A 51 mm f/4.9 Petzval astrograph with the field flattener built into the optical design — no spacers to calculate, no corrector to buy, round stars to the corners of a full frame.
Short verdict
A tiny, expensive, almost foolproof wide-field astrograph. You are paying for the fact that nothing about the optical train can go wrong.
Sold as an optical tube only. No mount, tripod or head is included. For imaging in particular, the mount is usually the larger part of the total cost.
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Best for
Very wide nebula fields and Milky Way panoramas
Travel imaging on a star tracker
A first astrograph for someone who wants the optical train to be a non-issue
Not ideal for
Any visual observing at all
Galaxies, planetary nebulae or anything small
Planetary and lunar imaging
Purpose scores
The same four purposes are scored for every telescope on this site, using the same weights. A telescope that is excellent for one is often poor for another — that is the point.
William Optics RedCat 51 is a camera-only instrument. There is no eyepiece position, so it cannot be used for visual observing at all — this is a property of the optical design, not a shortcoming to be scored.
Scores are relative to every telescope on this site, not absolute quality marks. A 6.5 for galaxy imaging and a 6.5 for visual observing do not mean the same thing. How scoring works.
Optical design
A Petzval puts the field-flattening elements inside the telescope rather than bolting them on
behind it. The practical consequence is that the RedCat has exactly one imaging configuration —
camera at 55 mm backfocus — and it is correct. There is no spacer stack to iterate on, no tilt
introduced by a third-party adapter, and no corner-star troubleshooting thread to read.
For a first astrograph that removes most of the failure modes that make beginners give up.
Visual observing
Not possible. The RedCat has no visual back and no eyepiece position: the design places the focal
plane where a sensor goes and nowhere else. Any comparison that scores it for visual observing is
comparing something that does not exist.
Nebula imaging
This is the entire purpose. At 250 mm and f/4.9 with a 44 mm image circle, it frames the Veil
complex, the Rho Ophiuchi region and the North America / Pelican pair with room to spare on a
full-frame sensor. Star shapes hold to the corners.
The constraint is aperture. 51 mm collects roughly half the light of a 72 mm doublet, so the same
signal takes about twice as long. In a light-polluted sky with a dual-band filter that difference
is measured in extra nights, not extra minutes — worth understanding before buying.
Galaxy imaging
250 mm gives an image scale far too coarse for galaxies. Andromeda and the Large Magellanic Cloud
work as landscape subjects; nothing smaller does.
Planetary imaging
No. 51 mm resolves to about 2.3 arcseconds, which is worse than typical seeing, and the focal
length would need a factor of ten of amplification to reach a usable planetary image scale.
Portability, setup and maintenance
1.6 kg and 280 mm long, with an Arca-Swiss compatible ring that mounts straight to a star tracker
or a photo head. It is the only telescope here that genuinely travels in a camera bag. Nothing to
collimate, nothing to cool, and the front cap doubles as a Bahtinov mask.
Full specifications
Optics
Optics specifications for William Optics RedCat 51
Aperture
51 mm (2")
Focal length
250 mm (9.8")
Focal ratio
f/4.9
Optical design
Four-element Petzval with integrated flattener
Type
Petzval refractor
Glass / mirror
FPL-53 extra-low-dispersion elements
Resolving power
2.27"Calculated from aperture (Dawes limit), not a manufacturer figure.
Limiting magnitude
≈ 10.5Estimated from aperture under dark skies. Real skies are usually worse.
Imaging compatibility
Imaging compatibility specifications for William Optics RedCat 51
Corrected image circle
44 mm
Backfocus
55 mm
Field correction built in
Yes
Usable visually
No — camera only
APS-C sensors
Supported
Full-frame sensors
Supported
Reducer
None offered
Corrector required
No
Autoguiding
Not essential
Mount and tracking
Mount and tracking specifications for William Optics RedCat 51
Mount included
No — optical tube only
Tracking
Depends on your mount
GoTo
Depends on your mount
Mounting interface
Integrated rotating tripod ring with Vixen and Arca-Swiss compatibility
Size and weight
Size and weight specifications for William Optics RedCat 51
OTA weight
1.6 kg (3.5 lb)
Tube length
280 mm (11")
Tube diameter
90 mm
Longest transport dimension
330 mm (13")
Product
Product specifications for William Optics RedCat 51
Brand
William Optics
Model
RedCat 51 Petzval
Status
Current
Price band
PremiumWe publish bands, not prices. Check the retailer for the current price.
Skill level
beginner, intermediate
Editorial notes
William Optics has shipped several RedCat variants at different focal ratios and apertures. These figures describe the original 51 mm f/4.9 model.
Strengths and limitations
Strengths
+The flattener is part of the optical design, so there is no corrector to buy and no spacing stack to get wrong beyond the standard 55 mm.
+Covers a full-frame sensor with round stars, which most telescopes at any price do not.
+Light enough for a battery-powered star tracker, which removes the largest single cost from a wide-field imaging setup.
Limitations
−No eyepiece position — this is a camera lens in a telescope tube and cannot be used visually.
−51 mm of aperture is very little light; deep results need many hours.
−Expensive per millimetre of aperture. You are paying for the correction, not the light grasp.
Accessories
In the box
Rotating tripod ring
Bahtinov mask front cap
Hard case
Required, sold separately
A tracking equatorial or star-tracker mount
A camera with the correct 55 mm backfocus adapter
Worth adding
A dual narrowband filter for suburban skies
A small guide scope for exposures beyond a few minutes
We list an accessory as included only when the product record says so explicitly. If a mount, eyepiece, camera, flattener, reducer, corrector, guider or adapter is not listed above, assume it is not in the box.