Best Telescope Diagonals in 2026: 9 Star Diagonals for Every Scope

The best telescope diagonal for most people is the Celestron 1.25-inch Twist-Lock Dielectric Star Diagonal. Its 99% rated dielectric mirror reflects more light than a standard aluminum diagonal, and the twist-lock holds the eyepiece centered. If your focuser takes 2-inch eyepieces, buy the Celestron 2-inch Twist-Lock. On a tight budget, buy the SVBONY SV188P. SCT and Maksutov owners should look at the Celestron 2-inch SCT XLT. Newtonian and Dobsonian owners do not need a diagonal.

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Not sure which size or type fits? Answer three questions below and the checker shows the diagonals that fit your telescope.

What is a telescope diagonal?

A telescope diagonal, also called a star diagonal, is a mirror or prism that turns the light path 90 degrees (or 45 degrees in an erecting prism) just before the eyepiece. Refractors, Schmidt-Cassegrains (SCTs) and Maksutovs put the eyepiece at the back of the tube, so pointing at a high target would mean lying on the ground. The diagonal moves the eyepiece to the side and lets you look down into it while seated.

The diagonal sits in the light path, so its quality affects every view. A cheap one can lose light, scatter it and soften the image. The one packed with an entry-level telescope is usually aluminum-coated and serviceable. An upgrade makes the most difference on small refractors and SCTs, where the stock part is often the weakest piece.

Newtonian and Dobsonian telescopes do not need a diagonal

If you own a Newtonian reflector or a Dobsonian, you do not need a star diagonal. These telescopes have the focuser on the side of the tube near the top, so the eyepiece already points sideways. The secondary mirror inside the tube does the job a diagonal does in other designs. This includes tabletop Dobsonians such as the Sky-Watcher Heritage models in our best tabletop telescopes guide.

Diagonals are for refractors, SCTs, Maksutovs and other scopes with a rear focuser. If you are not sure what you own, read Dobsonian vs Newtonian and types of reflecting telescopes. Every pick below is for a rear-focuser telescope.

How to choose a star diagonal

Five things decide which diagonal to buy: barrel size, mirror or prism, coating, how it holds the eyepiece and weight.

Barrel size: 1.25-inch or 2-inch

Match the diagonal to your focuser. Almost every beginner telescope has a 1.25-inch focuser. Larger refractors, SCTs and Maksutovs often have 2-inch focusers or visual backs that accept both sizes. Look at the eyepieces you own: the barrel size is usually engraved on them. You can also measure the opening of the focuser or check the telescope’s specifications.

A 2-inch diagonal takes 2-inch eyepieces, which pass a wider field of view, and most come with a 1.25-inch adapter, so one diagonal serves both sizes. The cost is weight. The 2-inch picks here weigh between about 455 and 690 grams, and a light focuser can sag under that. The 1.25-inch picks weigh between about 125 and 340 grams.

Mirror or prism

Mirror diagonals use a flat mirror and reflect light with no glass in the path. Prism diagonals use a glass prism and total internal reflection. Most modern 90 degree diagonals are dielectric mirrors, and they suit any telescope. Mirror diagonals flip the image left to right, which does not matter for the Moon, planets and deep-sky objects.

Prisms matter in two cases. An erecting prism, usually 45 degrees, corrects the flip and suits daytime viewing of birds and landscapes. A true 90 degree prism diagonal can work well on slower refractors but can add aberration on fast ones, roughly below f/7. We found no 1.25-inch 90 degree prism diagonal sold new on Amazon US in October 2026, so the 90 degree picks below are mirrors. Our prism vs mirror diagonals guide covers the trade-offs in detail.

Coating: aluminum or dielectric

A standard aluminum coating reflects roughly 90% of visible light. A dielectric coating is a stack of thin layers, and makers rate the good ones at 99%. Real-world numbers sit a little lower, but the gain over aluminum is still visible as a brighter, higher-contrast image, especially on faint objects. Seven of the nine picks here are dielectric. The exceptions are the Celestron SCT XLT, which is enhanced aluminum, and the erecting prism.

Eyepiece lock

Thumbscrews are the cheapest design. They can scratch barrels and push the eyepiece off center. Compression rings grip the barrel evenly. Twist-lock and ClickLock clamps are quicker and hold eyepieces square, which helps when you swap eyepieces in the dark or with gloves on.

Weight and focuser load

A heavy 2-inch diagonal with a heavy eyepiece can make a light focuser slip or tilt. Check the focuser’s load rating before you buy a diagonal at the heavy end of the range. If it sags, tighten the focuser tension or choose a 1.25-inch unit.

Will a star diagonal improve your image?

Only if the diagonal is the weak link. A better diagonal does not add aperture, so it does nothing for a telescope that gathers too little light. It helps most on small refractors and SCTs with a stock aluminum diagonal, and on any scope where the original has a poorly flat mirror or a loose eyepiece holder.

