How to Choose Telescope Eyepieces

Divide your telescope's focal length by the eyepiece. On a 1,200 mm Dobsonian, 25 mm gives 48x, 10 mm gives 120x and 6 mm gives 200x. Those three steps cover most nights.

Low power near 50x frames big clusters and the whole Moon. The middle step near 120x is the everyday planet view. High power near 200x is for steady nights. Past 406x, an 8 in scope has no more detail to show.

So you buy by focal length first. Field, eye relief and barrel size come second. Brand comes last.

Read the focal length on the tube label. It is printed in millimeters, such as 660 mm or 1,200 mm. Divide it by 50, 120 and 200. The three answers are your eyepiece focal lengths.

A 1,200 mm scope gives 24 mm, 10 mm and 6 mm. A 660 mm refractor gives 13 mm, 5.5 mm and 3.3 mm. The same 10 mm eyepiece is 120x on one scope and 66x on the other.

Two limits frame the range. The low end is aperture in mm divided by 7. For 203 mm that is 29x. The high end is twice the aperture in mm. For 203 mm that is 406x.

Most nights the air stops you first. From a suburb, 200x to 250x is often the useful top on an 8 in scope. That is why the high step sits at 200x, not at the ceiling.

Write your three numbers on a card. You will use them every time you shop. They also tell you which box eyepiece to keep and which to replace.

Read across your row. A box 25 mm and 10 mm already cover the first two steps on 1,200 mm. The gap is the 6 mm step at 200x.

On short scopes the high step gets tiny. A 450 mm reflector needs 2.3 mm for 200x. Few eyepieces go that short, so a 2x Barlow on a 4.5 mm is the usual route.

Long scopes flip the problem. A 2,032 mm SCT reaches 51x only with a 40 mm. The middle step is 17 mm and the high step is 10 mm. Short eyepieces add little there.

The middle step deserves a better design. You spend the most time at 120x to 133x. A 9 mm with a 60° field puts 0.45° of sky in view on 1,200 mm.

Apparent field is how wide the circle looks to your eye. A Plössl shows about 52°. Wide designs show 60° to 68°. Ultra-wide designs reach 82°.

True field is the patch of sky you see. Divide apparent field by power. A 68° eyepiece at 60x shows 1.13°. A 52° eyepiece at 200x shows 0.26°.

The Moon is about 0.52° wide. At 48x it fits with room on both sides. At 200x you see part of it, so you pan along the terminator.

Field also changes tracking. A Dobsonian has no motor. At 200x a planet crosses a 0.26° field in about a minute. At 133x with a 60° field you get about twice that time.

So a wider field buys time at high power. That matters more on a manual mount than on a GoTo mount. On a tracking mount, 52° is enough.

Wide fields also help you find things. At 48x a 60° eyepiece shows 1.25°. That is enough to star hop with a chart. A 44° eyepiece at the same power shows 0.92°.

Eye relief is the distance from the top lens to your eye. Under 10 mm feels cramped. With glasses you usually need 15 mm or more.

A Plössl's eye relief is about 0.7 times its focal length. A 25 mm gives about 17 mm. A 6 mm gives about 4 mm. That is tight even without glasses.

Modern designs fix short focal lengths with extra lenses. The X-Cel LX line keeps 16 mm across the range, from 25 mm down to 5 mm. The field stays at 60°.

If you wear glasses for astigmatism, keep them on. If you are only near or far sighted, you can take them off and refocus. Then eye relief matters less.

Exit pupil matters too. It is the eyepiece focal length divided by the focal ratio. On f/5.9, 25 mm gives 4.2 mm and 6 mm gives 1.0 mm.

Keep the exit pupil above about 0.5 mm. Below that the view goes dim and floaters show. On f/5.9 that floor is a 3 mm eyepiece at 400x.

A Plössl uses four lenses. It shows about 52° and costs little. It is sharp at f/6 and slower. Its weak point is eye relief below 10 mm.

A wide field design adds lenses for 60° to 82°. It suits slow and medium scopes. At f/4.5 and faster, cheaper ones stretch stars near the edge.

A zoom covers several steps in one barrel. An 8 to 24 mm zoom gives 50x to 150x on 1,200 mm. Its field runs from about 40° at 24 mm to 60° at 8 mm.

Zooms suit a first night and a smart phone adapter. You change power with a twist, so the target stays centered. The trade is a narrow 40° view at the low end.

Ultra-wide designs at 82° are heavy and costly. They make the most sense at 100x to 150x on a manual Dobsonian, where a wide field buys tracking time. On a tracking mount the gain is smaller.

So pick the step from Step 1, then the design. Budget eyepieces are fine at 48x. Spend on the 133x and 200x steps, where comfort shows. The three picks below cover three jobs on 1,200 mm.

Each pick fills a different step. The 6 mm is the 200x step. The 20 mm is a wide 60x step. The 25 mm is a comfortable 48x step for glasses.

Focal ratio is focal length divided by aperture. A 1,200 mm, 203 mm Dobsonian is f/5.9. A 660 mm, 102 mm refractor is f/6.5. A 2,032 mm SCT is f/10.

