William Optics RedCat 91 Review
I recently tested the William Optics RedCat 91 APO refractor. This is the fourth RedCat I’ve used in the RedCat series—five if you count the two versions of the RedCat 51 (version 1 and version 3).
With a healthy aperture, fast optics, and a super flat Peztval design, the William Optics RedCat 91 has a lot going for it. It includes several thoughtful features, such as a tilt adjuster, a camera angle rotator, and a robust WIFD focuser.
This telescope has two color options: the classic red and a new black version. William Optics sent me the black one and explained that it’s one of the best telescopes he’s ever produced.
Key Takeaways
- The RedCat 91 offers the legendary RedCat optical quality in a larger aperture, with thoughtful touches and a premium finish.
- The WIFD focuser is a dream to use manually, but it can also easily be upgraded with an autofocuser such as the ZWO EAF.
- The focal length, f-ratio (448mm at F/4.9), is perfect for a wide variety of deep-sky targets such as nebulae regions and larger galaxies.
William Optics RedCat 91
The Redcat 91 is the same proven high quality you have come to know and love in the Redcat series, in a larger size with a WIFD focuser. This refractor uses a 5-element Petzval design, eliminating the need for precise back focus adjustments.
In my test, the RedCat 91 produced sharp, edge-to-edge images with pin-point stars. Its large image circle (55mm) means it is truly full-frame compatible and potentially even medium-format compatible (I have not tested this).
You do not need to purchase an additional field flattener to utilize the optical potential of the RedCat 91. Its quintuple optical design produces a flat field out-of-the-box.
The RedCat 91 has a fast focal ratio of f/4.9, helping to increase light-gathering power on dim galaxies and nebulae. At a focal length of 448mm, it’s a great option for a variety of deep-sky objects.
I enjoy this focal length and the Redcat quality so much that I plan on installing my ZWO EAF (electronic auto focuser) and rotator on this telescope, which is a big commitment for me.
The William Optics RedCat 91 is available at High Point Scientific.
RedCat 91 Specifications
The Cat 91 is a 5-element Petzval refractor with a focal length of 448mm and a focal ratio of F/4.9. It uses a William Optics Internal Focus Design (WIFD) and has an image circle of 55mm.
This telescope is a great fit for your DSLR or mirrorless camera, especially if it uses a full-frame sensor.
- Focal Length: 448mm
- Focal Ratio: f/4.9
- Diameter: 91mm
- Image circle: 55mm
- Tube Length: 556mm
- OTA Weight: 4.72 kg / 10.41 lbs
- Total Weight: 6.3 kg / 13.89 lbs
- Lens Type: 5-Element Petzval Design
- Lens Elements: Super ED Glass x2 ED Glass x1 Lanthanum Glass x1 X-Element
- Focuser: WIFD R&P Focuser Compatible with EAF
- Focus travel: 0-36mm (Ruler to 40mm)
- Adapter Included: Tilt Adjuster, Rotator, M63 to M54 Adapter, M63 to M48 Adapter
- T-Mount Supported: Canon EF, RF Nikon F, Z Sony E Pentax K MFT (M4/3) FUJI
- Camera Format: Medium Format (needs additional adapter) Full Frame, APS-C, M4/3
The spot diagram shared on the William Optics product page.
What’s Included in the Box?
As usual, William put together an admirable package that includes everything you need to enjoy and store the RedCat 91. Standout items are the beefy mounting plate with a Vixen-style mount, as well as the handy mounting rings with a guide scope mount.
I also appreciate that an M48 and M54 adapter are included for those looking to take advantage of the massive 55mm image circle. While you hopefully won’t need to touch it, a tilt adapter is also included in the package.
- Optical tube assembly (OTA)
- Bahtinov mask cap
- M48 and M54 adaptor
- 290mm Vixen-style dovetail bar
- Mounting ring handle bar
- Riser blocks
- Soft carry case
- Allen key set
The Cat 91 package lives up to its predecessors with a complete mounting kit and useful add-ons.
Setting up the RedCat 91
At about 14 pounds, the Cat 91 isn’t exactly a compact telescope. This puts it just outside of the star tracker range but well within a modest strainwave mount like the ZWO AM3 or Sky-Watcher Wave 150i.
I mounted it to the ZWO AM5, which is attached atop a Starfield Portable Tri-Pier for better height and clearance. A more traditional EQ mount, such as the Sky-Watcher EQ6-R Pro, would also be a great fit.
I mounted the Cat 91 to an AM5 and tri-pier.
I attached a ZWO ASI2600MM monochrome CMOS camera with a 7-position filter wheel in front. This is my narrowband imaging camera system and a practical pairing for the Cat 91 in the backyard.
This camera and filter wheel also contain LRGB filters for broadband color projects, and this is the primary configuration I will be using with the Cat 91.