A simple test shows whether yours is the problem. Look at a distant daytime object or the Moon through the diagonal and note the detail and color. Then remove the diagonal and insert the eyepiece straight into the focuser. A straight-through view is awkward but works for this check. If the straight view is clearly sharper or brighter, upgrade the diagonal. If the two views match, spend the money on eyepieces first. Our best telescope eyepiece set guide covers that route.

The best telescope diagonals in 2026

These nine are the star diagonals sold on Amazon US in October 2026, chosen from what astronomy review sites, retailers and owner reviews recommend. We have not tested them hands-on. The notes draw on published specifications and independent reviews. Orion has closed, and no Orion diagonals appeared in the listings, so none are here.

Best value: Celestron 1.25-Inch Twist-Lock Dielectric Star Diagonal

Best value

Best for: The default upgrade from the diagonal that came with your telescope

Barrel

1.25"

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece

Eyepiece lock

Twist-lock

Weight

213 g

Pros

• Dielectric coating gives brighter, higher-contrast views than a stock aluminum mirror
• Twist-lock holds eyepieces centered without scratching the barrel
• Light at 7.5 oz, with 1.25-inch filter threads on the nosepiece

Cons

• Costs more than the SVBONY budget pick
• 1.25-inch barrel limits the field of view of wide eyepieces
• Mirror-reversed image, not an erect-image diagonal

Weight 7.5 oz (212 g). 1.25-inch eyepiece holder and nosepiece with filter threads. No SCT thread.

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This is the diagonal to buy if you have a 1.25-inch focuser and want a clear step up from the stock part. The BK7 mirror carries a 58-layer dielectric coating rated above 99% reflective, and the machined aluminum body adds 1.25-inch filter threads on the nosepiece. The twist-lock centers the eyepiece and has a stop so it cannot drop out.

At 7.5 oz it is light enough for any focuser. The image is mirror-reversed, as with every non-erecting diagonal. It costs more than the SVBONY, and the gain over a good budget dielectric is smaller than the gain over aluminum. It has a 1.25-inch nosepiece and no SCT thread, so on an SCT it needs a 1.25-inch visual back.

Best budget: SVBONY SV188P 1.25-Inch

Best budget

Best for: The cheapest dielectric diagonal that is worth buying

Barrel

1.25"

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece

Eyepiece lock

Compression ring

Weight

125 g

Pros

• Lowest price of any dielectric diagonal here
• Brass compression ring grips the barrel evenly
• Light at about 125 g

Cons

• Quality varies between units, and some reviewers report poor mirror flatness
• Compression ring is slower to use than a twist-lock

Weight about 125 g (4.4 oz). Brass compression ring, 1.25-inch filter thread, dust caps. The 99% reflectivity is the seller's rating.

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The SV188P is the cheapest dielectric diagonal we would buy. It carries a 99% dielectric coating and holds the eyepiece with a brass compression ring, at about 125 g. It suits small refractors and beginner telescopes whose stock diagonal is weak.

Quality control varies between units, and some reviewers report poor mirror flatness. Test it on a bright star or the Moon when it arrives and return it if the focus looks soft. If your telescope is a well-known brand with 100mm of aperture or more, the stock diagonal may be good enough, and the Celestron above is a safer step up. Our best refractor telescopes for beginners list shows which scopes pair well with a budget diagonal.

Best budget 2-inch: SVBONY SV188P 2-Inch

Best budget 2-inch

Best for: A 2-inch diagonal on a small budget, for wide-field eyepieces

Barrel

2" (takes 1.25" too)

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece

Eyepiece lock

Compression ring

Weight

463 g

Pros

• 2-inch barrel passes the full field of wide eyepieces
• 1.25-inch adapter included
• 99% rated dielectric coating at a low price

Cons

• Heavy at about 460 g for a light focuser
• Quality varies between units, as with the 1.25-inch model
• Compression ring, not a quick-release clamp

Weight about 463 g (16 oz) from a seller listing; another retailer quotes 650 g, probably with packaging. 2-inch nosepiece and 1.25-inch adapter. No SCT thread.

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The 2-inch SV188P is the cheapest way into 2-inch eyepieces. It uses the same 99% dielectric coating as the 1.25-inch model and comes with a 1.25-inch adapter, so your current eyepieces work today and wide-field ones fit later.

It is heavy, at about 460 g by one seller’s listing, so confirm your focuser can carry it. It shares the 1.25-inch model’s variable quality control and uses a compression ring instead of a quick-release clamp. For a clamp at a similar price, look at the SV223 below.

