William Optics Cat, MiniCat, Mighty-Cat and Ultra-Cat: Petzval astrographs from 178 to 648 mm

RedCat became Cat WIFD

On William Optics’ store, the RedCat 51 address now answers with a permanent redirect to the Cat 51 page, and the RedCat 61 address does the same for the Cat 61. The older RedCat 51 MK2.5 and RedCat 71 keep their own pages, tagged discontinued, and there is no RedCat 91: the 91 mm model is the Cat 91 WIFD.

The rename came with a larger published image circle. The Cat 51 WIFD keeps 250 mm at f/4.9 but lists 48 mm against the MK2.5’s 43 mm. The Cat 71 WIFD lists 350 mm and 48 mm against the RedCat 71’s 348 mm and 43 mm. If you compare reviews or sample images, check which generation they used.

Four branches of one design

The Cat 51, 61 and 71 are four-element Petzvals at 250, 300 and 350 mm, all f/4.9. The Cat 91 WIFD and the four Ultra-Cats use a five-element design with two Super ED elements, from 269 mm (Ultra-Cat 56) to 648 mm (Ultra-Cat 131) at f/4.8–f/4.9.

The two fast models trade focal length for speed. The MiniCat 51 (six elements) works at 178 mm and f/3.5, and the Mighty-Cat 71 (seven elements, a limited 30th-anniversary edition) at 230 mm and f/3.2. WO’s MiniCat page says f/3.5 lets in twice as much light as f/5; (5 ÷ 3.5)² = 2.04 is that ratio, per pixel and per unit time on an extended object.

The Mighty-Cat and the Cat 71 WIFD make the trade visible because both have a 71 mm objective. They collect the same light from a nebula. At 230 mm, a full-frame ASI6200MC Pro covers 8.95° × 5.98° at 3.37″ per pixel; at 350 mm it covers 5.89° × 3.93° at 2.22″. The shorter tube spreads the same photons over (350 ÷ 230)² = 2.32 times fewer pixels, which is where its speed comes from.

No flattener, a spacing range instead

Every Cat spec table says no flattener is required. The Cat 51 WIFD manual explains how spacing works on these Petzvals: rather than one exact back focus, they reach focus anywhere within a supported back spacing range, and focus may become unreachable outside it. For astrophotography WO recommends 55 ± 3 mm to limit drawtube extension and reduce light leakage. The spec table above shows 55 mm for that reason, not as a single required distance.

Some store pages publish the range itself: 30–58 mm for the MiniCat, and a maximum of 60.8 mm (M48) or 65.8 mm (M54) for the Cat 91. The back focus guide covers how a camera’s own flange distance and any filter drawer add up against those limits.

Which sensors the circles cover

A full-frame sensor has a 43.3 mm diagonal. The MiniCat and Mighty-Cat list 43.2 mm, right at that size; WO markets the Mighty-Cat as full-frame capable. The Cat 51, 61 and 71 WIFD (46–48 mm) and Ultra-Cat 56 and 76 (50 mm) leave some margin. The Cat 91 (55 mm) and Ultra-Cat 108 and 131 (60 mm) go past full frame toward medium format.

On a full-frame sensor the MiniCat frames 11.55° × 7.72° at 4.36″ per pixel; the Ultra-Cat 131 frames 3.18° × 2.12° at 1.20″. The shorter Cats are compared with other sub-400 mm refractors on the small refractors page.