser
- Threaded Focuser
- Adapter Included
- M54 → 2" Visual Back
- Optional Flattener
- FLAT 61A 1.0x FLAT 61R 0.8x
- Tube Length
- 340 mm
- Total Weight
- 1.44 kg / 3.17 lbs
Camera




ZWO ASI 533MC Pro
- Sensor
- Sony IMX533 CMOS ( color )
- Resolution
- 3008 × 3008
- Pixel Size
- 2.9μm
- Sensor Size
- 1″ ( 11.31 x 11.31mm )
- Max Frame Rate
- 20 fps
- Read Noise
- 1.0e
- Cooling Temp
- △35°C
- Quantum Efficiency
- 80%
- Full Well
- 50ke
- Exposure Range
- 32μs–2000s
- DDR3 Buffer
- 256 MB
- ADC
- 14 bit
- Flange Distance
- 17.5mm
- Camera Thread
- M42
- Adapter Included
- M48 → M42 Adapters ( 0 & 16.5mm ) M42 Extender ( 21mm ) M28.5 → M42 Adapter 1.25" Nosepiece → M42 Adapter
- Interface
- USB 2.0 Type-A USB 3.0 Type-B
- Cable Included
- USB 2.0 Type A↔B ( 0.5m ) x2 USB 3.0 Type A↔B ( 2m )
- Camera Weight
- 0.457 kg / 1 lbs
Camera Adapter



M48 T-mount for Pentax K
- Light-In Side
- M48 (female)
- Light-Out Side
- Pentax K Camera Mount
- Optical Length
- 9.3 mm



M48 T-mount for Canon EOS R Mirrorless
- Light-In Side
- M48 (female)
- Light-Out Side
- Canon EOS R Camera Mount
- Optical Length
- 34.4 mm



M48 T-mount for Nikon Z Mirrorless
- Light-In Side
- M48 (female)
- Light-Out Side
- Nikon Z Mirrorless Camera Mount
- Optical Length
- 38.4 mm






M48 T-mount for Canon EOS
- Light-In Side
- M48 (female)
- Light-Out Side
- Canon EOS Camera Mount
- Optical Length
- 10.6 mm



M48 T-mount for Fuji FX
- Light-In Side
- M48 (female)
- Light-Out Side
- Fuji FX Camera Mount
- Optical Length
- 37 mm



T-mount for Sony E
- Light-In Side
- M48 (female)
- Light-Out Side
- Sony E Camera Mount
- Optical Length
- 36.7 mm






M48 T-mount for Nikon F
- Light-In Side
- M48 (female)
- Light-Out Side
- Nikon F Camera Mount
- Optical Length
- 8.2 mm



M48 T-mount for Olympus Micro 4/3
- Light-In Side
- M48 (female)
- Light-Out Side
- Olympus Micro 4/3 Camera Mount
- Optical Length
- 35.6 mm


M48 → M42 Adapter
- Light-In Side
- M48 (female)
- Light-Out Side
- M42 (male)
- Optical Length
- 5.3 mm
EQ Mount









