Explore Scientific

Explore Scientific 20mm 100°

A 20mm Explore Scientific 100° Series: a low-power eyepiece for framing large nebulae, galaxies and open clusters, from a 100-degree hyper-wide that puts the field edge out past the corner of your eye.

86/ 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

100°

Barrel Size

Eye Relief

14.5mm

Focal Length

20mm

From$499

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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.

100° apparent field

At 100°, the edge slips into your peripheral vision, you stop looking at a field and start looking into the sky.

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

✓Argon-purged and waterproof, so it will not fog in cold, damp air.

✓Excellent correction across a field this wide.

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

Trade-offs

!Big and heavy; the long focal lengths need a sturdy focuser and can shift balance.

!Eye relief is tight for the field, and eyeglass wearers will miss the last few degrees.

!Expensive, and you pay for the field more than for on-axis sharpness.

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 4.2° 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

4.2°

Exit pupil

3.6 mm

24× across a 4.2° 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

1.7°

Exit pupil

3.3 mm

60× across a 1.7° 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

1.3°

Exit pupil

4.0 mm

75× across a 1.3° 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

59′

Exit pupil

2.0 mm

102× across a 59′ 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

About 60x in an 8 inch f/5.9 Dobsonian and 102x in an 8 inch SCT. Low enough to find and frame large objects, high enough to hold a dark sky background under suburban light.

Apparent fieldSpacewalk

100°

A 100 degree field at this focal length gives an enormous true field, enough to hold the Veil complex or a rich stretch of the Milky Way with the edges beyond your peripheral vision.

Eye reliefComfortable

14.5mm

Comfortable without glasses and usable with thin frames. At the longer focal lengths in this range the eye relief stops being the limiting factor, and the eyepiece's weight becomes the practical concern instead.

BarrelRequires 2 inch focuser

Every eyepiece in the 100 degree range uses a 2 inch fitting, so a 2 inch focuser or diagonal is required; only the 30 mm model steps up to 3 inch. These are also heavy pieces of glass, so expect to rebalance a Dobsonian and to check that a light focuser holds one securely.

OpticsFlagship wide-field

EMD fully multi-coated

Enhanced coatings on many elements are what keep an eyepiece this complex from washing out. Waterproof and argon purged, so damp nights do not fog the internal surfaces.

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 2-inch focuser

A 2-inch focuser or a 1.25-inch adapter; most Dobsonians and larger refractors qualify.

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.

86/ 100

Explore Scientific 20mm 100°

Judged as a low-power eyepiece for framing large nebulae, galaxies and open clusters, it earns its score on a 100-degree apparent field: the closest thing to a spacewalk short of an ethos. Tighter eye relief at this focal length is the main thing holding it back. A wide field is what this focal length is for, and it delivers.

Optical quality

Immersion

100

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.

✓Observers happy to take their glasses off at the eyepiece.

✓Deep-sky observers chasing the most immersive, wrap-around field they can get at this price.

Look elsewhere if you

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

!Eyeglass wearers, who will not comfortably reach the full field at this focal length.

!Planetary specialists who want the last ounce of high-power contrast from an orthoscopic.

05The numbers

Full specifications.

Optical

Focal Length20mm

Apparent Field100°

Comfort & handling

Eye Relief14.5mm

Fit & build

Barrel Size2"

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 100° Series is for.

— The Skynocular verdict

06Alternatives

For a different goal, start here.

Wider, lower power

Explore Scientific 100° Series 32mm

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

More magnification

Explore Scientific 100° Series 12mm

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

Cross-shop

Tele Vue Nagler

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

→

Explore Scientific 20mm 100°

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.