This configuration passed the “unattended meridian flip test,” although my cable management could use some improvement. It might be time to finally install my EAF (Electronic Automatic Focuser) on this rig.
A rotator will be the final piece of the puzzle, allowing me to have full control from inside the house.
- Camera: ASI2600MM Pro
- Telescope: William Optics RedCat 91
- Controller: ZWO ASIAIR Plus
- Mount: ZWO AM5
- Tri-Pier: Starfield Tri-Pier
Backfocus and Image Circle
The Petzval design of this telescope means that achieving price-based backfocus is less of an issue. I used the standard 55m back focus distance between my camera sensor and the connection threads at the rear of the telescope.
Using an APS-C-sized camera sensor (ZWO ASI2600MM Pro), I did not experience any vignetting around the edges of my image frame. With a 55mm image circle, I would expect the same for those using a full-frame sensor.
The website states that the Cat 91 is compatible with medium-format camera sensors, which is rare in amateur astrophotography telescopes. Unfortunately, I do not own a camera of this size to test this claim myself.
Adjusting Tilt
The Cat 91 includes tilt adjustment screws to make small adjustments manually. Tilt refers to a misalignment between the camera sensor and the optical axis of the telescope, which can cause stars to appear elongated or out of focus in certain areas of the image.
There are a number of ways to measure the amount of tilt in your image train of your camera and Cat 91. One method is to use software like Siril to measure tilt using 3 points on an image.
Cloudy Nights user Chen posted his tilt results using the Cat 91 out of the box (9% tilt across the image field).
Based on these results, I would not recommend adjusting the tilt in your Cat 91 unless you notice an obvious misalignment in your test images. The telescope should be well-aligned from the factory, and you may end up making it worse by over-analyzing the tilt in your system.
Results with the RedCat 91
In terms of image results and optical performance, so far, so good. In my limited testing time with the Cat 91, the star quality was great, and the image field was nice and flat.
There were no surprises in terms of odd star shapes or optical aberrations. For the video, my test subject was the Pleiades Star Cluster in Taurus, and I was able to collect a total exposure time of 2 hours in LRGB.
Following that night, I chose to capture the Boogeyman Nebula in Orion. Because the moon was nearly full, these exposures were using an H-alpha filter.
Pleiades Star Cluster (M45). William Optics RedCat 91 + ZWO ASI2600MM Pro (LRGB).
I applied this monochrome data to my existing color project to add some much-needed ‘pop’ to the image. The original color image was captured using the smaller William Optics Pleiades 68.
As I continue to test the RedCat 91, I will add new picture data captured with the scope, including broadband images taken with a color camera and no filter.
The RedCat 91 is a good match with the ZWO AM5. A Starfield Tri-Pier helps to create better clearance and additional height.
Sample Images
While I do my best to capture real-life example images from my light-polluted backyard in the city, the weather makes it challenging to produce test images in the winter months.
The following example image was shared on the William Optics website. This detailed portrait of the Pleiades Star Cluster showcases the native field of view through the Cat 91 with a full-frame camera sensor.
The Pleiades Star Cluster by Dear Bear. Stacked image using the Cat 91 with a ZWO ASI6200MC Pro.
Here is a star quality test using aberration inspector in PixInsight. This was created using a single image frame from the master image shared above.
The flat field performance of the RedCat 91. Notice the round stars at the very edges of the image frame.
Alternative Options
There are so many flat-field apochromatic refractor telescopes available that it can be difficult to choose which one to buy. In my mind, the Askar 107PHQ is a clear competitor to the William Optics RedCat 91.
While this model is cheaper than the Cat 91, there are some big reasons why. The Askar 107PHQ has a larger aperture and longer focal length, but the advantages over the Cat 91 end there.
The Cat 91 has a bigger image circle, faster f-ratio, a five-element Petzval design, and a more robust focuser. If these features are less important to you, and you want a little extra reach for your dollar, consider the Askar model.
The Askar 107PHQ 107mm f/7 Quadruplet Flat-Field Astrograph is worth considering in this category.
Unlike the RedCat 91, you will need to purchase an additional 0.7X focal reducer to reach f/4.9.
Conclusion
The RedCat 91 is a no-brainer for anyone looking for a premium APO in the 400-500mm range. While there are cheaper alternatives, William Optics’ complete package adds additional value and should be factored into your decision.
If you have ever used a William Optics telescope, you’ll know the attention to detail, and premium touches he puts on these scopes. The Cat 91 is no exception, and it is truly a ‘lifetime ‘scope that should be cared for and enjoyed.
I’ll use the William Optics RedCat 91 extensively in the backyard over the next few years. I look forward to sharing the amazing things I capture in the night sky with it.
Trevor Jones is an astrophotographer and a valued member of the RASC. His passion is inspiring others to start their astrophotography journey on YouTube so they can appreciate the night sky as much as he does. His images have been featured in astronomy books & online publications, including the NASA Astronomy Picture of the Day (APOD).
