Sky & Telescope - March 2021 - 67

a sample of the upgraded RedCat 51LX
introduced in early 2020.

RedCat? While its optics are certainly
superb, I found its mechanics are what
set it apart.

Optical Performance
The William Optics RedCat is a 51-mm
f/4.9 refractor using a Petzval design
with a doublet front element incorporating FPL-53 glass, plus a single
central element and a single rear lens,
which together serve as a ﬁeld ﬂattener.
While an instrument with only 51-mm
aperture is limited for visual observation, the RedCat is designed primarily
for imaging, making it an astrograph.
Think of it as a specialized telephoto
lens with a 250-mm focal length.
As such, my ﬁrst thought upon seeing the RedCat was: Why would I need
it? I already have a 200-mm telephoto
lens that, at f/2.8, is one and two-thirds
photographic stops faster. One night
I shot the same ﬁeld with both the
RedCat and my 200-mm, a premium
Canon L-series lens.
Sure enough, the RedCat did prove
sharper and with less chromatic aberration (in fact, none) than the telephoto
- but only when the Canon lens was
used wide open at f/2.8. Stopping the
200-mm lens down to f/5 to match
the RedCat's f/ratio leveled the playing ﬁeld. The lens now closely matched
the RedCat for both on- and off-axis
sharpness and lack of color aberration.
Indeed, I gained a new respect for the
Canon lens, which I had long considered to be a little soft. But extracting its
best performance required stopping it
down more than I usually do and also
precisely focusing the lens using the
RedCat's included Bahtinov mask.
Nevertheless, the RedCat's pinpoint
stars corner-to-corner on a full-frame
detector are impressive. And bright
stars don't exhibit the diffraction spikes
introduced by diaphragm blades when
shooting with a telephoto lens stopped
down. Only slight vignetting was visible
in the extreme corners of the ﬁeld.
The RedCat's $748 price is similar to
the Canon 200mm f/2.8 L-series lens
I compared it to, with the lens offering
more speed when needed, but comparable sharpness when stopped down.
So, the question remains: Why buy a

Mechanical Features
The RedCat is heavier and longer than
most 200-mm telephotos, but it has signiﬁcant advantages over a conventional
camera lens. Most importantly, the
RedCat's design includes two means of
easily rotating the camera for framing,
with no worries about the setup slip-

ping, as it can when mounting a camera
and telephoto lens tipped over on a ball
head or other form of tripod head.
First, the entire tube and camera
can be rotated within the mounting
cradle. Alternatively, the camera can be
rotated with respect to the tube using
an integral camera-angle adjuster that
is marked in one-degree increments.
This can facilitate shooting multi-frame
mosaics in which the camera has to be
rotated by a speciﬁc amount to keep

With a full-frame sensor, the RedCat provides a ﬁeld 8.3° by 5.5°, ideal for Milky Way
starﬁelds. Stars are very sharp across the ﬁeld, exhibiting just a trace of astigmatism
and lateral color when viewed at a pixel-peeping level in the extreme corners.

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Sky & Telescope - March 2021

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