William Optics is a company known for creating high-performance apochromatic refractors and constantly updating and refining their designs. The RedCat 51 Petzval APO is the latest creation from the company that can’t sit still, and it is bound to shake up the industry once more.
I am fortunate enough to have been granted early access to this exciting Petzval apochromat that debuts in early March 2019.
What makes the William Optics Redcat 51 so special? The 4-element Petzval design, unique focal length, and helical focuser. The sleek red finish of the RedCat 51 signals its individuality and charm. It is unlike any other astrophotography telescope on the market and one that I have been waiting to get my hands on since reviewing the early concept designs last year.
While shooting the unboxing video for the RedCat, I came to the conclusion that this is hands down the most beautiful looking telescope I have ever seen in terms of style and design. This is a signature quality of the William Optics brand and they continue to push the envelope with new and dramatic concepts.
In early 2020, William Optics announced a “Nightcat” version of their popular imaging refractor exclusive to OPT. The RedCat 51 and SpaceCat have become difficult to find, so the NightCat version that debut’s in June might be a good option.
It’s essentially the same telescope, but includes the new guide scope mounting hardware, a longer dovetail, and a slick new color scheme. If you are having trouble finding a RedCat in stock, you may also want to consider the Radian Raptor 61.
The William Optics RedCat 51
The design goals for the RedCat included creating an affordable refractor that uses the highest quality glass and delivers a flat imaging field with unmatched color correction. In this post, I’ll break down the specifications of the RedCat and explain why I think this little quadruplet will be one of the most sought after products in 2019.
Have you ever seen a more dramatic video for an astronomy product in your life?
The RedCat bridges the gap between an astrophotography telescope and a telephoto lens.
The 250mm focal length and F/4.9 focal ratio mean that the RedCat can be enjoyed as much by wildlife photographers as it is by amateur astrophotographers. The helical focuser makes focusing fast-moving targets such as birds much easier than ever before.
For those of you that don’t know, I am also an avid bird photographer. Once I discovered the clarity and sharpness provided by an apochromatic refractor telescope, I began using my astrophotography telescopes for bird photography. These telescopes were too heavy to use handheld and were challenging to focus on moving subjects.
Roughly 8 years later, William Optics releases a high-end apochromatic refractor that targets the wildlife photography market. The RedCat encapsulates two of my greatest passions (astrophotography and bird photography) in a single product.
The Andromeda Galaxy captured using the RedCat 51.
The RedCat 51 is small, beautifully designed, and versatile. The early astrophotography image examples shared by William Optics are breathtaking. Aside from looking good, and some promising looking results in terms of performance, the RedCat 51 has some handy features to improve the user experience.
Before I cover items such as the filter slot and modular mounting options, let’s dive into the core specifications of the RedCat 51.
RedCat 51 Specifications:
- Optical Design: Petzval Apochromatic Refractor (4 elements in 3 groups)
- Lens Type: Prime
- Diameter: 51mm
- Focal Length: 250mm
- F-Ratio: F/4.9
- Weight: 3.2 lbs
- Focuser: Calibrated Helical
- Mounting Style: Vixen/Arca-Swiss
The RedCat 51 mounted to an iOptron SkyGuider Pro.
Focal Length: 250mm
First off, the RedCat 51 has a focal length of 250mm. What does this mean for astrophotography? It means extremely wide-field deep sky images. If you consider a 480mm refractor to be a wide-field ‘scope, the RedCat is nearly twice as wide!
Massive deep sky objects such as the Carina Nebula will fit into the image frame in their entirety. Large nebulae that traditionally fill the frame in a typical wide-field setup are captured with plenty of surrounding space and additional star clusters and nebulae in the frame.
Until the RedCat came along, 250mm was a focal length reserved for those that employ a prime camera lens for astrophotography. Now amateur astrophotographers have the option of using a flat-field APO that easily mounts to their existing equatorial mount for deep sky imaging at this magnification range.
