William Optics SpaceCat 51 f/4.9 Petzval Refracting Telescope

  • This item has an extensive waitlist with the manufacturer and shipments are extremely limited! The expected availability for future orders is the end of 2020 or early 2021.
  • 51 mm Aperture.
  • f/4.9 Focal Ratio.
  • 43 mm Image Circle for use with Full-Frame Cameras.
  • Light Weight of 3.2 lbs.
  • The Ultimate Portable Astrophotography Telescope.
  • Includes a Patented Bahtinov Cap to Help Achieve Perfect Focus Quickly and Easily.
  • Includes the Iconic Soft Cat Carry Case.


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  • Regular price $748.00

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William Optics SpaceCat 51 f/4.9 Petzval Refracting Telescope

William Optics RedCat 51 Front Body

The Limited Edition SpaceCat 51 f/4.9 APO 250 mm Telescope!

Designed to be the best, affordable, small, lightweight and fast Apochromatic refactor for astrophotography. The unique optical design includes the highest quality glass for the best possible color correction and flat large image circle. The result is a 51 mm objective lens, in a sleek Petzval design, for a full-frame image.

Optical Design

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. The Limited Edition SpaceCat 51 will out resolve most high-resolution DSLR lenses of similar focal lengths.

William Optics RedCat 51 Front Objective

William Optics RedCat 51 Front Objective

William Opticas RedCat 51 Field Rotator

Field Rotator

Inspired by luxury watches, the telescope's filed rotator is laser etched with a  degree indicator. To ensure that you will have the most precise framing possible in a small portable system, the camera can be rotated to fit within a degree of accuracy.

Filter Adapter

 It is now easier to connect your filter to the Filter Adapter.  

Filter Index

The filter index marks let you know the estimated focusing site when you twist the focuser.

Camera Sensor Tilt Adjuster

The Tilt Adjuster can adjust the tilt of the image plane. 

William Opticas RedCat 51 tube Ring

William Optics Redcat 51 Dovetail Bar

Modular Mounting Ring

The single-tube Cat ring is sturdy enough to hold the optical tube and a large-sized DSLR without noticeable flexure. It is modular in the sense that you can add a finder scope or guide camera.

All New Mounting Plate

William Optics has reinvented the V-style dovetail. Not only will it fit into a Vixen style saddle plate, but if it's flipped over, it can be attached to any Arca-Swiss style photography tripod head. This will allow you to use the telescope for both day and night photography. The length of the new mounting plate is 210 mm.

Reversible Dew Shield

To maximize portability, the dew shield on this telescope is reversible. When stored backwards, it can shorten the telescope length by approximately 83 mm.

William Optics RedCat 51 Invertable Dew Shield

RedCat 51 Bahtinov Cap

Bahtinov Cap

The cap with this telescope not only protects the glass from the elements, but it also has one of the most useful tools when it comes to focusing.

The Bahtinov Cap creates three lines that pass through a bright star. There is a main large line and a smaller cross-section of two lines making an “X”. The goal is to get the large line centered between the X and centered on the star. Once that is done, you have perfect focus in as little as 2 to 3 seconds.


Aperture51 mm (2")
Camera/Eyepiece ConnectionMale M48x0.75
Dawes Limit0.46 arcseconds
Focal Length250 mm
Focal Ratiof/4.9
Free ShippingYes
Glass TypeFPL53 and FPL51
Highest Magnification102x
Light Gathering Power53x
Limiting Magnitude12.3
Optical DesignPetzval
Tube ColorSpace Gray
Tube Diameter51mm
Tube Length225mm/ extended is 305mm
Tube Weight3.2 lbs


  • SpaceCat 51 f/4.9 Petzval Refracting Telescope.
  • M48 Camera Mount.
  • Patented Bahtinov Mask.
  • Soft CAT Carry Case.


Customer Reviews
5.0 Based on 5 Reviews
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Scott B.
United States United States

I have been looking at the stars for a long time. 10" 8" 6" every kind of scope, binoculars etc. Straight out of the box, I got a picture of the orion nebula. 1 - 10 second shot absolutely Amazing . I was so excited and when I looked at it and shared it with the Astronomy club I belong to, they were amazed. Right out of the box. focusing took maybe 5 seconds. Yes, it took some time to receive it. but, it's here and i love it.

Rod C.
United States United States
WO Space Cat

This is a very well built telescope. Very easy to use. I use it with a Canon 60Da. The images are very sharp. Excellent for photographing larger deep sky objects.

Kurt A.
United States United States
This is one sharp kitty!

Seriously I could go on and on about this but I'll tell you what! This is a killer scope. Don't let its size fool you. It's the ultimate lightweight portable scope on the market. It's a quadruplet which means unlike most Apo triplets and Ed doublets, you will have no need to spend the extra money on a field flattener. The stars a pinpoint and if they ever did get out of wack you'll notice a nice little tilt adjuster they added to help if that time ever comes. If you want to see some images with the spacecat just check out my Instagram @gorilla_astro I always share my equipment used in my posts. Tell them Kurt sent ya

United States United States
William Optics SpaceCat 51 with a attitude!

First of all thanks to Chris for excellent customer service. Chris called me back in less than 10mins made sure the SpaceCat is in stock. Ordered it ASAP and the Cat came to my doorstep pretty fast! Excellent APO, super sharp like a Cats claw with lots of nice features. I like every single detail they put it. It feels really good and sturdy. You will love it and never get disappointed. Excellent quality from William Optics.

United States United States
Very nice scope

Very simple, well built, excellent features, beautiful to look through