William Optics

RedCat 51mm f/4.9 II-U

SKU: L-RC51II-U


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  • Regular price $865.00

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Version II-U RedCat (2023) with upgraded angle tilt adjuster. This has now been replaced with the WIFD variant, so once these sell out, they are gone. We still consider this version to be a superior choice for a lightweight package with a camera tracker mount like the iOptron SkyGuider Pro or SkyWatcher GTI. (The WIFD variant is two pounds heavier, and better suited for use with a focus motor for a more automated setup.)

Accepts 55mm photo filters on the front, including 55mm thread on solar filters.

Probably the very sharpest 250mm camera lens you will find!

  • Newest version (including integral angle adjuster, cat paw scope clamp, DSD dovetail bar, and focus tensioner)
  • 51mm quadruplet ED APO f/4.9 (250mm fl)
  • FPL53 & FPL51 glass 
  • 3.2 lbs tube weight.
  • >43mm imaging circle with a flat field edge to edge for full-frame sensors
  • Dual Vixen and Arca Swiss Dovetail
  • M48 thread to accept wide t-rings/t-mounts


The best value, highest quality 250mm lens you will find for your camera.

51 mm lens, in a sleek Petzval design, using Synthetic Fluoride (FPL53 and FPL51) glass for a full-frame flat field image. The optical quality provides exceptional performance at a fast focal ratio of f/4.9. 

Filter Adapter allows use of 48mm filters. Focusing is made easy with a bahtinov mask integrated into the lens cap. 


Aperture 51 mm (2")
Camera/Eyepiece Connection Male M48x0.75
Dawes Limit 0.46 arcseconds
Focal Length 250 mm
Focal Ratio f/4.9
Glass Type FPL-53 and FPL-51
Highest Useful Magnification 102x
Light Gathering Power 53x
Limiting Magnitude 12.3
Optical Design Petzval
Tube Color Red
Tube Diameter 51mm
Tube Weight 3.2 lbs
Dew Shield Outer Diameter (for soar filter sizing) 80.5mm



  • RedCat 51mm Telescope
  • Bahtinov Mask Cap
  • 210mm Dovetail
  • Upgraded Mount Ring (with cat paw logo)
  • Soft Carry Case


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