The RedCat 91 is a refractor everyone should want

Wide-field imaging is a passion of mine. Something about capturing a large swath of the night sky filled with delicate billows of ionized hydrogen stimulates the imagination and creates a sense of awe at the beauty of our universe. For two years, my favorite wide-field refractor has been my William Optics RedCat 71, but I’ve sometimes wished it could produce a little more resolution on some of those intricate patterns of nebulosity. When I heard that William Optics was releasing a 91mm f/4.9 version of the RedCat, I wondered if this might be the refractor that gives me what I wanted.

My first impression after receiving my scope is that the RedCat 91 is a well-crafted astronomical imaging refractor, with a machined aluminum finish and quality feel. Its size (less than 22 inches [56 centimeters] long) and weight make it portable. It comes mounted in a ring system with a Vixen dovetail plate on the bottom and a handle on the top for easy carrying. The handle also serves as a mounting shoe for a guide scope.

Optically, the RedCat 91 is superb. Its focal ratio of f/4.9 makes it a fast photographic system and its 55mm image circle easily covers a full frame sensor. It also utilizes the Petzval design, which produces a nice flat field without the need for a special “field flattener.” For me, that is a major feature. Standard apochromatic refractors produce curved light cones, which result in warped stars near the edge of the field. This curvature makes the use of a field flattener a necessity — and they can be quite frustrating to use. Getting the proper flattener-to-sensor distance involves trial and error and the tolerances are often less than 1 millimeter.

The RedCat 91’s field of view is not quite as broad as my RedCat 71. However, a quick test using the Framing Wizard of the popular automation application Nighttime Imaging ’N’ Astronomy shows the RedCat 91 still covers a respectable field of 4.6° by 3.1° with a full frame camera. This is enough sky real estate to easily frame all of the Andromeda Galaxy (M31), or the rich field in Auriga that encompasses the Flaming Star Nebula (IC 405), the Tadpoles (IC 410), and the Spider and the Fly (IC 417 and NGC 1931).

Extras included

One of the most attractive features of the RedCat is its unique focuser, the patented William Optics Internal Focus Design (WIFD). This design places the focuser within the optical tube near the center of balance, providing two major benefits. First, it secures the focuser in a dust-free environment, thus eliminating the need for frequent cleaning. But most importantly, it eliminates balance issues when adding or changing imaging equipment. And it is just as easy to attach an electronic focusing mechanism to the WIFD as it is to a traditional rack-and-pinion focuser.

This RedCat comes with two imaging camera adapters: an M54 for full-frame sensor cameras and an M48 for APS-C sized sensor cameras. These adapters also provide T-mount support, making the RedCat compatible with various brands of DSLR and mirrorless cameras when paired with a matching T-ring.

The RedCat 91 has a manual field rotator and image tilt adjuster, which is very useful if the sensor is not perpendicular to the imaging axis. Finally, the lens cap contains a built-in Bahtinov mask for easy manual focus.

Standing ovation

My RedCat 91 came with a promotional Uniguide, a 32mm f/3.75 guide scope that connects to the handle on top of the refractor’s rings. To complete my setup, I attached my full-frame CMOS camera with its 50mm filter wheel using the supplied M54 adapter. I was pleased to discover that there was enough back focus to add an electronic rotator, with about 8mm to spare.

On my first night out, I aligned the system and easily brought it to focus using the Bahtinov mask. The RedCat 91 holds focus well; I didn’t notice any shift as the temperature dropped through the night.

Clouds shut down my evening activities, so first light would have to wait. When I got my clear night, I chose IC 2177, also known as the Seagull Nebula, as my target. For my imaging run I took a set of RGB light frames — 12 frames per filter at 300 seconds each. Next I took a series of Hydrogen-alpha (Hα) frames — nine at 900 seconds each. Finally I shot a series of Oxygen-III (OIII) frames — 12 at 1,200 seconds each. All data captured was binned 2×2 to limit the file size of each light frame. In total, I spent two nights collecting my data.

The first thing I wanted to check was the star geometry: How do the stars look across the entire frame of the image? For that test, I processed the RGB data first, creating a simple RGB image. After enlarging the image, I checked the stars at the extreme edges, and was pleased to see that the stars remained perfectly round across the entire field.

Then, to see this scope’s full capabilities, I added the narrowband Hα and OIII data to the image. The RedCat 91 easily pulled in fine structural details only visible in narrowband light to create an absolutely impressive result.

Trial and little to no error

I am impressed with the performance of the RedCat 91. It produces wonderful deep-sky images that can cover a full-frame sensor, showing no aberration to star geometry at the edges of the field. Its Petzval design produces a truly flat field and eliminates the need for special flatteners and the issues surrounding them, such as proper flattener-to-sensor spacing.

The RedCat 91 does not come with an external guide scope, though William Optics was offering a free promotional 32mm UniGuide when I received mine and at the time of writing this review. I found star focusing a challenge with the 32mm UniGuide and would personally recommend purchasing the larger 50mm UniGuide, as it has a longer focal length and larger aperture, making the stars easier to bring to focus.
For those who prefer off-axis guiding instead of an external guide scope, back focus will be an issue. Using a standard CMOS camera with a filter wheel, there is about 24mm of back focus available. That’s not enough for a large off-axis guider (OAG), which would be needed with a full-frame camera. The OAG itself may fit, but the guiding camera unit atop the OAG spans 50mm and will not clear the back of the RedCat when installed. On the other hand, the RedCat’s focal length is short enough at 448mm and the image scale used is such that an external guide scope works just fine. The refractor offers plenty of room to add an electronic field rotator, which is a necessity if you wish to make the RedCat 91 the centerpiece of a robotic system.

William Optics provides a manual for the RedCat 91 available online and downloadable as a PDF file. This manual covers all aspects of the scope, including how to use the tilt adjuster and mount external devices such as an electronic focuser.

In conclusion, the RedCat 91’s portability makes it perfect for field use as a wide-field imaging refractor. For those who have the luxury of a backyard observatory, the RedCat lends itself well to full automation as a semi-permanent observatory telescope, or even as part of a robotic imaging system at a remotely hosted observatory.