Celestron Omni 40mm — image 1
Celestron

Celestron Omni 40mm

A 40mm Celestron Omni Plössl: a wide-field, low-power eyepiece for the biggest objects and rich star fields, from a clean, honest 43-degree Plössl that does one job well for very little money.

74/ 100
Polaris Score
Optical quality · Immersion · Comfort · Value
See the breakdown ↓
What it's for
Low power · wide fieldHigh power · narrow field

Low power · wide field. At 40mm this is a wide-field, low-power eyepiece for the biggest objects and rich star fields; it is not the eyepiece for high-power planetary detail.

Apparent Field
43°
Barrel Size
1.25"
Eye Relief
28mm
Focal Length
40mm
From$52.95

Lowest in 90 daysLowest ever tracked

Price tracked by PolarisObserved 101 timesTracking since Jul 2026

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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.

43° apparent field

At 43°, 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.

Inexpensive enough to build a whole starter set around.
Best at
✓Inexpensive enough to build a whole starter set around.
✓Light and small; no strain on any focuser.
✓At 40mm it lands squarely in low power · wide field territory, exactly where wide star fields and large objects belong.
Trade-offs
!Short eye relief at the short focal lengths, uncomfortable below about 10mm.
!A 43-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.

12× in a small refractor, 51× in a long SCT

An eyepiece has no magnification of its own — the telescope decides. The same eyepiece becomes 12× and 3.6° of sky in a little refractor, or 51× 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
12×
True field
3.6°
Exit pupil
7.3 mm
12× across a 3.6° field at a 7.3mm exit pupil — wider than a dark-adapted pupil, so the outer light is wasted and a reflector may show its secondary shadow.
8-inch Dobsonian
1200 mm · f/6
Magnification
30×
True field
1.4°
Exit pupil
6.7 mm
30× across a 1.4° field at a 6.7mm exit pupil — wider than a dark-adapted pupil, so the outer light is wasted and a reflector may show its secondary shadow.
12-inch Dobsonian
1500 mm · f/5
Magnification
38×
True field
1.1°
Exit pupil
8.0 mm
38× across a 1.1° field at a 8mm exit pupil — wider than a dark-adapted pupil, so the outer light is wasted and a reflector may show its secondary shadow.
8-inch SCT
2032 mm · f/10
Magnification
51×
True field
51′
Exit pupil
4.0 mm
51× across a 51′ field at a 4mm exit pupil — right in the deep-sky sweet spot, bright and easy on the eye.
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.

40mm FL · 43° AFOV · 1.25"
Focal lengthLow power · wide field
40mm
About 30x in an 8 inch f/5.9 Dobsonian and 51x in an 8 inch SCT. In the fast Dobsonian that works out to a 6.8 mm exit pupil, wider than most adult eyes open at night, so some of the light never reaches your retina. In the f/10 SCT it settles at a well matched 4 mm.
Apparent fieldStandard
43°
Narrower than the 52 degrees of the shorter Omnis, and that is a consequence of the format rather than a fault. At 40 mm the 1.25 inch barrel clips the field stop, so you look through a smaller window than the rest of the line offers.
Eye reliefGlasses-friendly
28mm
Nearly three centimetres, comfortable enough to observe in spectacles and to hand around at an outreach night. The trade is that your eye has to sit well back before the whole field appears.
BarrelUniversal fit
1.25"
The reason this is a 43 degree design. A 1.25 inch barrel caps the field stop near 27 mm, so a 40 mm Plössl shows almost the same patch of sky as a 32 mm one, just at lower power. More true field needs a 2 inch eyepiece.
OpticsFour-element Plössl
multi-coated optics
Four multi-coated elements: few air to glass surfaces, little scatter, and sharp on axis for very little money. At low power the simple design is an asset, since there is no complex glass to soften the view.
04What comes alive

The targets this eyepiece was born for.

The Veil Nebula, The North America Nebula, The Pleiades (M45), Milky Way star clouds, The…
The Veil NebulaComes alive
The Veil Nebula
With an OIII filter the lacework wraps across the field.
The North America NebulaExcellent
The North America Nebula
A wide, faint target that only fits at long focal length.
The Pleiades (M45)Comes alive
The Pleiades (M45)
The full cluster framed with room around every star.
Excellent
Milky Way star clouds
Sweeping the summer Milky Way at its richest.
The Andromeda Galaxy (M31)Excellent
The Andromeda Galaxy (M31)
The full span of the galaxy and its dust lanes.
The Double ClusterComes alive
The Double Cluster
Both clusters with a generous star field between them.
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.

40 mmThis one
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
7 mm
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.
Slower scopes suit it best
At 40mm a fast f/5 scope pushes the exit pupil past 8mm, wider than your eye can use. An f/8 or f/10 scope puts it back in range.
Pair it
What extends its range
A high-power eyepiece (~10mm)
Locate and frame with this one, then swap to high power for planets and small objects.
Filter it
What transforms the view
A UHC or OIII nebula filter
At low power this eyepiece frames big emission nebulae; a narrowband filter makes the Veil and North America jump out.
07The Polaris Score

How it scores, and why.

74/100
74/ 100
Celestron Omni 40mm

Judged as a wide-field, low-power eyepiece for the biggest objects and rich star fields, it earns its score on simple four-element plössl optics: sharp on-axis and high in contrast. The comfortable eye relief lifts it further. A wide field is what this focal length is for, and it delivers.

Optical quality
70
Immersion
56
Comfort
94
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 wide-field, low-power eyepiece for the biggest objects and rich star…
Buy it if you
✓Observers who want a wide-field, low-power eyepiece for the biggest objects and rich star fields.
✓Anyone who observes in eyeglasses; the eye relief here is long enough to see the whole field.
✓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 43-degree tunnel.
!Grab-and-go observers watching every gram, since this is not the lightest option.
!Planetary specialists who want the last ounce of high-power contrast from an orthoscopic.
09The numbers

Full specifications.

4 measured and stated values
Optical
Focal Length40mm
Apparent Field43°
Comfort & handling
Eye Relief28mm
Fit & build
Barrel Size1.25"
Editor’s Take

Sweep it across a rich star field and the reason to own this focal length is obvious in a second. That is what the Omni Plössl is for.

— 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 56mm

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

→
More magnification

Celestron Omni 32mm

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.

→