William Optics
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
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Step up your astrophotography game with the William Optics Cat 91, an advanced apochromatic refractor tailored for full-frame and medium-format cameras. This first addition to the Ultra Cat 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 Cat 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 Cat 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 Cat 91 provides precise, stable focusing without additional adjustments, enhancing the ease of use for both seasoned astrophotographers and beginners.
The Cat 91 blends the trusted Cat heritage with William Optics’ innovative design, making it the ultimate choice for those ready to elevate their night-sky explorations.
- 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
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
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We Source It
Delivered within 4 weeks.
Enjoy It
Be happy with your gear.
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what we get asked most often
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Yes. The price you see is the price you'll pay. All taxes are included. Free shipping on all orders.
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We list only in-stock items on our website that can be shipped right away. Sold out products are out of stock, so you can't place an order.
Selected products are available for Preorder, with a guaranteed delivery within 30 days or your money back.
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We process and ship all orders within two business days. Shipping time is determined by the speed of the shipping partner.
Is there a guarantee on your products?
Yes. On top of the manufacturer's warranty, we also offer a Replacement Guarantee on all online orders. In the event that the product is defective or damaged during shipping, you will receive a replacement.
Can I cancel my order?
We'll refund you 90% if you cancel before dispatch. Cancellations after dispatch are complicated. Take a look at the our Policy here
What's your return policy?
We have strict return/refund policies as a company that deals with fragile products. Casual returns are not permitted. Items that are not damaged in transit or defective cannot be returned.So please take a moment to review our Refund Policy.
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