The 91mm Redcat refractor wears an elegant black design with sharp red details, perfect for combining its higher performance. Credit: William Optics
Great field imaging is my passion. Something in capturing a large band of the night sky full of delicate ionized hydrogen waves stimulates the imagination and creates a sense of reverential fear for the beauty of our universe. For two years, my big favorite refractor was my William Optics Redcat 71, but sometimes I wanted it to be produced a little more resolution on some of those intricate reasons of nebulosity. When I heard that William Optics was releasing an F/4.9 version of 91 mm of Redcat, I wondered if this could be the refractor that gives me what I wanted.
My first impression after receiving my field of application is that the Redcat 91 is a well -made astronomical image refractor, with a worked aluminum finish and a sensation of quality. Its size (less than 22 inches [56 centimeters] long) and the weight make it portable. It is mounted in a ring system with a Vixen tail tail plate on the bottom and a handle on the top for easy transport. The handle also acts as a assembly shoe for a guidance.
Optically, Redcat 91 is superb. Its focal ratio of f/4.9 makes it a fast photographic system and its 55 mm images circle easily covers a complete frame sensor. It also uses Petzval’s design, which produces a nice flat field without the need for a special “Field Flattenner”. For me, this is an important feature. Standard apocrylomatic refractors produce curved light cones, which cause deformed stars near the field. This curvature makes the need for a field flattening – and can be frustrating enough to use. Obtaining the flattened sensor distance involves test and error and the tolerances are often lower than 1 millimeter.
The visual field of Redcat 91 is not as large as my Redcat 71. However, a rapid test using the guarding procedure for framing the popular night imaging of the automation application ‘N’ Astronomy shows that Redcat 91 still covers a respectable field of 4.6 ° for 3.1 ° with a complete frame camera. This is quite motionless Sky to easily frame the whole Andromeda galaxy (M31), or the field rich in Auriga which contains the nebulose flaming star (IC 405), the Girini (IC 410) and the spider and the Moscow (IC 417 and NGC 1931).
Extras included
One of the most attractive features of Redcat is its unique focus, the William Optics Internal Focus Design (WIFD) palestick. This design places the focuster inside the optical tube near the center of balance, offering two important benefits. First of all, fix the focuster in an environment without dust, thus eliminating the need for frequent cleaning. But above all, eliminates balance problems during the addition or modification of the imaging equipment. And it is equally easy to connect a mechanism of electronic focus to the Wifd as to a traditional rack-and-pier focusing.
This Redcat is provided with two imaging camera adapters: an M54 for full-frame frame sensor cameras and an M48 for APS-C size sensors cameras. These adapters also provide T-Mount support, making Redcat compatible with various DSLR brands and Mirrorless cameras if combined with a corresponding T ring.
Redcat 91 has a manual field rotor and an image inclination regulator, which is very useful if the sensor is not perpendicular to the imaging axis. Finally, the lens hood contains a bahtinov mask incorporated for easy manual focus.
Standing ovation
My Redcat 91 arrived with a promotional uniguide, a 32 mm F/3.75 Guide scope that connects to the handle on the upper part of the refractor rings. To complete my configuration, I connected my complete frame CMOS camera with its 50 mm filter wheel using the M54 adapter supplied. I was happy to find that there was quite focus on the back to add an electronic rotator, with about 8 mm to save.
During my first evening outside, I aligned the system and I did it easily focusing it using the Bahtinov mask. Redcat 91 keeps concentration well; I didn’t notice any shift while the temperature fell overnight.
The clouds closed my evening activities, so the first light should have waited. When I had my clear night, I chose IC 2177, also known as Nebula Seagull, like my goal. For my imaging race I took a series of RGB light frames – 12 filter frames at 300 seconds each. Subsequently I took a series of hydrogen-alfa frames (Hα) -nove to 900 seconds each. I finally shot a series of frames of Oxygen-III (OIII) -12 to 1,200 seconds each. All the data acquired were grouped 2 × 2 to limit the size of the file of each light frames. In total, I spent two nights to collect my data.
The first thing I wanted to check was star geometry: how do you look at the stars through the entire frame of the image? For that test, I first processed the RGB data, creating a simple RGB image. After enlarged the image, I checked the stars at the extreme edges and I was happy to see that the stars remained perfectly around the whole field.
So, to see the complete skills of this area, I added the Hα and Banda OIII data close to the image. Redcat 91 has easily attracted the fine structural details visible only in the narrow band light to create an absolutely impressive result.
Try and little or no mistake
I am struck by the performance of Redcat 91 Its Petzval design produces a truly flat field and eliminates the need for special flatteners and the issues that surround them, like a correct flat sensor spacing.
Redcat 91 is not provided with an external driving area, even if William Optics offers a promotional 32 mm promotional one when I received mine and at the time of writing this review. I found stars that focused a challenge with the Uniguide of 32 mm and personally recommend the purchase of the 50 mm uniguide bigger, as it has a longer focal length and a bigger opening, making the stars easier to focus on.
For those who prefer out of axis guide instead of an external driving area, the rear focus will be a problem. Using a standard CMOS camera with a filter wheel, about 24 mm of rear heat is available. It is not enough for a great off-axis guide (OAG), which would be necessary with a full-top camera. The OAG itself can adapt, but the drive unit leads to the top of the OAG extends for 50 mm and will not erase the rear of the Redcat when installed. On the other hand, the focal length of the Redcat is quite short at 448 mm and the image scale used is such that an external driving area works perfectly. The refractor offers a lot of space to add an electronic field rotor, which is a necessity if you want to make the Redcat 91 the fulcrum of a robotic system.
William Optics provides a Redcat 91 manual available online and downloadable as PDF file. This manual covers all aspects of the scope, included how to use the inclination regulator and external assembly devices such as an electronic focuer.
In conclusion, the portability of the Redcat 91 makes it perfect for the use of the field as a refractor of imaging on Campo Largo. For those who have the luxury of a courtyard observatory, the Redcat lends itself well to full automation as a semi-permanent observatory telescope, or even as part of a robotic imaging system in a distance observatory.