ZWO AM3 RedCat Limited Edition
ZWO x William Optics Co-branded Limited Edition
Traditional equatorial mounts often require heavy counterweights, making it challenging to balance the right ascension (RA) and declination (DEC) axes. They are cumbersome to transport and store, causing significant inconvenience for amateur astrophotographers during outdoor sessions. Additionally, some strain wave gear mounts have limited functionality, insufficient payload capacity, and lack stability during use. Addressing these issues, ZWO has developed the ASI Mount series, which eliminates the need for counterweights.
The AM3 weighs only 3.9 kg and supports payloads ranging from 8 kg to 13 kg. While portability is crucial, ZWO prioritizes stability to ensure a reliable user experience. The optimized internal structure incorporates harmonic drive reducers and timing belts, delivering precise control and an impressive 300:1 reduction ratio. The mount is versatile, functioning as either an equatorial or an alt-azimuth mount, and can be controlled via Wi-Fi, Bluetooth, or a handheld controller.
Key Features
Portability and Stability
The optimized internal structure, featuring harmonic drive reducers and timing belts, ensures precise control and stability, achieving a remarkable 300:1 reduction ratio.
Custom Strain Wave Gears
ZWO developed specialized strain wave gears tailored to astrophotography needs, offering extremely low periodic error and high torque. Each mount undergoes factory testing and comes with a unique PE curve to guarantee quality.
Lightweight Design
At just 3.9 kg, the AM3 strikes a balance between portability and stability, prioritizing dependable performance over extreme weight reduction.
8 kg to 13 kg Payload Capacity
Without Counterweights: Supports up to 8 kg.
With Counterweights: Supports up to 13 kg.
(Counterweights should be mounted on a bar no longer than 20 cm.)
Dual Modes: Equatorial and Alt-Azimuth
Equatorial Mode: Ideal for tracking and imaging celestial objects.
Alt-Azimuth Mode: Best suited for visual astronomy.
Easily switch modes using the ASI Mount App or the handheld controller.
Full 0°–90° Altitude Adjustment
Gear 1: Adjust the handle to set angles between 0° and 60°.
Gear 2: Loosen the hex screws to adjust the clamp for angles between 30° and 90°.
Wireless Control
Enjoy features like Tonight’s Target, GOTO, and Live Preview directly from your smartphone via Wi-Fi.
Joystick Hand Controller
Ergonomically designed for comfort and ease of use, the controller integrates with the Wi-Fi network for wireless operation. Firmware updates and installation can be managed effortlessly via a web interface.
Seamless ASIAIR Integration
The AM3 can be controlled via wired or wireless connection with ASIAIR, offering ZWO’s customers an optimized astrophotography experience.







ZWO AM5N RedCat Limited Edition
ZWO x William Optics Co-branded Limited Edition
Traditional equatorial mounts often require heavy counterweights, making them cumbersome to transport and balance. The AM5N addresses these challenges with a compact, counterweight-free design, offering enhanced portability and stability for amateur astrophotographers. Its optimized internal structure incorporates harmonic drive reducers and timing belts, delivering precise control and a 300:1 reduction ratio. The mount operates in both equatorial and alt-azimuth modes and can be controlled via Wi-Fi, Bluetooth, or a handheld controller.
Key Features
Enhanced Portability and Stability
Weighing only 5.5 kg, the AM5N supports payloads up to 15 kg without counterweights and up to 20 kg with counterweights, maintaining a balance between portability and performance.
Custom Harmonic Drive System
The AM5N features a custom harmonic drive system with a synchronous belt, ensuring high torque, zero backlash, and low periodic error of ±10 arcseconds.
Integrated Cable Management
Internal cable routing through the DEC saddle allows for seamless power and control connections, reducing cable clutter and the risk of snags during operation.
Dual Mount Modes
Switch effortlessly between equatorial and alt-azimuth modes using the ASI Mount App or the included hand controller. A status LED indicates the current mode: red for equatorial and green for alt-azimuth.
Wide Latitude Adjustment
The mount offers a full 0°–90° latitude adjustment range, accommodating various observing locations and setups.
Wireless and Wired Control Options
Control the AM5N via Wi-Fi, Bluetooth, or USB-C connections. The mount is compatible with the ASIAIR system and supports ASCOM and INDI protocols for versatile operation.
Power Failure Protection
An RA axis brake system engages automatically during power failures, safeguarding your equipment from potential damage.
Seamless ASIAIR Integration
The AM5N integrates seamlessly with ZWO's ASIAIR system, providing an optimized astrophotography experience through wired or wireless connections.




Tripod





Tripod Pouch
Featuring durable Velcro straps for secure attachment, this pouch offers ample storage space to make your stargazing experience more convenient.







Pier Mount 90
- Adapter
- 3/8" Thread ( for ZWO AM3/AM5 )
More EQ Mount types are optional - Weight
- approx. 7 kg
- Loading Capacity
- 90 kg / 198 lbs
- Height
- Stance: 89.3 cm Closed: 79.3 cm Foot Adjustable Height Range: 15 mm
- Material
- Aluminum Alloy
- Pillar Diameter
- 100 mm / 4"
![[Bundle] ZWO Mount Upgrade Kit](http://williamoptics.com/cdn/shop/files/ZWOMountUpgradeKit.jpg?v=1724140371&width=2400)
[Bundle] ZWO Mount Upgrade Kit
Make your ZWO AM3/5 tripod kit load-bearing and stable.


Tripod Extrapadding Backpack
This Tripod Extrapadding Backpack is specifically designed for astro EQ mounts tripod, offering added support and protection for your equipment.
- Thickness: 22mm
- Size: approximately 20 × 20 × 67 cm, with a measurement tolerance of ±3 cm.