For wildlife photography, this focal length is also quite useful, especially when you consider the all-important f-ratio of this lens. 250mm is enough reach for many larger birds such as hawks and owls, but will require a steady hand and gimbal head for the best chance of a sharp shot.
For nature and wildlife photography, the RedCat 51 has a minimum focus distance of 3.7 meters. This can be improved to under 3 meters with the use of the 2″ extension nose piece from the diagonal. By adding even more extension tubes, the RedCat can even be used for macro photography of small insects.
The FPL-53 Objective lens of the RedCat 51 Petzval APO.
Focal Ratio: F/4.9
When it comes to photography (of filming) birds, it’s all about light. Fast shutter speeds are required to capture a bird in motion, and this demands a fast lens to adequately expose the shot. F/4.9 isn’t incredibly fast in the world of camera lenses, but when you consider that this is essentially a refractor telescope – it’s about as much light-gathering power as you’ll find on the market.
For comparison, the RedCat is almost a stop faster than the extremely popular Canon EF 400mm F/5.6L lens. William Optics has released some incredible wildlife footage shot using the RedCat, a testament to this quality. Low light situations such as a cloudy day, made wildlife photography tough with my old F/6 William Optics Z72. The speedy RedCat is a completely different animal.
51mm Lens Diameter
As the name eludes to, the RedCat has a 51mm objective lens. In the world of astrophotography telescopes, this is absolutely tiny! If you thought the adorable little Zenithstar 61 was cute, wait until you see the RedCat. But this kitten has claws (I couldn’t resist).
The 51mm objective lens on the RedCat is made from top quality FPL-53 and FPL-51 glass (synthetic fluorite) which creates a flat frame image from corner to corner, even when utilizing the entire image circle with a full-frame camera.
The small size of the RedCat means keeping the overall weight to a minimum, despite the extra glass. The RedCat 51 weighs only 3.2 pounds even through the design requires 4 elements in 3 groups. For owners looking for a high-end telescope to mount on the iOptron SkyGuider Pro, the RedCat is an ideal candidate.
Lens Structure: Petzval
A Petzval lens design involves a low-dispersion doublet in combination with two elements farther down the optical path to both speed up the f-ratio of the telescope, and flatten the image field. There is no need to use a field flattener with the RedCat, adding to its simplicity and practicality in the field.
The Petzval quadruplet lens design is well corrected and incredibly sharp. You can expect pinpoint stars to edges of your image. If you don’t believe me, have a look at this image of the Witch Head Nebula taken using the RedCat 51 by Mehmet Ergün.
These traits will surely make the RedCat a popular choice for high-resolution deep sky astrophotography imaging. Portability and design aside, the RedCat is an affordable option for those looking to own a top-of-the-line astrophotography telescope. A comparable refractor on a much larger scale is the Takahashi FSQ106.
Example images taken using the William Optics RedCat 51 on Flickr.
In mid-February 2019, Wei-Hao Wang shared an incredible image of the Running Chicken Nebula using the RedCat 51 with a modified Nikon D800 on Astrobin. The full-size image really showcases the color correction and flat-field qualities of the optics.
I had a chance to test the RedCat 51 on the night of the full moon. This is not an ideal time to capture deep sky objects in broadband true color, so I used an Optolong 7nm Ha filter inside of the RedCat. The image below was captured using a modified Canon EOS Rebel T3i (600D) through the RedCat 51 APO.
I stacked 33 sub-exposures of 4-minutes @ ISO 1600 each to create a total integrated exposure of 2 hours and 12 minutes. The 4-minute sub-exposures were accomplished thanks to the accurate tracking of the iOptron SkyGuider Pro.
The Rosette Nebula in Ha – RedCat 51 + Canon DSLR
Yes, a helical focuser! This design aspect completely changes the user experience of the RedCat, whether you use it to photograph the night sky or a Black-crowned night heron. I have used a number of apochromatic refractors for daytime photography in the past, but none of them felt natural because of the rack and pinion focuser.
