William Optics RedCat 51V2 AP 51/250 OTA

Description

Packing down to just 225mm for lightweight airline carry-on travel, the William Optics RedCat 51V2 AP 51/250 OTA is a portable full-frame petzval astrograph optical tube assembly built for wide-field imaging on star trackers. Operating at a fast f/4.9 focal ratio with a 250mm focal length, this 51mm (2.01 in) aperture instrument delivers a 2.27 arcseconds theoretical resolving power and reaches magnitude 10.3, capturing extended emission nebula complexes and functioning as a high-resolution telephoto lens for daylight wildlife observation.

The optical system features a modified Petzval-style quadruplet flat-field layout utilizing an air-spaced doublet objective and a doublet rear corrector made from Ohara FPL-53 Super ED glass and synthetic fluorite elements, treated with ultra-high transmission broad-band anti-reflection multi-coatings with an unobstructed 0% central profile. Built inside an aluminum chassis measuring 225mm in retracted length with an 80mm outer dew shield diameter, the optical system supplies 59.7mm of mechanical back focus from the rear M48 thread, with standard 55mm sensor spacing achieved via the included threaded M48 spacer. Supported by a precision calibrated helical focuser with a tension ring, a Vernier index scale, and an integrated 360-degree camera rotator, this 1.45 kg (3.20 lbs) astrograph covers a 44mm full-frame flat field without external corrector lenses.

Key Features of William Optics RedCat 51V2 AP 51/250 OTA

Engineered for ultra-fast wide-field deep-sky astrophotography, this 51mm Petzval astrograph couples fluorite-grade optics with a calibrated helical focuser. Its reversible Vixen and Arca-Swiss dovetail bar, patented Bahtinov focus mask cap, internal M48 filter cell, and soft padded carry case deliver a complete grab-and-go imaging setup.

• 51mm (2.01 in) clear aperture with 250mm focal length and fast f/4.9 optical speed
• Petzval-style quadruplet design with Ohara FPL-53 and synthetic fluorite elements
• Ultra-high transmission broadband multi-coatings with an unobstructed 0% central profile
• Precision calibrated helical focuser with tension ring, Vernier scale, and 360-degree rotator
• Standard 59.7mm back focus from M48 thread (55mm achieved with included M48 spacer)
• True 44mm edge-to-edge full-frame flat field without requiring separate correctors
• Reversible Vixen/Arca-Swiss dovetail bar with CNC mounting ring and reversible saddle handle
• Patented clear Bahtinov focus mask cap, M48 filter cell, and soft padded case included

Modified Petzval Quadruplet & Fluorite Glass Array

The four-element modified Petzval optical architecture pairs Ohara FPL-53 glass with synthetic fluorite elements to cancel chromatic aberration, coma, and field curvature across modern full-frame sensors. Star profiles remain pinpoint out to the extreme edges of the 44mm image circle, while an unobstructed light path supports practical visual magnifications from 7.3x up to 102x.

Calibrated Helical Focuser & Vernier Scale Alignment

Critical focusing at f/4.9 is delivered through a calibrated helical focuser equipped with an adjustable tension ring and an engraved Vernier millimeter scale for repeatable focal indexing. The rear camera connection incorporates a 360-degree manual rotator and an internal M48 filter thread, while the reversible mounting foot clamps directly onto Vixen saddles or Arca-Swiss tripod heads.

• Precision helical focuser with Vernier scale and tension lock to prevent focal shift during exposures
• Built-in 360-degree camera field rotator for framing celestial targets without unthreading equipment
• Patented clear Bahtinov focus mask integrated into the metal lens cap for rapid focus confirmation

William Optics Cat Series Travel Warranty

The William Optics RedCat 51V2 AP 51/250 OTA is backed by a 2-year limited warranty against material and manufacturing defects. The complete factory package contains the RedCat 51 V2 OTA, integrated patented Bahtinov focus mask cap, CNC mounting ring, dual-sided dovetail bar, M48 filter thread cell, and a soft padded transport case.