William Optics

William Optics RedCat 51

Flat-field 51mm Petzval astrograph, pin-point stars for widefield deep-sky imaging.

93/ 100

Polaris Score

#1 of 12 premium astrophotography scopes

See the breakdown ↓

Aperture

51mm

Focal Length

250mm

Focal Ratio

f/4.9

Mount Type

None (OTA only)

Optical Design

Petzval Refractor

From$888

Lowest in 90 days

Price tracked by PolarisObserved 61 timesTracking since Jun 2026

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Where to buy

Agena AstroProductsPartnerBest price$888→High Point Scientific$888→

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What it's built forOpen these in the simulator →

This telescope was built for the camera.

A fast f/4.9 astrograph designed to throw a wide, flat, well-corrected field onto a sensor. It is an optical tube first — give it a tracking mount and a camera and it rewards long exposures, not eyepieces.

f/4.9 — built to imageNeeds a tracking mount

The view people remember

M57, a perfect smoke ring.

A tiny, flawless grey ring floating in a rich star field — the blown-off shell of a dying star, and one of the most satisfying small targets in the sky.

See Ring Nebula through this telescope →

Where else it shines

Needs patience

Orion Nebula

Colorful nebula captured with live stacking — wings and filaments

Open view →

Rewards a dark sky

Andromeda Galaxy

Long-exposure capture revealing galaxy core and outer structure

Open view →

Easy first target

The Pleiades

Field rich with stars — long exposure pulls in fainter members

Open view →

Also excellent on

Saturn

Processed planetary capture with rings or bands clearly visible

Open view →

✦

There is no eyepiece experience here. Its reward shows up on a sensor, after the exposure — not in the moment.

01What it's good at

What it's best at — and the trade-offs.

Best at

✓Designed specifically for astrophotography with built-in field correction

✓Produces sharp, high-quality wide-field images

✓Compact and travel-friendly

✓Minimal setup complexity compared to other imaging scopes

Trade-offs

!Useless for visual astronomy, imaging only

!A quality equatorial mount (significant extra cost)

!Very short focal length, not suitable for planetary targets

!Premium price compared to similar-aperture refractors

02In plain English

What These Numbers Mean

Aperture

51 mm

Two inches of glass, so this is not a telescope for faint or small targets. It exists to deliver an exceptionally clean wide field, and it reaches dim nebulosity through long total integration rather than raw aperture.

Focal length

250 mm

Very wide. The Orion complex end to end, the Cygnus loop and large sweeps of the Milky Way fit in a single frame, which is closer to a fast telephoto lens than a conventional telescope.

Focal ratio

f/4.9

Fast and extremely forgiving. At 250 mm even modest tracking accuracy gives round stars, which is why this is one of the most recommended first astrophotography telescopes.

Optical design

Petzval quadruplet astrograph

A four-element Petzval with the flattener built in, so full-frame corners stay clean with no accessory to buy or space. Its helical focuser and compact body are built around camera use rather than eyepieces.

Weight

~1.6 kg tube

Around 3.5 lb. Any star tracker carries it, and it fits in a camera bag, which makes it realistic to take on a plane for a dark-sky trip.

Mount

OTA only

You supply the mount, and almost anything that tracks will do. This is the cheapest complete imaging setup in the William Optics range once a mount is counted.

02The Polaris Score

How it scores, and why.

93/ 100

William Optics RedCat 51

#1 of 12 premium astrophotography scopes

Performance

Ease of use

Portability

Upgrade potential

Polaris weighs optical capability, how easy the scope is to live with, how portable it is, and how far it takes you before you outgrow it — not list price or brand.

03Who it's for

An honest fit check.

Buy it if you

✓Beginners who want high-quality astrophotography results

✓Users building portable imaging rigs

✓Wide-field imaging enthusiasts

Look elsewhere if you

!Budget-conscious beginners

!Visual astronomy users

!Planetary imaging specialists

04Build your rig

What to buy next — and where it leads.

Imaging path

Better tracking & light imagingCelestron Advanced VX (AVX) Equatorial Mount✓ Compatible

Commonly paired with this telescope

$1,099View details →

Portable astrophotographyZWO AM5N Harmonic Equatorial Mount✓ Compatible

Start capturing images of the Moon and planets

$2,299View details →

Permanent-setup astrophotographySky-Watcher EQ6-R Pro✓ Compatible

Commonly paired with this telescope

$1,899View details →

Start imagingZWO ASI533MC Pro

Start capturing images of the Moon and planets

$799View details →

Show all compatible mounts (5) →

Accessories

A sharper everyday viewEthos 13mm

Balanced magnification, great for general observing

$745View details →

Wide-field deep-sky imaging

Excellent, flat field

Lunar & planetary

Small image scale

Visual use

Possible, but an imaging tube

05The numbers

Full specifications.

Optics

Optical DesignPetzval Refractor

Aperture51mm

Focal Length250mm

Focal Ratiof/4.9

Mount & drive

Mount TypeNone (OTA only)

More

weight lbs3.5lbs

Editor’s Take

The telescope that has probably started more astrophotography journeys than any other premium refractor. It makes beautiful images almost effortlessly, which is why so many experienced imagers still keep one even after buying much larger scopes.