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Cat 91 WIFD

Aug 21, 2025 by WO Team

Specification

Lens Type
5 Elements Advanced Petzval Design
Focal Length
448 mm
Diameter
91 mm
Focal Ratio
f/4.9
Image Circle
55mm
Focuser
WIFD R&P Focuser
Focuser Travel Length
0-36 mm (Ruler to 40mm)
Adapter Included
Tilt Adjuster
Rotator
M63 → M54 Adapter
M63 → M48 Adapter
Optional Flattener
No Flattener Required
Camera Format
Medium Format (needs additional adapter)
Full Frame
APS-C
M4/3
Total Weight
6.3 kg / 13.89 lbs
OTA Weight
4.72 kg / 10.41 lbs
Tube Length
556 mm

Diagram Reports

Spot Diagram

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Longitudinal Aberration

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Learn how to read Diagram Reports

Size Diagram

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Parts

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  1. Front Bahtinov Mask Cover
  1. Lens Cover with Bahtinov Mask
  1. Dew Shield
  1. WIFD Focuser [ Tension Adjustment Guide ]
    1. Focus ruler scale Window
    2. Two Speed 1:10 Ratio Fine Knob
    3. Fine Knob Protecting Cover
    4. M4 Screw Holes for Finder Bracket
    5. Focuser Knob with Temperature Gauge ( Can be removed to attach EAF ) [ EAF Installation Guide ]
  1. Sensor Tilt Adjuster [ Adjustment Guide ]
    1. Tilt Adjuster Protecting Cover
  1. Camera Angle Rotator (CAA)
    1. CAA Locking Knob
  1. M63 → M48 Adapter with M48 Filter Thread [ Filter Installation Guide ] ( M63 → M54 Adapter interchangeable )
  1. M48 ( M54 ) Rear Dust Cap
  1. Handle Bar with Mini Vixen-style Mounting Base (HCRC-L)
    1. M6 Screw Holes
    2. M4 Screw Holes For Locking Screws
    3. Mounting Base Locking Screws (M4)
  1. Mounting Ring
    1. M6 Screw Holes for Finder Bracket
    2. Mounting Ring Knob
    3. 20mm Riser Blocks (BK100)
  1. Vixen-style Dovetail Plate (PV-LII)

Hardware Parts

Code Name
Image
Quantity
Usage
M4K12/10
cdn.shopify.com
2
Secure the guider to the handlebar
M6H5/16
cdn.shopify.com
2
Spare screws for mounting

Operation Guide

Focuser Tension Adjustment

William Optics focusers are typically smooth and ready to use without any adjustment. For larger telescopes, tightening the Quick Tension Adjustment Knob can help prevent tube slippage. Other than this, we recommend not touching any screws on the focuser. If adjustment is necessary, please consult a qualified technician.
 
There’s two side to build WIFD focuser tension:

BOTTOM / Rack & Pinion Gear

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  • Quick tension adjustment knob
  • Smoothness adjustment - Pull in (3mm Hex)
  • Smoothness adjustment - Push out (2mm Hex)
  • Please Don’t Touch
  • Top cover screw Can be removed to attach the electronic devices such as EAF (refer to Installing EAF) or be used as a spare.
      • Telescope models from June 2025 no longer include the top cover screw.
       

TOP / Focuser Draw Tube

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  • Focuser Draw Tube Tension Screws Please Don’t Touch
  • Guider/Finder bracket mounting screws (2.5mm Hex)
Notice
  • When operating the telescope with a motorized focuser, do not overtighten the focuser tension. Excessive tension may damage the focuser mechanism.
  • Make small and slow adjustments. Do not turn anything too much at once.

Sensor Tilt Adjustment

  1. Locate the pair of diagonally opposing screws corresponding to the affected corners.
Where is the tilt adjuster?
1. Remove the rotator knob.
2. Remove the ring cover.
3. The tilt adapter is inside.
  1. Use the 2mm hex key to loosen the pull-in screw of the screw group closest to side where stars appear too close, and loosen the push-out screws of the group on the opposite side (too far).
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  1. Adjust the push-out screws in the "too close" group and the pull-in screw in the "too far" group until the tilt is corrected.
  1. Gently snug all screws to hold the final position.
Note
  • Make small adjustments at a time—1/8 turn is recommended for fine control.
  • Adjust other screws if necessary to maintain overall balance.
  • If the tilt becomes worse or is severely misaligned, perform a reset:
      • 1. Loosen all push-out screws
      2. Tighten all pull-in screws fully
      3. Back out each pull-in screw by the same number of turns
      4. Begin tilt adjustment again from this neutral position

Installing EAF

  1. Turn the telescope over and remove the Tension screw.
 
  1. Remove the top cover screw near the temperature gauge with 2mm Allen key.
Starting from June 2025, new telescope models no longer include the top cover screw. (Skip Step 2)
 
  1. Release (don’t remove) two inner knob screws.
  1. Remove the temperature gauge knob.
  1. Find the flat side on the shaft and turn it to face the hole.
  1. Put the 5-6mm Flexible coupling on the shaft and lock the two inner screws.
  1. Put the EAF body on and lock the two inner screws.
On Step 6 & 7:
Please make sure one of the screws is lock on the flat side (D-cut profile)
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  1. Align the screw holes.
  1. Lock the four M4 bracket screws.

Installing Filter

Only applies to the M48 camera adapter ( There are no filter threads on the M54 adapter, filter wheel is used instead )
  1. Lock the Rotator
 
  1. Take off the M48 adapter
  1. Lock the filter ( male thread ) inside the M48 adapter ( female thread )
  • It is recommended to apply some lubricating oil on the threads first to avoid being permanently tightened.
 
  1. Put the adapter back

Care & Safety Instructions

Solar Safety Warning

Never point your telescope or finder scope at the Sun without a proper solar filter securely mounted to the front of the telescope.
Even a brief, unfiltered glimpse of the Sun can permanently damage your eyes or your camera’s image sensor.
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Use only front-mounted solar filters made by trusted manufacturers. These filters must cover the front of the telescope’s dew shield securely.
Do not use solar eyepiece filters. They are unsafe and can fail under sunlight.
With the correct solar filter in place, you can safely observe and photograph the Sun without damaging your eyes, your camera, or your telescope.

Telescope Care & Storage

To preserve the optical quality and lifespan of your William Optics telescope, please follow these essential care and storage practices:
  • Allow time to acclimate After use, let the telescope adjust to room temperature before storage. During this period, keep the optical tube open and uncovered to allow moisture to evaporate naturally.
  • Ensure dryness Make sure the entire telescope is completely dry, especially the optics and interior tube, and free of dew or moisture before putting it away.
  • Use proper storage conditions Store your telescope in a dry, dust-free environment with a stable temperature. Avoid high heat or humidity. Dehumidifiers or silica gel are recommended for long-term protection.
  • Prevent dust accumulation Always place dust caps on both ends when not in use, or cover the telescope with a clean plastic sheet if it’s not stored in a case.
Following these steps will help maintain optimal performance and prevent damage to sensitive components such as lenses, mirrors, and coatings.
For a detailed cleaning and maintenance guide, please refer to the full article: Cleaning and Maintenance of Telescopes

Clearance Around WIFD Focuser

To avoid interference between the focuser knobs and the mount, make sure the telescope is properly aligned on the mounting rings.
Loosen the rings slightly and adjust the telescope position so that both focuser knobs are level and parallel to the mount. This ensures sufficient clearance from the saddle plate and prevents accidental contact or obstruction during use.
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