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William Optics RedCat 51 f/4.9 Petzval Refracting Telescope

  • 51mm f/4.9 Petzval design makes the RedCat 51 the ultimate portable astrophotography telescope
  • Includes a patented Bahtinov Cap to help achieve perfect focus quickly and easily
  • 43mm corrected image circle for use with full-frame cameras

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  • Regular price $698.00

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William Optics RedCat 51 f/4.9 Petzval Refracting Telescope

William Optics RedCat 51 Front Body

From the beginning, the goal was to create the best possible design for an 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 results is a 51mm 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 RedCat 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 within a degree of accuracy.

Modular Tube Ring

The single tube 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.

William Opticas RedCat 51 tube Ring

William Optics Redcat 51 Dovetail Bar

All New Dovetail

William Optics has reinvented to 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, allowing you to use the telescope for both day and night photography.

Reversible dew shield

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

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 these 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 2-3 seconds.


Aperture51 mm (2")
Camera/Eyepiece ConnectionMale 2" Filter/Wide T—Ring (M48x0.75)
Corrected Image Circle43mm
Dawes Limit2.27 arcseconds
Focal Length250 mm
Focal Ratiof/4.9
Free ShippingYes
Glass TypeFPL53
Hightest Magnification102x
Light Gathering Power53x
Limiting Magnitude12.3
Optical DesignPetzval
Tube Diameter80mm
Tube Length225mm
Tube Weight3.2 lbs


RedCat 51 included items

Customer Reviews

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Stu Parker

First impressions

I ordered the redcat in March.I finally received it in June after 2 delays by William Optics.Was it worth the wait?????? Holy cow YES. I did my unboxing and was so impressed with the quality of this telescope.Everything was packed well and came in a stylish black case.Thanks to Utube clips the un boxing was easy and I was able to start using it straight away on our local birds that were near the house. I was impressed with how easy it was to focus and the images were very crisp using the 1.25" inch RedCat Erecting Prism and 10mm eyepiece that were optional extras that I also purchased. I had to wait for a few days for a clear night.I tried 3 cameras with the red cat to compare the differences. I used a Canon 6D,Sbig 8300C and a ZWO 174mm. For each camera I imaged the same target and had no bother at all with focus.I just had to add 1 spacer for the Sbig camera to achieve focus which wasn't a problem.The great feature which made things so much easier was the Bahtinov Cap that is attached as part of the lens cap.This was a breeze to focus and I was able to start with in a couple of minutes.The lock down nut made for a firm fit and the focus didn't seem to change for most of the night.One feature that I thought I wasn't going to use was the field rotator.But I must admit I used it all the time to get the correct position in the following night for the same target it was very smooth and worked like a breeze this is a really nice feature.The tube ring is very well made and I added a WO ***** Style Mounting Base to the ring and added a guide scope.This fitted like a glove and worked very well. The first images looked fantastic they were crisp images and tight stars right to the edges and all 4 corners.It is fast and the images are bight and well defined.The internal filter housing is great and I added a narrowband filter for even better images. Overall the scope is easy and fun to use.If you want a portable fast telescope that will give fine images this is the one for you.It also works very well with daytime observing. This has been my first William Optics scope and other than the delays in getting it I cannot complain about anything else I am sure I will get many years of easy use. I would highly recommend the REDCAT