Celestron Omni 4mm — image 1
Celestron

Celestron Omni 4mm

A 4mm Celestron Omni Plössl: a dedicated high-power eyepiece for planets, the Moon and tight double stars, from a clean, honest 52-degree Plössl that does one job well for very little money.

67/ 100
Polaris Score
Optical quality · Immersion · Comfort · Value
See the breakdown ↓
What it's for
Low power · wide fieldHigh power · narrow field

High power. At 4mm this is a dedicated high-power eyepiece for planets, the Moon and tight double stars; it is not meant for framing large deep-sky objects.

Apparent Field
52°
Barrel Size
1.25"
Eye Relief
3mm
Focal Length
4mm
From$31.95

Lowest in 90 daysLowest ever tracked

Price tracked by PolarisObserved 101 timesTracking since Jul 2026

Try in simulator →
The spacewalk

You don’t look at a field.
You look into the sky.

Every number on the spec sheet matters less than one feeling: how much sky wraps around your eye. Widen the apparent field and watch the black edge of the eyepiece slip away.

50° apparent field

At 52°, you are aware of the field edge, like looking down a short, comfortable tube.
Try this eyepiece on your own scope in the Simulator →
01What it's good at

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

Simple four-element Plössl optics: sharp on-axis and high in contrast.
Best at
✓Simple four-element Plössl optics: sharp on-axis and high in contrast.
✓Simple four-element Plössl optics: sharp on-axis and high in contrast.
✓At 4mm it lands squarely in high power territory, exactly where planetary and lunar detail lives.
Trade-offs
!Short eye relief at the short focal lengths, uncomfortable below about 10mm.
!A 52-degree field feels narrow once you have looked through anything wider.
!Basic coatings and build compared with premium glass.
02In your telescope

One eyepiece. A different job in every scope.

120× in a small refractor, 508× in a long SCT

An eyepiece has no magnification of its own — the telescope decides. The same eyepiece becomes 120× and 26′ of sky in a little refractor, or 508× and a tight frame in a long SCT. Here’s what it does across common scopes.

Small apo refractor
480 mm · f/5.5
Magnification
120×
True field
26′
Exit pupil
0.7 mm
120× across a 26′ field at a 0.7mm exit pupil, high power for planets, the Moon and tight doubles.
8-inch Dobsonian
1200 mm · f/6
Magnification
300×
True field
10′
Exit pupil
0.7 mm
300× across a 10′ field at a 0.7mm exit pupil, very high power, worth it only on the steadiest nights.
12-inch Dobsonian
1500 mm · f/5
Magnification
375×
True field
8′
Exit pupil
0.8 mm
375× across a 8′ field at a 0.8mm exit pupil, high power for planets, the Moon and tight doubles.
8-inch SCT
2032 mm · f/10
Magnification
508×
True field
6′
Exit pupil
0.4 mm
508× across a 6′ field at a 0.4mm exit pupil, very high power, worth it only on the steadiest nights.
Exit pupil is how wide the beam of light leaving the eyepiece is. Around 5–6 mm is bright and immersive under dark skies; push much past 6 mm and you waste light, and under light pollution a smaller exit pupil shows more contrast.
03In plain English

The numbers, in plain English.

4mm FL · 52° AFOV · 1.25"
Focal lengthHigh power
4 mm
Divided into your telescope this gives 120x in a small refractor, 300x in an eight inch Dobsonian and 508x in an eight inch Schmidt-Cassegrain. That is planetary territory, where the limit is almost never the eyepiece but the atmosphere: on an average night the seeing will blur detail before this magnification runs out of sharpness.
Apparent fieldTraditional
52°
Fifty two degrees is the classic Plossl field, and it is narrower than the modern wide-field designs by a visible margin. The practical consequence is drift: in an eight inch Dobsonian this frames about 10 arcminutes, so on an undriven mount an object crosses the view and needs nudging back roughly every 1 minute or so.
Eye reliefTight
3 mm
This is the Plossl design's real weakness. Eye relief runs at roughly seven tenths of the focal length, so at 4 millimetres you get about 3, which means pressing your eye close to a small lens and fogging it with every breath. It is entirely usable and it is not comfortable, and it is why most people reach for a Barlow and a longer eyepiece instead.
Optical designSimple and sharp
4-element Plössl
Four elements in two cemented pairs, which is the arrangement that has defined this eyepiece type since the 1860s. It is sharp on axis, colour neutral and cheap to make well. What it does not do is hold the field flat: in a telescope faster than about f/6 the outer fifth of the view softens visibly, which is the difference between this and the flat-field designs that cost three times as much.
BarrelUniversal
1.25 inch
Fits the 1.25 inch focuser on virtually every telescope made, which is the format's whole advantage. Nothing about this eyepiece requires an adapter, a 2 inch diagonal or a specific focuser, so it works in a beginner refractor and in a large Dobsonian without any thought.
04What comes alive

The targets this eyepiece was born for.