Best quick-release: SVBONY SV223 2-Inch ClickLock

Best quick-release

Best for: Fast, one-handed eyepiece swaps at a mid-range price

Barrel

2" (takes 1.25" too)

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece

Eyepiece lock

ClickLock

Weight

690 g

Pros

• ClickLock clamp swaps eyepieces quickly and holds them square
• Thick 11.5 mm mirror substrate
• Includes a 1.25-inch adapter

Cons

• Heavy, listed at about 690 g
• Fewer independent reviews than the Celestron and Baader units
• No SCT thread

Listed weight 0.69 kg (retailer figure, may include packaging). Optical length 109 mm, 118.6 mm with the adapter. Stainless steel compression rings.

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The SV223 adds a ClickLock clamp: a lever locks the eyepiece in one motion, with no thumbscrew to loosen. The mirror is a thick 11.5 mm substrate rated 1/10 wave, with a 99% dielectric coating, and a 1.25-inch adapter is included.

Weight is listed at about 690 g, and that figure may include packaging. It has fewer independent reviews than the Celestron and Baader units, and no SCT thread. Pick it for fast eyepiece changes on a refractor with a sturdy focuser.

Best all-round 2-inch: Celestron 2-Inch Twist-Lock

Best all-round 2-inch

Best for: One 2-inch diagonal that fits both a refractor and an SCT

Barrel

2" (takes 1.25" too)

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece and SCT thread

Eyepiece lock

Twist-lock

Weight

539 g

Pros

• Includes both a 2-inch barrel and an SCT thread adapter
• 58-layer dielectric coating rated above 99%
• Twist-lock 1.25-inch adapter included

Cons

• About 19 oz with the adapter, heavy for light focusers
• Costs more than the SVBONY 2-inch units
• Mirror-reversed image

Weight 19 oz (539 g) with the 1.25-inch adapter. BK7 mirror, dual compression rings. Ships with the 2-inch barrel, SCT adapter and 1.25-inch stepdown.

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This is the 2-inch diagonal for people who may change telescopes. It ships with a 2-inch barrel for refractor focusers, an SCT adapter for Schmidt-Cassegrain visual backs and a twist-lock 1.25-inch stepdown. It uses the same BK7 mirror and 58-layer dielectric coating as the 1.25-inch version.

It weighs about 19 oz with the adapter, so it is heavy for a light focuser. It costs more than the SVBONY 2-inch units, and the image is mirror-reversed. If you are not sure whether your next telescope will be a refractor or an SCT, this one covers both.

Best for SCT and Mak: Celestron 2-Inch SCT Diagonal XLT

Best for SCT and Mak

Best for: SCT and Maksutov owners who want a 2-inch diagonal that threads on directly

Barrel

2" (takes 1.25" too)

Type

Enhanced aluminum mirror

Reflectivity

Not stated

Angle

90°

Image

Mirror-reversed

Connection

SCT thread

Eyepiece lock

Thumbscrew

Weight

482 g

Pros

• Threads directly onto the SCT visual back
• Includes a 1.25-inch adapter and a storage case
• Lighter than the 2-inch dielectric units at about 17 oz

Cons

• Enhanced aluminum coating, not dielectric
• No 2-inch barrel, so it does not fit a refractor
• Mirror-reversed image

Weight about 17 oz (482 g) per retailer data. SCT male thread, 2-inch eyepiece holder, 2-inch to 1.25-inch adapter and storage case.

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This diagonal screws straight onto an SCT or Maksutov visual back and carries Celestron’s StarBright XLT enhanced aluminum coating. It includes a 2-inch to 1.25-inch adapter and a storage case. It is the simplest route for an SCT owner replacing a stock diagonal.

The coating is enhanced aluminum, not dielectric, so the 2-inch Twist-Lock above has the better coating and also fits SCTs through its adapter. This model does not fit a refractor focuser, so buy it only for an SCT or Mak.

Best for daytime viewing: Celestron 1.25-Inch 45 Degree Erect Image Diagonal

Best for daytime viewing

Best for: Birdwatching and landscapes, where the image must be the right way round

Barrel

1.25"

Type

Erecting prism

Reflectivity

Not stated

Angle

45°

Image

Correct (upright)

Connection

Nosepiece

Eyepiece lock

Thumbscrew

Weight

170 g

Pros

• Upright, left-to-right correct image for land viewing
• 45-degree angle suits a spotting-scope posture
• Light at about 6 oz

Cons

• A prism passes less light than a dielectric mirror
• Some sharpness loss at high power
• Mostly a daytime accessory

Weight 6 oz (170 g). 1.25-inch eyepiece holder and nosepiece, thumbscrew lock.

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Use this one if you want to watch birds, landscapes or boats. The 45 degree erecting prism gives an upright image that is correct left to right, which a mirror diagonal cannot do. The 45 degree angle also gives a comfortable spotting-scope posture. It works on a refractor or Maksutov with a 1.25-inch focuser.

