William Optics UltraCat 56 f/4.8 WIFD with Elite Drawer OAG Kit

5-Element Petzval Optics: A Flat 50mm Image Circle

William Optics designs the UltraCat 56's 5-element Petzval optics to correct field curvature across its 50mm image circle, so no separate field flattener is required. The design uses two Super ED glass elements, one ED element, one lanthanum element and one X-Element. William Optics optimises the UltraCat 56 for full-frame, APS-C and M4/3 sensors.

The 269mm focal length gives a wide field: with a full-frame sensor (36 x 24 mm) the field is roughly 7.7 x 5.1 degrees, enough to frame the North America Nebula (NGC 7000) together with the Pelican Nebula (IC 5070). The f/4.8 focal ratio collects light about 50% faster than an f/6 system, which shortens the exposure needed on faint targets.

WIFD Focuser, STX Rotator and Field-Ready Build

The UltraCat 56 uses the William Optics Internal Focuser Design (WIFD), a rack-and-pinion internal focuser with 0-30 mm of travel, listed by William Optics as compatible with an EAF (Electronic Automatic Focuser).

• Sensor Tilt Xterminator (STX) rotator: rotate the camera 360° to frame a target, and dial out sensor tilt if stars are elongated in one corner.
• M54 and M48 camera adapters: included with an adapter wrench.
• Bahtinov mask cap: the lens cap doubles as a focusing mask.
• Dual Vixen/Arca-Swiss plate: fits Vixen-style saddles and Arca-Swiss clamps.

The optical tube weighs 2.56 kg (5.64 lbs); the total weight is 3.21 kg (7.08 lbs) with ring and plate.

Elite Drawer OAG: Guiding and Filters in 37.5 mm

The Elite Drawer OAG merges an off-axis guider and a filter drawer into one solid body, so you have fewer threaded joints in the imaging train and save back focus compared with stacking two separate accessories.

• Adjustable prism: the prism can be raised or lowered to pick off guide stars at the edge of the field without shadowing the main sensor.
• Helical guide focuser: the guide port accepts M42-threaded and 1.25" push-in guide cameras and focuses them without touching the main camera focus.
• Magnetic M48 filter drawer: holds one M48 x 0.75 (2-inch) mounted filter that you can swap without taking the imaging train apart.
• Adapters included: M48, M54 and M68 on the telescope side, M48 and M54 on the camera side.

The drawer holds a single filter, so it does not replace a filter wheel for automated filter changes.

Getting Started

• Mount the dual-side plate in a Vixen-style saddle or an Arca-Swiss clamp.
• Thread the Elite Drawer OAG to the UltraCat 56 with the matching M54 or M48 adapter, then attach your main camera to the OAG.
• Fit your guide camera in the guide port, place a mounted M48 filter in the magnetic drawer if you use one, and balance the telescope with everything attached.
• Focus the main camera on a bright star with the Bahtinov mask cap and the WIFD focuser, then remove the cap.
• Adjust the prism height and the helical guide focuser until the guide camera shows sharp stars, then start guiding and imaging.

Never aim the telescope at the Sun without a proper solar filter fitted over the front of the telescope.