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William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD
William Optics Cat 91 WIFD

William Optics Cat 91 WIFD

by William Optics
SKU WO-T-C-91
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CA$4,137.00
CA$3,741.00 - CA$4,137.00
CA$4,137.00
CA$3,930.00
CA$3,554.00 - CA$3,930.00
CA$3,930.00
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  • 91mm Aperture
  • 448mm Focal Length
  • f/4.9 Focal Ratio
  • 5-Element Advanced Petzval Design
  • 55mm Corrected Image Circle
  • WIFD Rack & Pinion Internal Focuser
  • 4.72 kg Optical Tube Assembly Weight
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • William Optics Cat 91 WIFD Astrograph

    The William Optics Cat 91 WIFD is a 91mm, 448mm, f/4.9 apochromatic refractor with a 5-element advanced Petzval design and a 55mm image circle, and no flattener is required. The optical tube weighs 4.72 kg and is 556 mm long. It suits full-frame cameras and, with an additional adapter, medium format.

    A 5-Element Advanced Petzval with Super ED Glass

    The lens elements are Super ED glass x2, ED glass x1, lanthanum glass x1 and an X-Element x1, in a 5-element advanced Petzval design, and the Cat 91 is an apochromatic refractor.

    The Petzval design eliminates the need for backfocus adjustments and achieves flat-field images without an additional flattener. No separate flattener is required.

    91mm Aperture at a Fast f/4.9 Focal Ratio

    A 91mm objective has a Dawes limit of about 1.27 arcseconds. At f/4.9 each sub-frame needs roughly half the exposure of an f/7 instrument for the same signal, which helps on faint nebulae and galaxies.

    A Flat 55mm Image Circle for Large Sensors

    The Cat 91 has a 55mm image circle. A full-frame sensor (36 x 24mm, 43.3mm diagonal) fits inside it. Supported camera formats are medium format (needs additional adapter), full frame, APS-C and M4/3.

    WIFD Focuser: 36mm of Travel, EAF Compatible

    Focus comes from the William Optics Internal Focus Design (WIFD), a WIFD R&P focuser compatible with EAF (an electronic automatic focuser). Travel is 0 to 36 mm, with a ruler marked to 40mm.

    The WIFD focuser provides precise, stable focusing. No further details of the internal mechanism are published.

    M48 & M54 Adapters with 60.8mm & 65.8mm Back Spacing

    The Cat 91 includes a tilt adjuster and rotator, plus an M63 to M48 adapter and an M63 to M54 adapter. Back spacing is a maximum of 60.8mm with M48 and 65.8mm with M54. T-mount support covers Canon EF and RF, Nikon F and Z, Sony E, Pentax K, MFT (M4/3) and FUJI X.

    Field of View at 448mm

    With a full-frame sensor, 448mm gives about 4.6 x 3.1 degrees; with APS-C (about 23.5 x 15.6mm) about 3.0 x 2.0 degrees. That frames the Andromeda Galaxy (M31), the North America Nebula (NGC 7000) or the star-forming regions of Cygnus.

    There is no matching reducer or flattener for the Cat 91. Ask us before adding any corrector.

    Mechanical Build: Ring, Handle Bar and Vixen-Style Dovetail

    The tube sits in a mounting ring with a handle bar on top and a Vixen-style dovetail plate below; riser blocks are included. The dovetail fits a Vixen-style saddle.

    Ergonomics and Weight: 4.72 kg OTA for Portable Setups

    The optical tube weighs 4.72 kg (10.41 lbs); the total weight is 6.3 kg (13.89 lbs). The tube is 556 mm long. Add your camera and accessories and compare the total with your mount's rated payload.

    Included Accessories: Bahtinov Mask, Adapters, and Carry Case

    The box holds the main tube, a Bahtinov mask lens cap for focusing on a bright star, the mounting ring and handle bar, the Vixen-style plate, the tilt adjuster and rotator, M63 to M54 and M63 to M48 adapters, riser blocks, an Allen key set and a soft carry case.

    Visual Use

    William Optics presents the Cat 91 as an astrograph and does not describe a visual configuration for it. Ask us before buying a diagonal or eyepieces.