Aside from the impressive lens design, the helical focuser is the single biggest differentiating factor between the RedCat and a typical imaging APO. The focuser drawtube is calibrated and features printed mm spacing marks for precise adjustments. The black textured focuser ring is made out of soft rubber for a comfortable grip.
Adjusting focus during frantic wildlife photography moments is now much more fluid and responsive, while the precision and rigidity needed for deep sky astrophotography are retained. The focuser tension ring allows you to precisely control the level of friction desired, and also can also lock the tube in place when needed.
Some of the deep sky images I have captured using the William Optics RedCat 51 and a DSLR camera.
The field rotator resembles the face of a luxury brand watch, which is exactly the inspiration William Optics used when designing the rotator markings on the RedCat. Every degree of the field rotator is marked to help aid in the process of creating a mosaic. There is a small white arrow on the M48 adapter to use as a reference point when setting your camera orientation.
This level of attention to detail is noteworthy, as this subtle feature indicates input from actual amateur astrophotographer needs.
You’ll find that many of the hidden “extras” on the RedCat 51 follow this mindset as well, including the small white Teflon rings inside of the mounting ring. This small, yet thoughtful detail allows the user to smoothly rotate your imaging train.
The field rotator includes markings for each degree of rotation.
The M48 thread adapter allows you to fasten your DSLR or dedicated astronomy camera to the RedCat for astrophotography or daytime photography. The imaging circle completely covers a full-frame camera sensor for edge to edge illumination with a flat-field.
Speaking of covering your camera sensor, the M48 adapter includes an internal thread for 48mm (2-inch) threaded filters. This is a convenient location to place your favorite light pollution or narrowband astrophotography filter.
Owners of Canon, Nikon, Sony or Pentax cameras will be happy to know that their camera bodies are a perfect fit for the RedCat with the necessary t-mount and adapter hardware. The M48 end adapter must be removed to apply the matching red William Optics erecting diagonal.
Mounting Base and Lens Collar
The base of the RedCat 51 was specially designed to avoid adding extra weight to the telescope, yet provides a reliable platform for the demanding positions of astronomical imaging. The matching red low profile dovetail bar can be used with either a standard Vixen mount saddle or the photography-based Arca-Swiss style mounting bracket.
The dovetail plate includes standard ¼ inch threads and simply needs to be flipped over to accommodate your desired mounting configuration. Deciding on the configuration of this aspect of the telescope will depend on your primary use of the RedCat.
The optical tube itself can also rotate easily thanks to the lens collar and release knob. This is similar to the design you’ll find on high-end telephoto camera lenses, but with 2 extras. There are modular mounting options on the lens collar. Here, you can fasten accessories such as a shotgun microphone or red dot finder scope.
Integrated Bahtinov Focusing Mask
To top things off, William Optics has included their signature diffraction spikes Bahtinov mask in the lens cap. This feature is handy for DSLR astrophotography imaging sessions when you need to quickly and accurately confirm the focus of your target. You simply need to aim the telescope towards a bright star to create the diffraction spike pattern.
The elongated star patterns displayed when pointed at a bright star create an obvious and distinct guideline to reference when using adjusting the helical focuser. The clear acrylic design of these masks makes them much easier to use than the traditional opaque black Bahtinov masks.
You may have also noticed the color-matched steel William Optics Vixen-style base attached to the iOptron SkyGuider Pro. This base mount also fits the Sky-Watcher Star Adventurer Pro and adds some serious stability to and improved aesthetics over the original base. The package also includes a 4.5″ red extension bar for the counterweight, that I regretfully forgot to install before filming the video.
You may be asking yourself if there are alternatives to the RedCat to consider, with a comparable optical design and size. Although the Vixen FL55 does not share a Petzval design, it is an ultra-wide field fluorite refactor with a 55mm lens diameter. The focal length of the Vixen FL55 is 239mm, with a focal ratio of F/4.3 (with the optional reducer used). The Vixen model does not include a carry case orBahtinov mask in the price, and requires an additional reducer for maximum usefulness.