Jupiter’s cloud belts, Saturn’s rings, Lunar craters at the terminator, Izar and tight…
Jupiter’s cloud beltsComes alive
Jupiter’s cloud belts
The two main belts and festoons resolve on steady nights.
Saturn’s ringsComes alive
Saturn’s rings
The Cassini Division opens up at this magnification.
Lunar craters at the terminatorExcellent
Lunar craters at the terminator
Crater walls and central peaks throw hard shadows.
Excellent
Izar and tight double stars
Splits pairs under an arcsecond apart when the air is calm.
Mars at oppositionExcellent
Mars at opposition
Polar cap and dark albedo markings when Mars is close.
The Ring Nebula (M57)Excellent
The Ring Nebula (M57)
Tightens the smoke ring and darkens the sky around it.
05Role in a collection

Where it sits in your eyepiece case.

Where it sits across 3 focal lengths

A good eyepiece collection isn’t a pile of glass — it’s two or three focal lengths that hand off from wide to tight. This is the anchor at the bottom of the ladder: the one you find and frame with. You buy it once, and it stays your low-power eyepiece for life.

32 mm
Low power · wide field
Find, frame and sweep. Your widest true field and the lowest useful magnification, the eyepiece you reach for first.
Veil Nebula · Pleiades · Milky Way star clouds
13 mm
Medium power · the workhorse
Where most deep-sky objects look their best, galaxies, globulars and planetary nebulae at a comfortable scale.
M13 · Ring Nebula · most galaxies
4 mmThis one
High power · detail
Planets, the Moon at the terminator, tight double stars and the smallest planetary nebulae. Steady-air nights only.
Jupiter · Saturn · lunar craters
06Build around it

What it pairs with.

The telescope · Pair it · Filter it
The telescope
What it goes into
Any 1.25-inch focuser
Fits essentially every telescope made.
Longer focal length scopes reward it
The higher the scope’s focal length, the more planetary scale you get from this eyepiece.
Pair it
What extends its range
A low-power finder eyepiece (~6mm)
Use the long eyepiece to locate and frame, then drop in this one for detail.
Filter it
What transforms the view
A neutral-density Moon filter
Tames the glare of a bright gibbous Moon at high power so detail returns.
07The Polaris Score

How it scores, and why.

67/100
67/ 100
Celestron Omni 4mm

Judged as a dedicated high-power eyepiece for planets, the Moon and tight double stars, it earns its score on simple four-element plössl optics: sharp on-axis and high in contrast. Tighter eye relief at this focal length is the main thing holding it back. It is honest about being a specialist, not an all-rounder.

Optical quality
70
Immersion
60
Comfort
42
Value
84

Polaris weighs how well the eyepiece does the job it was designed for — optics, immersion, comfort and value — never list price or brand.

08Who it's for

An honest fit check.

Observers who want a dedicated high-power eyepiece for planets, the Moon and tight double…
Buy it if you
✓Observers who want a dedicated high-power eyepiece for planets, the Moon and tight double stars.
✓Observers happy to take their glasses off at the eyepiece.
✓Observers who value on-axis contrast over a sprawling field.
Look elsewhere if you
!Anyone who has looked through a wide-field eyepiece and cannot go back to a 52-degree tunnel.
!Eyeglass wearers, who will not comfortably reach the full field at this focal length.
!Anyone hoping to frame large nebulae; that is a job for a low-power eyepiece.
09The numbers

Full specifications.

4 measured and stated values
Optical
Focal Length4mm
Apparent Field52°
Comfort & handling
Eye Relief3mm
Fit & build
Barrel Size1.25"
Editor’s Take

Point it at a planet on a steady night and the case makes itself. This is where the Omni Plössl earns its keep.

— The Skynocular verdict
10Alternatives

For a different goal, start here.

3 alternatives, each mapped to the goal it wins at
Wider, lower power

Celestron Omni Plössl 6mm

The same optics with more true field, for framing larger objects and finding your way around.

→
More magnification

Celestron Omni Plössl 3mm

The same line at higher power, for planets and the smallest deep-sky detail.

Cross-shop

Celestron X-Cel LX

A natural alternative at a nearby price point, if you want to compare field and feel before committing.

→