Ultra-Cat 108 · William Optics Support

Specification

Lens Type	5 Elements Advanced Petzval Design
Focal Length	518 mm
Diameter	108 mm
Focal Ratio	f/4.8
Image Circle	60 mm
Focuser	WIFD R&P Focuser
Focuser Travel Length	0-30 mm (Ruler to 31mm)
Adapter Included	Sensor Tilt Xterminator Rotator M79 → M68 Adapter M79 → M54 Adapter M79 → M48 Adapter
Optional Flattener	No Flattener Required
Camera Format	Medium Format Full Frame APS-C M4/3
OTA Weight	7.82 kg / 17.24 lbs
Tube Length	512.9 mm - 617.9 mm

Diagram Reports

Spot Diagram

Longitudinal Aberration

MTF Chart

Relative Illumination

Learn how to read Diagram Reports

Size Diagram

Parts

Front Bahtinov Mask Cover

Lens Cover with Bahtinov Mask

Dew Shield, retractable – 105 mm

Dew Shield Locking Screw

WIFD Focuser [ Tension Adjustment Guide ]

Focus ruler scale window
Two Speed 1:10 Ratio Fine Knob
Fine Knob Protecting Cover
M4 Screw Holes for Finder Bracket
Focuser Knob with Temperature Gauge ( Can be removed to attach EAF ) [ Installation Guide ]

Sensor Tilt Xterminator (STX) with integrated Camera Rotator (CAA) [ Adjustment Guide ]

STX Auto Tension Screw
CAA Locking Knob ( Can be replaced with the H4 set screw when using an off-axis guide system )
CAA 360° engraved scale

M79 → M48 Adapter ( M79 → M54, M68 Adapter interchangeable )

M48 ( M54, M68 ) Rear Dust Cap

Handle Bar with Mini Vixen-style Mounting Base (HCRC-L)

M6 Screw Holes
M4 Screw Holes For Locking Screws
Mounting Base Locking Screws (M4)

Mounting Ring

M6 Screw Holes for Finder Bracket
Mounting Ring Knob

Vixen-style Dovetail Plate (PV-LII)

Hardware Parts

Code Name	Image	Quantity	Usage
M4K12/10		2	Secure the guider to the handlebar
M6K15/08		1	Secure the dew shield
M6H5/16		6	Spare screws for mounting
M8H4/12		1	Replacement for CAA locking knob (in case of interference with accessories)

Operation Guide

Sensor Spacing

To ensure optimal light control and image quality, set the distance from the telescope’s rear connection to the camera sensor to 52–58 mm. This spacing allows proper use of the front dew shield, prevents stray light from entering through the tilt adjuster, and maintains effective internal baffling.

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

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

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 (STX)

Locate the pair of diagonally opposing screws corresponding to the affected corners.

Use 2mm hex key to release (turn clockwise) the knob near the corner where the sensor is too far.

Push out (turn counterclockwise) the opposite knob accordingly.

Do not touch the Auto Tension Screws. It may disrupt balance or cause loosening.

Make small adjustments — turn each knobs only about 1/8 turn at a time.

Any 2mm rod or hex key can be used to twist the knob mechanism.

After each adjustment, perform a test exposure to check the star shape and alignment. Fine-tune as needed until the stars are round and sharp across the entire frame.

Tips & Notice

Typical adjustment sessions take 2–4 test exposures to reach optimal flatness. However, fine-tuning may vary depending on your camera, filter, and backfocus setup.

Tilt does not need to be adjusted frequently. Once set, it should remain stable unless major components (e.g., camera, tilt plate, or spacing) are changed.

For best results, perform tilt adjustments in a stable environment (minimal temperature change, fully cooled optics and sensor).

Avoid overtightening, which may deform the sensor platform or affect collimation.

Installing EAF

Turn the telescope over and remove the Tension screw.

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)

Release (don’t remove) two inner knob screws.

Remove the temperature gauge knob.

Find the flat side on the shaft and turn it to face the hole.

Put the 5-6mm Flexible coupling on the shaft and lock the two inner screws.

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)

Align the screw holes.

Lock the four M4 bracket screws.

Installing Off-axis Guide System

When installing an off-axis guiding system, the CAA Locking Knob may interfere when rotating the camera. In this case, you can replace it with the included H4 set screw to avoid the conflict.

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.

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.

Accessories Map

WILLIAM OPTICSCamera Adapter

Camera Adapter

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WILLIAM OPTICSUniGuide 50mm f/4.0

UniGuide 50mm f/4.0

Elevate your astrophotography with the William Optics UniGuide 50mm f/4.0, designed for precision tracking and stellar clarity. This guide scope features a 50mm aperture and a fast f/4 focal ratio, offering a wide 5.7º field of view for easily locating guide stars.Unique to this model is the 1.25" RotoLock adapter, which securely attaches guide cameras and other accessories with a simple twist, ensuring stable, rotation-free connections. The fully multi-coated lens ensures bright, clear images, and at just 460g, it adds minimal weight to your setup.Perfectly compatible with the latest William Optics telescopes, this guide scope can be directly slid into their carry handles for seamless integration. It also includes an M48 male thread under the RotoLock for greater accessory compatibility.With its robust build and excellent optical quality, the UniGuide 50mm f/4.0 is an essential tool for both amateur and professional

William OpticsZWO EAF Mounting Bracket

ZWO EAF Mounting Bracket

Manufacturer: K-ASTEC Compatibility Telescope Focuser: All of WO focuser with modular focusing mechanism WIFD focuser (Recommended) Focusers other than the WIFD may have slight dimensional differences, which may require a bit of force during installation. Focuser Motor: ZWO EAF - Key Features: Aligned with OTA BodyPositions the EAF motor in line with the telescope tube for improved balance and convenient storage without disassembly. Precise Belt Drive MechanismAdopts a belt-driven layout for flexible motor positioning and smooth power transmission, replacing the stock direct-coupling method. Lightweight & Rigid Aluminum BuildCNC-machined from anodized aluminum alloy, combining strength, stability, and reduced weight.

WILLIAM OPTICSRed dot finder Kit

Red dot finder Kit

Centering your objects in the sky has never been easier! Look through the Red Dot Finder (R.D.F.) with one eye leaving the other open, and you will see one of the four selected LED shapes in the sky. Quality Make Optimized anti-reflection coating. Alt-azimuth adjustment controls. Very compact size, but no plasticky feel. Quick Release Seven different brightness adjustments make it suitable for any lighting condition, including daylight. Quick-release Vixen/Synta-style bracket/base included in the price. Multiple Reticles Four different reticles to select from: dot, crosshair, circled dot, circled dot with crosshairs. Ideal for Many Scopes Included cross-slot base will mount on many scopes (min. 3.5" base curve diameter; not suitable for SCTs unless you mount using only one screw). 3V lithium battery included. Features Reticules: 4 different, selectable Brightness: 7-level brightness adjustments Battery: CR3023 Fully adjustable in alt-az Quick release bracket included

WILLIAM OPTICSLosmandy-style Dovetail Plate

Losmandy-style Dovetail Plate

Weight: 1.43kgLength: 40cm long Space Grey Special offer