William Optics RedCat 51 Specs and Review
The William Optics RedCat 51 is a 51 mm (2 inch) petzval apochromatic refractor with a 250 mm focal length, giving f/4.9, a maximum useful magnification of about 102x and a Dawes resolution limit of 2.27 arcseconds. It typically sells in the $700 to $800 band.
The William Optics RedCat 51 is a 51 mm petzval apochromatic refractor on a optical tube only, with a focal length of 250 mm and a focal ratio of f/4.9. A four element Petzval design that needs no separate flattener, at a focal length short enough that a star tracker can carry it without guiding. It is a camera lens that happens to be a telescope, and it is useless for visual work.
What are the full specifications?
Aperture and focal length come from the manufacturer. Focal ratio, magnification limits, resolution and light gathering are calculated from those two figures rather than transcribed, so they cannot disagree with each other. Any specification we could not verify is left out of this table entirely rather than estimated.
| Specification | Value |
|---|---|
| Optical design | Petzval apochromatic refractor |
| Aperture | 51 mm (2 in) |
| Focal length | 250 mm |
| Focal ratio | f/4.9 |
| Mount type | Optical tube only |
| Weight | 3.2 lb |
| Maximum useful magnification | 102x |
| Lowest useful magnification | 7x |
| Resolution, Dawes limit | 2.27 arcsec |
| Light gathering vs the naked eye | 53x |
| Typical price band | $700 to $800 |
Two of those numbers do most of the work. Aperture sets how much light the telescope collects and how fine a detail it can resolve, and nothing else on the spec sheet can compensate for it. Focal ratio, which is focal length divided by aperture, decides how wide a field you can reach and how forgiving the telescope is of cheap eyepieces. At f/4.9 this is a fast telescope, which means a wide field and shorter exposures, at the cost of showing edge aberrations with simple eyepieces and needing more careful collimation.
What magnification does the William Optics RedCat 51 support?
Magnification is not a property of the telescope. It is telescope focal length divided by eyepiece focal length, so it changes every time you swap an eyepiece. The telescope sets the ceiling, and the ceiling is set by aperture: roughly 2x per millimetre of aperture, which is the same rule as 50x per inch. For the William Optics RedCat 51 that is about 102x.
There is a floor as well, and it is less well known. Exit pupil is the width of the light cone leaving the eyepiece, equal to eyepiece focal length divided by focal ratio. A dark-adapted adult pupil is about 7 mm across and shrinks with age, so once the exit pupil exceeds roughly 7 mm the eye cannot accept the whole cone and the surplus aperture is thrown away. For the William Optics RedCat 51 that puts the longest sensible eyepiece at about 34.3 mm and the lowest useful magnification at around 7x.
| Eyepiece | Magnification | Exit pupil | Verdict |
|---|---|---|---|
| 32 mm | 8x | 6.5 mm | Low power, finding and wide fields |
| 25 mm | 10x | 5.1 mm | Low power, finding and wide fields |
| 20 mm | 13x | 4.1 mm | Low power, finding and wide fields |
| 15 mm | 17x | 3.1 mm | General purpose, most deep sky work |
| 12.5 mm | 20x | 2.6 mm | General purpose, most deep sky work |
| 10 mm | 25x | 2 mm | High power, planets and double stars |
| 9 mm | 28x | 1.8 mm | High power, planets and double stars |
| 6 mm | 42x | 1.2 mm | High power, planets and double stars |
| 5 mm | 50x | 1 mm | High power, planets and double stars |
| 4 mm | 63x | 0.8 mm | High power, planets and double stars |
Work out the same figures for any other eyepiece with the magnification calculator, or plan a whole set at once with the eyepiece calculator.
What can you actually see through it?
Before the list, the thing that decides whether somebody enjoys a telescope or abandons it: a telescope does not show colourful nebulae to the eye. Human night vision runs on rod cells, which are nearly monochrome, so the Orion Nebula appears as grey-green mist and galaxies appear as faint grey ovals no matter how large the aperture. Photographs are long exposures stacked from hours of data. What does look genuinely spectacular through an eyepiece is the Moon, the planets, double stars, open clusters and, in enough aperture, globular clusters.
The Moon
Dozens of large craters, the maria, and the rugged terminator. The Moon is the one target that looks genuinely spectacular in any aperture.
The planets
The rings of Saturn as an obvious shape, the Galilean moons of Jupiter, and the phases of Venus. Planetary surface detail is at the edge of what this aperture can deliver.
Deep sky objects
Open clusters, the brighter nebulae as grey patches, and the larger galaxies as faint ovals. Anything faint needs a dark sky far more than it needs magnification.
What it will not show
Galaxies as anything but faint smudges, colour in any nebula, or the moons of Saturn beyond Titan. Set expectations here or the telescope ends up in a cupboard.
Aperture is only half the story for faint objects. Sky darkness is the other half, and it is usually the larger of the two: moving from a suburban Bortle 7 sky to a rural Bortle 4 sky buys about as much as doubling the aperture, for the cost of a drive. The Bortle scale chart sets out what each class actually means at the eyepiece, and what you can see by aperture breaks the targets down band by band.
What does the optical tube only mean in practice?