    Care and Solar Safety

    Never aim the telescope or finder at the Sun without proper solar filters installed on the front of the telescope; doing so for even a moment may permanently damage your vision. Let the telescope dry fully before storing it in its soft case.

  • Does the William Optics Cat 91 require a separate field flattener?

    No. Its 5-element advanced Petzval design needs no flattener and covers a 55mm image circle.

    What is the back spacing for my camera with the Cat 91?

    Maximum back spacing is 60.8mm with the M48 adapter and 65.8mm with the M54 adapter.

    How does the Cat 91 frame the Andromeda Galaxy (M31)?

    On full frame, 448mm covers about 4.6 x 3.1 degrees, so M31 (about 3 degrees long) fits across the long side. At f/4.9 each sub-frame needs roughly half the exposure of an f/7 instrument.

    Can I fit an EAF to the Cat 91?

    Yes. The WIFD R&P focuser is compatible with EAF (an electronic automatic focuser).

    Can I use a full-frame camera with the Cat 91?

    Yes. The 55mm image circle covers full frame (43.3mm diagonal). T-mount support covers Canon EF and RF, Nikon F and Z, Sony E, Pentax K, MFT and FUJI X, connecting to the included M48 or M54 adapter.

    What mount does the Cat 91 need?

    The tube weighs 4.72 kg and the total is 6.3 kg. Add your camera, guide scope and accessories, and choose a mount whose rated payload covers the total with margin.

    How would the Cat 91 WIFD frame the North American Nebula (NGC 7000) with a full-frame camera?

    With its 448mm focal length, the Cat 91 WIFD paired with a full-frame sensor (36x24mm) creates a field of view of approximately 4.6° x 3.1°. This is an excellent framing for the entire North American Nebula (NGC 7000) and the neighboring Pelican Nebula (IC 5070) in a single shot.

    What is the difference between the OTA weight (4.72 kg) and total weight (6.3 kg)?

    4.72 kg is the optical tube alone. 6.3 kg is William Optics' total weight figure; it does not itemise what that includes.

    What does "WIFD" mean?

    WIFD stands for William Optics Internal Focus Design. On the Cat 91 it is a rack-and-pinion focuser with 36 mm of travel, compatible with EAF.

    Can I use a focal reducer with the Cat 91?

    There is no matching reducer for the Cat 91. Ask us before adding any corrector.

    Which colours are offered on this page?

    Black and red. Both are the same Cat 91 WIFD optical tube.

  • Lens Type 5-Element Advanced Petzval
    Lens Elements Super ED Glass x2, ED Glass x1, Lanthanum Glass x1, X-Element x1
    Diameter 91 mm
    Focal Length 448 mm
    Focal Ratio f/4.9
    Image Circle 55 mm
    Focuser WIFD R&P Focuser, EAF Compatible
    Focuser Travel 0-36 mm
    Back Spacing (M48) 60.8 mm (maximum)
    Back Spacing (M54) 65.8 mm (maximum)
    Flattener Required No
    Supported Camera Formats Medium Format (needs additional adapter), Full Frame, APS-C, M4/3
    T-Mount Support Canon EF/RF, Nikon F/Z, Sony E, Pentax K, MFT, FUJI X
    Dovetail Vixen-style
    Tube Length 556 mm
    OTA Weight 4.72 kg / 10.41 lbs
    Total Weight 6.3 kg / 13.89 lbs
    Rear / camera-side connection M48 or M54 camera adapter (on M63)
    Mount No
    Back Focus 60.8 mm

    • Cat 91 Main Tube

      × 1

    • Bahtinov Mask Lens Cap

      × 1

    • Mounting Ring & Handle Bar

      × 1

    • Vixen-style Dovetail Plate

      × 1

    • M63 to M54 Adapter

      × 1

    • M63 to M48 Adapter

      × 1

    • Tilt Adjuster & Rotator

      × 1

    • Riser Blocks (set)

      × 1

    • Allen Key Set

      × 1

    • Soft Carry Case

      × 1

  • William Optics 2-Year Warranty