Slow scopes forgive eyepieces. At f/10 almost any design stays sharp to the edge. At f/5.9 a Plössl and most 60° to 68° designs still work well.

Fast scopes ask more. At f/4.5, a cheap wide field shows stars as short lines in the outer 20% of the view. The center stays sharp.

So match the design to the ratio. On f/4 to f/5, spend on the wide step. On f/8 and slower, a 52° Plössl at 38x already looks clean. That saves money for the 200x step.

Most focusers take 1.25 in eyepieces. Many 8 in and larger Dobsonians also take 2 in, with an adapter in the box. Check the focuser before you buy.

The barrel caps the field. A 1.25 in barrel allows a field stop near 27 mm. A 2 in barrel allows about 46 mm. True field is field stop ÷ focal length × 57.3.

On 1,200 mm, 27 mm gives 1.29°. 46 mm gives 2.20°. So only the low step gains from 2 in. Above 60x, 1.25 in does the job.

A 34 mm with a 72° field shows about 2.0° at 35x. That frames large star fields in one view. It also needs a firm focuser.

A 2 in eyepiece can weigh 2 or 3 times a 1.25 in one. Tighten the altitude tension on a Dobsonian. Keep the 1.25 in adapter on the table for the next swap.

A 2x Barlow doubles the power of every eyepiece in it. Two eyepieces and a Barlow give 4 steps. Pick focal lengths that do not collide.

A 25 mm and a 12 mm with a 2x Barlow give 48x, 96x, 100x and 200x on 1,200 mm. Two steps nearly match. A 25 mm and a 9 mm give 48x, 96x, 133x and 267x. That spreads better.

A Barlow also keeps eye relief. A 12 mm in a 2x Barlow acts like a 6 mm but keeps the 12 mm's relief. For glasses, that can settle it. Our Barlow guide covers 2x versus 3x.

A kit bundles four or five eyepieces, a Barlow, filters and a case. It suits a first upgrade if you own only the box eyepieces. The cost is overlap.

Run the division first. Five Plössls at 6, 8, 13, 17 and 32 mm give 200x, 150x, 92x, 71x and 38x on 1,200 mm. A 2x Barlow turns the 17 mm into 141x.

The short kit eyepieces have tight eye relief. Plan to replace the 6 mm first if you wear glasses. The filters are mostly colored glass. A neutral Moon filter is the one most people keep using at 38x near full Moon. The 32 mm Plössl shows 44°, so the case still holds a useful low step.

If you already own a 25 mm and a 10 mm, one 6 mm adds more than most kits. If you own nothing beyond the box, a kit covers 38x to 200x at once. Our kit comparison lists 6 sets by focal length.

Here is one path for an 8 in f/5.9 Dobsonian. Keep the box 25 mm for 48x. Add a 9 mm for 133x. Add a 6 mm for 200x.

Then add one 2 in low power eyepiece if the focuser allows it. A 34 mm at 35x shows about 2.0°. Four eyepieces then cover 35x to 200x.

On a 660 mm refractor, use shorter numbers. Keep a 13 mm for 51x. Add a 5.5 mm for 120x. A 2x Barlow on the 5.5 mm gives 240x, past the 204x ceiling, so skip that step.

On a 1,325 mm Maksutov, use longer ones. A 26.5 mm gives 50x and an 11 mm gives 120x. The 102 mm ceiling is 204x, so 6.6 mm is the top step.

Three is enough for most people. Four covers almost every target. Past five, you swap more than you look.

Spacing matters more than count. Each step should be about 1.5 to 2.5 times the last. On 1,200 mm, 48x, 120x and 200x sit 2.5x and 1.7x apart. That spacing feels natural at the eyepiece.

A 4th step fills the widest view. A 5th step at 300x sees use only a few nights a year. From a suburb, skip it until the first 4 feel limiting.

Count your box eyepieces first. Many kits ship a 25 mm and a 10 mm. Those are already 2 of the 3 steps on 1,200 mm.

Start with focal length in mm. Then read apparent field in degrees. Then eye relief in mm. Then barrel size, 1.25 in or 2 in.

Skip claims like 675x on a 60 mm scope. That number comes from 900 mm × 3 ÷ 4 mm. The 60 mm ceiling is 120x, so the top 555x is empty power.

Check the field stop if it is listed. Divide it by your focal length and multiply by 57.3. That gives the true field without any guess.

If a listing gives no focal length, skip it. If it gives no apparent field, assume about 50°. If eye relief is missing, a short Plössl will be tight, near 0.7 times its focal length.

Weight matters on small mounts. A 2 in eyepiece can pass 500 g. A light tabletop scope may tip at high altitude with it.

Keep caps on both ends between uses. Dust is the main problem. A blower bulb clears most of it in 10 seconds.

For fingerprints, use 1 drop of lens fluid on a folded tissue. Wipe from the center out with light pressure. Do it rarely.

Dew forms on cold nights, often from your breath. Keep spare eyepieces in a closed case until you need them. Never wipe a dewed lens. Let it dry indoors with the caps off, then cap it.