Apertura

Apertura Super Wide Angle 20mm

A 20mm Apertura Super Wide Angle: a low-power eyepiece for framing large nebulae, galaxies and open clusters, from a budget 70-degree wide-field, a big step up from a kit Plössl for little money.

79/ 100

Polaris Score

Optical quality · Immersion · Comfort · Value

See the breakdown ↓

What it's for

Low power · wide fieldHigh power · narrow field

Low power. At 20mm this is a low-power eyepiece for framing large nebulae, galaxies and open clusters; it is not the eyepiece for high-power planetary detail.

Apparent Field

70°

Barrel Size

1.25"

Eye Relief

18mm

Focal Length

20mm

From$79.95

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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.

68° apparent field

At 70°, the black rim sits out at the corner of your attention rather than in front of you.

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

✓Inexpensive enough to fill out a focal-length range cheaply.

✓Good on-axis sharpness for the price.

✓At 20mm it lands squarely in low power territory, exactly where wide star fields and large objects belong.

Trade-offs

!Eye relief is tight at the short focal lengths.

!The edge of the field softens in fast scopes.

!Basic build compared with premium wide-fields.

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 24× and 2.9° of sky in a little refractor, or 102× 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

24×

True field

2.9°

Exit pupil

3.6 mm

24× across a 2.9° field at a 3.6mm exit pupil, right in the deep-sky sweet spot, bright and easy on the eye.

8-inch Dobsonian

1200 mm · f/6

Magnification

60×

True field

1.2°

Exit pupil

3.3 mm

60× across a 1.2° field at a 3.3mm exit pupil, right in the deep-sky sweet spot, bright and easy on the eye.

12-inch Dobsonian

1500 mm · f/5

Magnification

75×

True field

56′

Exit pupil

4.0 mm

75× across a 56′ field at a 4mm exit pupil, right in the deep-sky sweet spot, bright and easy on the eye.

8-inch SCT

2032 mm · f/10

Magnification

102×

True field

41′

Exit pupil

2.0 mm

102× across a 41′ field at a 2mm exit pupil, crisp medium power for galaxies, clusters and planetary nebulae.

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 lengthLow to mid power

20mm

Around 60x in an 8 inch f/5.9 Dobsonian and 102x in an 8 inch SCT. Low enough to locate objects comfortably and high enough to show real detail once you are there.

Apparent fieldWide

70°

At 20 mm a 70 degree apparent field gives a true field wide enough to frame most open clusters whole. That combination makes it a natural main eyepiece for a night of deep-sky observing.

Eye reliefGlasses-friendly

18mm

Eighteen millimetres is enough to see the full field while wearing glasses, which matters if you have astigmatism that a telescope's focuser cannot correct. It also makes long observing sessions less tiring.

BarrelStandard

1.25"

Fits every standard focuser and diagonal. At 20 mm and 70 degrees the field stop is close to the practical limit of the 1.25 inch format, so longer focal lengths in this series move to 2 inch.

OpticsBudget wide-field

Multi-coated

Multi-coating keeps transmission and contrast respectable through a wide-field design with several elements. Expect a sharp centre and some edge softness in fast telescopes.

03What comes alive

The targets this eyepiece was born for.

Comes alive

The Orion Nebula (M42)

Wings of nebulosity fill the field with the Trapezium embedded.

Excellent

The Andromeda Galaxy (M31)

The bright core with its two companion galaxies framed.

Excellent

The Pleiades (M45)

The whole cluster with dark sky to spare around it.

Comes alive

The Double Cluster

Two open clusters glittering side by side in one view.

Excellent

The Beehive (M44)

A loose scatter of bright stars, best at low power.

Excellent

Large open clusters

Bright, wide targets that want field, not magnification.

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.

20 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

04Build around it

What it pairs with.

The telescope

What it goes into

Any 1.25-inch focuser

Fits essentially every telescope made.

Fast, short scopes love this field

A fast Newtonian or refractor turns this focal length into a rich, wide sweep.

Pair it

What extends its range

A high-power eyepiece (~12mm)

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.

02The Polaris Score

How it scores, and why.

79/ 100

Apertura Super Wide Angle 20mm

Judged as a low-power eyepiece for framing large nebulae, galaxies and open clusters, it earns its score on a 70-degree field that feels wide open after a kit eyepiece. The comfortable eye relief lifts it further. A wide field is what this focal length is for, and it delivers.

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 low-power eyepiece for framing large nebulae, galaxies and open clusters.

✓Anyone who observes in eyeglasses; the eye relief here is long enough to see the whole field.

✓Observers who want a comfortably wide field without paying ultra-wide prices.

Look elsewhere if you

!Anyone chasing the absolute widest field, who should look at a 100-degree eyepiece.

!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.

05The numbers

Full specifications.

Optical

Focal Length20mm

Apparent Field70°

Comfort & handling

Eye Relief18mm

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 Super Wide Angle is for.

— The Skynocular verdict

06Alternatives

For a different goal, start here.

Wider, lower power

Apertura Super Wide Angle 32mm

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

→

More magnification

Apertura Super Wide Angle 12mm

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

Cross-shop

Explore Scientific 68°

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

→

Apertura Super Wide Angle 20mm

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.