Camouflage Tripod Pouch
Featuring durable Velcro straps for secure attachment, this pouch offers ample storage space to make your wildlife observation experience more convenient.
- Lens Type
- 5-Element Advanced Petzval
- Lens Elements
- Super ED Glass x2 ED Glass x1 Lanthanum Glass x1 X-Element x1
- Focal Length
- 448 mm
- Diameter
- 91 mm
- Focal Ratio
- f/4.9
- Image Circle
- 55 mm
- Focuser
- WIFD R&P Focuser Compatible with EAF
- Focuser Travel
- 0-36 mm (Ruler to 40mm)
- Back Spacing
- Maximum: M48 (60.8mm),M54 (65.8mm)
- Adapter Included
- Tilt Adjuster Rotator M63 → M54 Adapter M63 → M48 Adapter
- Optional Flattener
- No Flattener Required
- T-Mount Support
- Canon EF, RF Nikon F, Z Sony E Pentax K MFT (M4/3) FUJI X
- Camera Format
- Medium Format ( needs additional adapter ) Full Frame APS-C M4/3
- Dovetail Type
- Vixen
- Tube Length
- 556 mm
- OTA Weight
- 4.72 kg / 10.41 lbs
- Total Weight
- 6.3 kg / 13.89 lbs
Choose options














Step up your astrophotography game with the William Optics RedCat 91, an advanced apochromatic refractor tailored for full-frame and medium-format cameras. This latest addition to the RedCat series combines portability with superior optical performance, making it a versatile tool for capturing breathtaking detail in deep-sky objects.
Highlights:
Petzval Design, No Backfocus Hassles
The 5-element Petzval design eliminates the need for backfocus adjustments, providing sharp, edge-to-edge images with pinpoint stars—ideal for wide-field and detailed astrophotography alike.
Full-Frame & Medium-Format Ready, No Extra Flattener Needed
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.
Fast f/4.9 Focal Ratio
The RedCat 91's fast f/4.9 focal ratio is designed for bright, vivid captures of galaxies, nebulae, and beyond, ensuring excellent light-gathering power and sharp detail even on faint celestial objects.
Seamless WIFD Focusing
Equipped with William Optics' WIFD focuser, the RedCat 91 provides precise, stable focusing without additional adjustments, enhancing the ease of use for both seasoned astrophotographers and beginners.
The RedCat 91 blends the trusted RedCat heritage with William Optics’ innovative design, making it the ultimate choice for those ready to elevate their night-sky explorations.
What's in the box

Main Tube

Bahtinov Cap

M48, M54
Camera Adapter

Vixen-style Plate

Mounting Ring & Handle bar

Riser Blocks

Soft Carry Case

Allen Key Set


The cat with medium-format soul
An advanced apochromatic refractor tailored for full-frame and medium-format cameras. This latest addition to the RedCat series combines portability with superior optical performance, making it a versatile tool for capturing breathtaking detail in deep-sky objects.

Advanced Petzval
Extra-low Dispersion (ED) glasses


Needs NO Flattener
Never easier to capture deep sky objects


Compatible with Electronic Motorized Devices
Hassle Free Installation
( No Mounting Bracket Kit required )
Leading the market
Wo patented design
The Cat 91 holds three unique patents, ensuring easy and perfect focusing along with precise coaxial image quality, preventing draw tube tilt issues caused by optical misalignment.
:
Internal Focuser


Integrated Bahtinov mask
CAT Handle Bar
Crafts for enhanced portability and versatility.
Mounting Ring
Designed for seamless functionality without compromising on stability. Engineered with wide surface contacts, this innovative ring ensures a secure fit while enabling fast, easy detachment.


comprehensive configuration
Precision Imaging Adjustment

Tilt Adjuster
The built-in tilt adjuster makes it easy to correct camera sensor tilt issues.

Camera Angle Rotator
Gallery
Swipe left and right to view the Original & After processed image.

Original

After Process

▲ IC434 (APS-C)
Cat 91 + ZWO ASI 2600MC Duo
Courtesy of Mehmet Ergün
- Stacked ( Original & After Process )
- Original Single Shot Corner Grid


▲ M45 (Full-Frame)
Cat 91 + ZWO ASI 6200MC Pro
Courtesy of Dear Bear
- Stacked ( After Process )
- Original Single Shot Corner Grid
















