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A 91mm astrograph that covers full frame and medium format with no flattener
The William Optics Cat 91 WIFD, first announced as the RedCat 91, is a 91mm f/4.9 apochromatic astrograph with a 448mm focal length. Its 5-element advanced Petzval design (2 Super ED, 1 ED, 1 lanthanum and 1 special X-element) produces a large, flat 55mm image circle, so it covers full-frame sensors edge to edge and reaches medium format cameras with an additional adapter, all without a separate flattener. It's available in black, red and space grey, and it's on display in our Denver store.
Why imagers choose the Cat 91
- A 55mm image circle: covers full frame with room to spare, and 44 x 33mm medium format sensors with an extra adapter.
- No flattener to buy or space: the flattening is built in. Set your camera at the recommended 55mm (±3mm) of back focus.
- 448mm at f/4.9: enough reach for galaxies like M81 and M82 and nebula detail, while staying fast and forgiving to guide.
- WIFD internal focuser: a rack-and-pinion focuser with 36mm of travel that moves the optics inside the sealed tube, so your camera never shifts or sags. The knob removes to fit an electronic autofocuser.
- Tilt adjuster and rotator: correct sensor tilt and frame targets without removing the camera.
- Built-in Bahtinov mask: the lens cap doubles as a focusing mask.
Field of view with popular cameras
| Camera sensor | Field of view | Image scale |
|---|---|---|
| Medium format (44 x 33mm) | 5.6° x 4.2° | 1.73"/pixel (3.76µm pixels) |
| Full frame (36 x 24mm), e.g. ZWO ASI6200 | 4.6° x 3.1° | 1.73"/pixel (3.76µm pixels) |
| APS-C (23.5 x 15.7mm), e.g. ZWO ASI2600MC Pro or ASI2600MM Pro | 3.0° x 2.0° | 1.73"/pixel (3.76µm pixels) |
With an APS-C camera, the Pleiades, the Rosette Nebula or the Triangulum Galaxy fill the frame nicely. Full frame takes in the whole Andromeda Galaxy or the Heart Nebula.
M45, the Pleiades, with a ZWO ASI6200MC Pro full-frame camera. Courtesy of Dear Bear.
Corner grid from a single unprocessed full-frame exposure.
NGC 2237, the Rosette Nebula, by Reynold Edwards
M31, the Andromeda Galaxy, by Reynold Edwards
What you'll need to complete your setup
- A mount: at 13.89 lbs with its ring, handle and dovetail, the Cat 91 fits well on a strain-wave mount like the ZWO AM5N or Sky-Watcher Wave 150i.
- A camera: a cooled astronomy camera such as the ZWO ASI2600MC Pro, a full-frame astronomy camera, or a DSLR or mirrorless camera. Use the included M48 adapter with a wide M48 T-ring for your camera, such as the Canon EF, Canon RF, Nikon F, Nikon Z or Sony E T-mounts.
- Autofocus (optional): the WIFD focuser accepts an electronic focuser such as the ZWO EAFN or EAF Pro.
- Guiding (optional): the handle bar's mini Vixen base takes a UniGuide 50 or UniGuide 32 guide scope.
Compare the William Optics Cat astrographs
| UltraCat 56 | UltraCat 76 | Cat 91 | UltraCat 108 | UltraCat 131 | |
|---|---|---|---|---|---|
| Aperture | 56mm | 76mm | 91mm | 108mm | 131mm |
| Focal length | 269mm (f/4.8) | 365mm (f/4.8) | 448mm (f/4.9) | 518mm (f/4.8) | 648mm (f/4.9) |
| Image circle | 50mm | 50mm | 55mm | 60mm | 60mm |
| Total weight | 7.08 lbs | 10.98 lbs | 13.89 lbs | 21.12 lbs | 30.8 lbs |
| Best for | Very wide fields in a light, portable package | Wide nebula fields with more aperture | Medium reach for full frame and medium format | Galaxy groups and smaller nebulae, medium format | Maximum aperture and reach in the Cat line |
Why buy from Mile High Astronomy
- No surprise import duties: our price includes all import duties. Telescopes ordered directly from overseas are often listed without duties, which are then billed separately before delivery.
- Ships from the US: your Cat 91 ships from our Denver store, not from overseas.
- Expert help from astrophotographers: we can help you choose a mount, camera and focuser for your setup, and we're here to help after the sale.
What's in the box
- William Optics Cat 91 WIFD f/4.9 astrograph with tilt adjuster and camera angle rotator
- Bahtinov mask cap for precise focusing
- M63 to M48 and M63 to M54 camera adapters
- Mounting ring and handle bar with mini Vixen base
- 20mm riser blocks
- Vixen-style dovetail plate
- Soft carry case
- Allen key set
Cat 91 FAQ
Is the Cat 91 the same as the RedCat 91?
Yes. William Optics announced it as the RedCat 91 and now sells it as the Cat 91 WIFD, available in black, red and space grey. The optics are the same in every color.
What is the Cat 91's focal length?
448mm at f/4.9, with a 91mm aperture. That's about 1.8 times the focal length of a RedCat 51, for a closer view of galaxies and smaller nebulae.
Do I need a field flattener?
No. The 5-element Petzval design has the flattening built in. Set your camera sensor 55mm (±3mm) behind the telescope using the included adapters and the right T-ring or spacers.
Will it cover a medium format camera?
Yes. The 55mm image circle covers a 44 x 33mm medium format sensor. Medium format cameras need an additional adapter to connect to the M63 thread.
What mount do I need?
Plan for about 14 lbs for the telescope with its ring, handle and dovetail, plus your camera and guiding gear. A strain-wave mount like the ZWO AM5N or Wave 150i, or a mid-size German equatorial mount, is a good match. The included riser blocks help the focuser clear dual-saddle mounts.
Can I add an electronic focuser?
Yes. The focus knob removes so an electronic focuser like the ZWO EAFN can drive the WIFD focuser with a flexible coupling. Installation steps are in the online manual.
Can I see one before I buy?
Yes. The Cat 91 is on display in our Denver store.
Optical performance
Specifications
| Optical design | 5-element advanced Petzval apochromat |
| Glass | 2 Super ED, 1 ED, 1 lanthanum, 1 X-element |
| Aperture | 91mm |
| Focal length | 448mm |
| Focal ratio | f/4.9 |
| Image circle | 55mm |
| Camera formats | Medium format (with additional adapter), full frame, APS-C and Micro Four Thirds |
| Field flattener | None required |
| Back focus | 55mm (±3mm) recommended |
| Camera connection | M63, with M48 and M54 adapters included |
| Focuser | WIFD internal rack and pinion, EAF compatible |
| Focuser travel | 36mm |
| Included adapters | Tilt adjuster, camera angle rotator, M63 to M48 and M63 to M54 adapters |
| Dovetail | Vixen style |
| Tube length | 556mm |
| OTA weight | 10.41 lbs (4.72 kg) |
| Total weight with ring, handle and dovetail | 13.89 lbs (6.3 kg) |
| Dawes limit | 1.3 arcseconds |
| Colors | Black, red, space grey |
| Warranty | 2 years from William Optics |
| Manufacturer part numbers | T-C-91 (black), T-C-91-RD (red), T-C-91-SG (space grey) |
Online manual: back focus, tilt adjustment, electronic focuser installation and mounting.