The Takahashi FS-60 CB is also a direct competitor of the RedCat, although its design mirrors the WO Z61 more so than the RedCat 51. This is another compact fluorite refractor, and features a focal length of 335mm, at F/5.9. The Takahashi name carries with it a premium price. The optical performance of this 60mm refractor is indeed impressive, yet it does not have the unique features of design of the RedCat in a complete package.
RedCat 51 Alternatives, the Vixen FL55 and Takahashi FS-60CB.
The Radian Raptor 61 has comparable specs to the William Optics RedCat, at a 275mm focal length at F/4.5. The Raptor 61 uses a 10:1 rack and pinion focuser in place of a helical focuser and includes high-quality hex rings to mount the telescope. You can watch my video explaining my involvement with this project here: I Designed My Ultimate Telescope.
The Radian Raptor 61 with the optional electronic focuser.
When I mentioned that the RedCat 51 can be used as a daytime wildlife photography lens, you may have wondered whether the RedCat will work with the autofocus function of your DSLR. I have some good news. Yes, the RedCat 51 can be used with autofocus!
But there is a catch (two actually). You will need to use the TechArt Pro adapter that includes a built in motor for manual lenses. This system only works with certain Sony DSLR cameras, which means for Canon or Nikon shooters, you’ll have to make due with manual focusing of the RedCat 51.
The system relies on the phase-detect autofocus system found on modern Sony cameras such as the Sony a6300, a6500, A7ii, and A7Rii.
The RedCat mounted to a Sony a7R and the TechArt Pro adapter.
The package William Optics sent to me included a number of extras, including the Canon EOS T-Mount adapter, and the dedicated erect image diagonal. The diagonal will hold a 1.25″ eyepiece for visual observing, or spotting scope purposes. The Canon EOS T-ring included with the RedCat is the most stylish looking camera adapter I have experienced to date.
The diagonal is of little importance to those that will use the RedCat 51 exclusively for deep sky astrophotography, but it is a nice option for those that like the choice of using this telescope for visual use. This is of particular interest to anyone who purchases the RedCat with plans on using the telescope as a spotting scope for wildlife photography.
The RedCat can be mounted to a traditional photography tripod for crisp views at 250mm.
The dedicated RedCat erect image diagonal fastened to the telescope.
Removing the M48 Adapter
To install the dedicated erect image diagonal on the RedCat 51, you need to remove the 48mm adapter. Use a small Allan key (0.7mm) to unscrew the three grub screws just enough so that you can turn the M48 adapter counter-clockwise with the field rotator locked. You should eventually feel the thread release so you can unscrew the adapter.
Remove the M48 Adapter on the RedCat to fasten the diagonal or install a 2-inch filter.
The diagonal can then be threaded directly to the base of the field rotator for visual observing or spotting scope purposes.
The 48mm filter threads are also located between the M48 adapter and the field rotator. This is a discreet location in the imaging train to thread a light pollution filter inside of the RedCat 51.
The Bottom Line
The price tag of the William Optics RedCat 51 may seem a bit steep at first, but considering the pedigree of this refractor, it’s right on the mark. Creating an affordable option for those looking for a premium imaging APO in a small package was the overall goal of the RedCat design. I believe the RedCat is poised to have a big year, and look forward to the official unveiling at NEAF.
If you have interests in both wildlife photography and deep-sky astrophotography as I do, you might feel like the RedCat was designed specifically for you. William Optics is a brand that continues to innovate and create original products. In a world full of copycats, this little APO stands in a category of its own.
The RedCat 51 can be very difficult to find. In 2020, William Optics introduced the “Nightcat”, a special edition version of the telescope that is the successor to the newer “Spacecat” version (see below).
See the RedCat 51 in action on YouTube: Deep Sky Astrophotography with the RedCat 51
- William Optics RedCat 51 Official Product Page
- William Optics Zenithstar 73 APO Review
- Astrophotography in Costa Rica (Carina Nebula)