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

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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.

See this field in your own scope in the Simulator →

01What it's good at

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

Best at

✓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.

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.

02In plain English

The numbers, in plain English.

Focal lengthHigh power

4mm

Sets the magnification: shorter is higher power. At 4mm this is a dedicated high-power eyepiece for planets, the Moon and tight double stars.

Apparent fieldStandard

52°

A traditional field; the object drifts through faster on an undriven mount.

Eye reliefTight

3mm

You will get the full field more easily with glasses off.

Barrel1.25-inch

1.25"

Fits the 1.25-inch focuser on virtually every telescope.

OpticsCoatings

multi-coated optics

Multi-coatings hold contrast and cut internal reflections on bright objects.

03What comes alive

The targets this eyepiece was born for.

Comes alive

Jupiter’s cloud belts

The two main belts and festoons resolve on steady nights.

Comes alive

Saturn’s rings

The Cassini Division opens up at this magnification.

Excellent

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.

Excellent

Mars at opposition

Polar cap and dark albedo markings when Mars is close.

Excellent

The Ring Nebula (M57)

Tightens the smoke ring and darkens the sky around it.

04Role in a collection

Where it sits in your eyepiece case.

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

04Build around it

What it pairs with.

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.

02The Polaris Score

How it scores, and why.

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

Immersion

Comfort

Value

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

03Who it's for

An honest fit check.

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.

05The numbers

Full specifications.

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

06Alternatives

For a different goal, start here.

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.

→

Celestron Omni 4mm

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

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

One eyepiece. A different job in every scope.

The numbers, in plain English.

The targets this eyepiece was born for.

Where it sits in your eyepiece case.

What it pairs with.

How it scores, and why.

An honest fit check.

Full specifications.

For a different goal, start here.

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