This is an optical tube only, which is the single most misread line on a telescope listing. There is no mount and no tripod in the box, so the price you see is not the price of a working telescope. Budget a mount rated for at least twice the tube weight for visual use, and considerably more than that for imaging. A capable mount frequently costs more than the tube it carries, and that is the correct way round.
Whatever the mount, the general rule holds and it is the one beginners most often ignore: the mount matters more than the telescope. A shaking image at 150x is unusable no matter how good the optics are, and a cheap department store telescope on a wobbly tripod is the single most common reason people give up on the hobby. Check any candidate against the mount payload calculator before buying.
What maintenance does it need?
Almost none, and that is a real advantage. The lens cell is set at the factory and stays aligned through normal handling, and the sealed tube keeps dust off the optical surfaces. The one recurring job is dew: a closed tube facing the sky radiates heat and the front element fogs, which a dew shield delays and a dew heater strip prevents outright.
For any design, storage matters more than cleaning. Keep the telescope somewhere dry with the caps on, let it reach room temperature before putting it away so condensation does not form inside a cold tube, and resist cleaning optics until they are genuinely dirty, since every clean carries more risk of a scratch than a little dust costs in contrast. The maintenance guide covers the routine in full.
What else should you look at in this class?
These are the closest telescopes in the database by aperture. Aperture is the fairest first comparison because it sets the physical limits, and everything else on a spec sheet is a choice about how to use those limits.
| Telescope | Type | Aperture | Focal ratio | Price band |
|---|---|---|---|---|
| ZWO Seestar S50 | Smart telescope, apochromatic triplet | 50 mm | f/5 | $450 to $550 |
| Celestron AstroMaster 70AZ | Achromatic refractor | 70 mm | f/12.9 | Under $200 |
| ZWO Seestar S30 Pro | Smart telescope, apochromatic triplet | 30 mm | f/5 | $600 to $750 |
| Celestron FirstScope 76 | Newtonian reflector | 76 mm | f/3.9 | Under $100 |
| Celestron StarSense Explorer LT 80AZ | Achromatic refractor | 80 mm | f/11.3 | $200 to $300 |
| SVBONY SV503 80ED | ED doublet refractor | 80 mm | f/7 | $400 to $450 |
If you are still deciding between optical designs rather than between models, the refractor versus reflector comparison covers the trade properly, and how to choose a telescope works through the whole decision from budget to storage space. If you already know roughly what you want and need the rest of the kit around it, the complete builds price out three full setups with running totals.
Is the William Optics RedCat 51 the right buy?
It suits wide field imaging on a star tracker. A four element Petzval design that needs no separate flattener, at a focal length short enough that a star tracker can carry it without guiding. It is a camera lens that happens to be a telescope, and it is useless for visual work.
The honest test is not whether a telescope is good on paper. It is whether you will carry it outside on a Tuesday in the cold, because the telescope that gets used beats the larger one that stays in a cupboard every single time. Weigh the 3.2 lb this weighs against where it will live and how far it has to travel to reach the sky. That single consideration decides more about how much observing somebody does than aperture ever will.
Frequently asked questions
What magnification can the William Optics RedCat 51 actually reach?
The optical maximum is about 102x, which is two times the aperture in millimetres. In practice the atmosphere decides: on a typical night the image degrades somewhere between 180x and 250x no matter how large the telescope is, so a realistic working ceiling for the William Optics RedCat 51 is around 102x. Pushing past it makes the image larger, dimmer and blurrier without adding any detail the aperture never collected.
What eyepieces should I use with the William Optics RedCat 51?
Three cover almost everything. A 32 mm gives roughly 8x for finding targets and sweeping star fields, a 15 mm gives 17x as the general workhorse, and a 9 mm gives 28x for planets and double stars. Anything shorter than about 2.5 mm exceeds the useful maximum of this telescope and will only make the image dimmer.
Can you do astrophotography with the William Optics RedCat 51?
Lunar and planetary imaging works well, since those are bright video targets stacked from thousands of frames. Long exposure deep sky imaging is limited by field rotation on an alt-azimuth mount, which smears the corners of a frame after roughly 30 to 60 seconds.
Does the William Optics RedCat 51 need collimation?
Effectively no. A refractor has its lens elements fixed in a cell at the factory and they stay put through normal handling. This is one of the genuine practical advantages of a refractor over a reflector of the same price.
How much does the sky quality matter with a 51 mm telescope?
More than the telescope does for faint objects. Moving from a Bortle 7 suburban sky to a Bortle 4 rural sky typically gains around two magnitudes of reach, which is roughly the same gain as doubling the aperture, and it costs a drive rather than a purchase. Planets and the Moon are almost unaffected by light pollution, so a city observer loses very little on those targets.
Is the William Optics RedCat 51 worth its price band of $700 to $800?
A four element Petzval design that needs no separate flattener, at a focal length short enough that a star tracker can carry it without guiding. It is a camera lens that happens to be a telescope, and it is useless for visual work. Judge it against what else that money buys in the same aperture class rather than against a larger telescope you would not carry outside. The telescope that gets used is always the one that earns its price.
How we choose: we compare published manufacturer specifications, optical figures we can verify, and reviews from owners who have used the equipment under real skies. We do not test gear in person. Never point any telescope, finder or binocular at the Sun without a certified full-aperture solar filter fitted over the front of the instrument.
Recording your own eyepieces, exit pupils and sessions? The Observing & Astrophotography Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.