William Optics RedCat 51 Review

written by Zane Landers
TTB score


The Good

  • Good Optics
  • An Extremely Easy-To-Use Astrophotography Scope For Beginners
  • Lightweight And Portable
  • Huge Field Of View Is Great For Smaller Sensors

The Bad

  • Very Short Focal Length Limits Target Choices And Image Scale
  • Not Really Usable With Monochrome Cameras
  • Rather Expensive
Check price on↓

The William Optics RedCat 51 is a great scope for beginners starting out in astrophotography or more experienced imagers looking for a lightweight, portable, and/or wide-field option without the drawbacks of a regular telephoto lens.


The William Optics RedCat 51 is, to some, barely a telescope at all. In many ways, this telescope has more in common with a telephoto lens than a traditional refracting telescope. You can use it on even a small star tracker mount, the focal length is very short, it focuses like a telephoto lens, and it really isn’t designed for looking through with an eyepiece (though you can attach a diagonal that William Optics sells separately to do so if you wish). 

The RedCat is different from a telephoto lens that one might also use for astrophotography in its innately astronomical features. You won’t find an iris for the aperture – nor the annoying diffraction spikes that come with one. The RedCat easily attaches to a variety of astronomical and traditional DSLR or mirrorless cameras, has a massive 43mm image circle, and the field of view is flat by default.



51mm (2”) Aperture

250mm Focal Length


Petzval Doublet Optics

The RedCat 51 uses a doublet objective lens with an ED glass element and a 2-element flattener/reducer lens built into the scope with another ED element. This is known as a Petzval design. By starting off with a slower objective lens and then building in a flattener inside the optical tube, a Petzval has less chromatic aberration than a natively fast focal ratio refractor would, along with a flatter field and lower costs. You also avoid the pitfalls of spacing, quality, dust particles, and extra costs associated with a separate field flattener. 

The RedCat 51 illuminates a 43mm image circle, enough for most ZWO astronomical cameras or a full-frame DSLR sensor. The field of view is flat all the way to the edge. Focusing is easy with the included Bahtinov mask, though a motorized auto focuser always helps.



The RedCat 51 focuses by twisting the center of the tube pretty much exactly like a standard telephoto lens. If you’re getting into astrophotography but have experience with regular daylight photography, it should be second nature to you. Motorization options are available too. 

The scope has a sliding dew shield, an included Bahtinov focusing mask that fits in the dew shield, and a slot to put a 48mm (2”) threaded filter such as a broadband light pollution or IR cut filter in between your sensor and the rear lens elements. There’s also a rotator to optimize the tilt of your camera sensor, and a dovetail bar compatible with ARCA Swiss clamps, Vixen dovetail saddles, or standard ¼ 20 tripod threads. 

William Optics provides a carry case with the RedCat 51 which is well-made and has extra room for accessories, a guide scope etc.

You can also buy a combination handle/guide scope saddle to attach William Optics’ tiny 32mm guide scope to the top of the RedCat 51 if you wish, though guiding the RedCat is arguably unnecessary if you are using a remotely acceptable quality mount with decent polar alignment.

The RedCat’s rear cell has an M48 threaded rear mount, which either mates directly to your astronomical CMOS/CCD camera or can have a T-ring installed for DSLR or mirrorless camera use. 

Mounting Options

The RedCat 51 will attach to basically anything with a Vixen-style saddle and weighs very little, but many astronomical mounts, with their huge counterweights, massive steel tripods, and autoguiding capability are arguably unnecessary. Something like an iOptron SkyGuider Pro, Sky-Watcher Star Adventurer, or perhaps Sky-Watcher’s Star Adventurer GTi mount is really all you need. If you want to upsize to a bigger telescope in the future, however, consider the HEQ5 Pro from Sky-Watcher or a similar EQ5-class mount with autoguiding capability and high quality.

What can you see if you look through it?

You can look through the RedCat 51 using a separately sold 1.25” erecting prism diagonal (and of course eyepieces, but a 51mm aperture instrument is essentially a finder scope – or one half of a 10×50 binocular – and the 1.25” format means you get a field of view narrower than many inexpensive 7×50 or 10×50 options. The RedCat 51 isn’t designed for high magnification views of the Moon and planets, and the diffraction spikes induced by the Amici prism diagonal don’t help. So think of this as an astrophotography-only instrument. You can see the moons of Jupiter, the phases of Venus, and the rings of Saturn, but Mercury, Mars, and the ice giant planets will be fuzzy at best. Large open star clusters look nice; most other deep-sky objects are too dim to see with a 51mm telescope, especially under light-polluted conditions.




Short Focal Length

Slightly More Costly

The short focal length of the RedCat 51 enables it to have a huge field of view, along with minimal mounting/guiding requirements, and makes it very compact. However, these also limit you to imaging the largest deep-sky objects – mainly the Milky Way and huge nebular complexes. If you want to image globular star clusters, galaxies, or smaller nebulae, a longer focal length and larger aperture instrument – probably a Newtonian reflector – is a better choice. A larger refractor is expensive and heavy, while Schmidt-Cassegrains and Ritchey-Chretiens are poor choices for beginners due to their stringent guiding, mounting, and focusing tolerances. You can always start with the RedCat and upgrade later.

The RedCat 51 is also a little more expensive than some cheaper and larger aperture doublet refractors, though by the time you add in a flattener and account for the cost of a beefier mount and auto-guiding there’s little difference in how much you actually spend.


The William Optics RedCat 51 is a great scope for beginners starting out in astrophotography or more experienced imagers looking for a lightweight, portable, and/or wide-field option without the drawbacks of a regular telephoto lens. We highly recommend it.