A prism passes less light than a dielectric mirror, and it can lose some sharpness at high power, so it is a daytime accessory first. At about 6 oz it is light.

Best of the best: Tele Vue 1.25-Inch Everbrite

Best of the best

Best for: The diagonal other brands are compared to, for owners of premium eyepieces

Barrel

1.25"

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece

Eyepiece lock

Compression ring

Weight

340 g

Pros

• Pyrex mirror with a 99% rated dielectric coating
• Brass compression ring holds eyepieces square
• Long track record and strong resale value

Cons

• Costs several times the SVBONY budget pick
• The visible gain over the Celestron twist-lock is small on most scopes
• 1.25-inch barrel only

Weight 12 oz (340 g). Brass compression ring, 1.25-inch nosepiece.

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The Everbrite is the diagonal others are measured against. It has a Pyrex mirror with a 99% dielectric coating and a brass compression ring, built to Tele Vue tolerances. At about 12 oz it is heavy for a 1.25-inch unit, which also keeps it stable.

It costs several times as much as the SVBONY, and on most telescopes the visible gain over the Celestron twist-lock is small. Buy it if you use premium eyepieces on a high-quality refractor, or if you want a diagonal that holds its resale value.

Best premium 2-inch: Baader MaxBright 2-Inch ClickLock

Best premium 2-inch

Best for: Serious refractor and wide-field users who want a premium 2-inch diagonal

Barrel

2"

Type

Dielectric mirror

Reflectivity

99%

Angle

90°

Image

Mirror-reversed

Connection

Nosepiece

Eyepiece lock

ClickLock

Weight

455 g

Pros

• 99% rated all-dielectric coating on a 1/10 wave mirror
• ClickLock clamp holds eyepieces tight and square
• Lighter than the Celestron 2-inch at 455 g

Cons

• Most expensive pick in this guide
• Check the listing for a 1.25-inch adapter
• Needs an extra lock ring for SCT use

Weight 0.455 kg, optical length 109 mm. M48 thread on the telescope side, 2-inch ClickLock on the eyepiece side.

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The MaxBright has a mirror ground to 1/10 wave with an all-dielectric coating rated at 99% and a ClickLock clamp that grips the eyepiece centered without marring it. At 455 g it is lighter than the Celestron 2-inch. It fits a 2-inch focuser by M48 thread. On an SCT it needs a separate Baader SCT lock ring.

It is the most expensive pick here. Check the listing for a 1.25-inch adapter before you buy, and add one from the accessories below if it is missing. Choose it for a high-end refractor and wide-field eyepieces.

Best diagonal for SCT and Maksutov telescopes

An SCT or Maksutov takes a diagonal on its visual back, which is a threaded fitting at the rear of the tube. You have three routes. The Celestron 2-inch SCT XLT screws on directly. The Celestron 2-inch Twist-Lock ships with an SCT adapter and has the better coating. Any 2-inch nosepiece diagonal, such as the SVBONY 2-inch models, can connect through a separate SCT to 2-inch adapter. All three accept 1.25-inch eyepieces through an adapter. Our guide to Schmidt-Cassegrain telescope pros and cons explains what to expect from the scope itself, and our travel and portable telescopes list covers compact scopes that use diagonals.

Celestron star diagonals compared

Celestron makes four of the nine picks, and they differ in useful ways. The 1.25-inch Twist-Lock is the default upgrade for a refractor or small scope. The 2-inch Twist-Lock fits refractors and SCTs and is the best all-round Celestron. The 2-inch SCT XLT is the direct-thread option for SCT and Mak owners and uses an enhanced aluminum coating instead of dielectric. The 45 degree erect image diagonal is for daytime viewing. The two twist-lock models share the same mirror and coating, so the choice between them comes down to barrel size and telescope type.

Star diagonal brands

Baader Planetarium makes high-end optics in Germany and has no entry-level line. Its ClickLock clamp is widely copied. Tele Vue makes premium diagonals and eyepieces with a long track record. Celestron covers the mid-range, with dielectric diagonals that cost less than the premium brands. SVBONY sells budget dielectric diagonals, including 2-inch models, at prices the others do not match, with more variation between units.

Star diagonal comparison

This table lists all nine diagonals side by side. On a phone, scroll sideways.

Useful accessories for a star diagonal

These four help when you add a diagonal to your setup.

A 2-inch to 1.25-inch adapter lets a 2-inch diagonal take the eyepieces you already own. An SCT to 2-inch adapter connects a 2-inch nosepiece diagonal to an SCT or Maksutov visual back. A wide-field 10mm eyepiece is a good first test for a new diagonal. A 3x Barlow triples the magnification of any 1.25-inch eyepiece, and a diagonal adds light path, so check that your scope still reaches focus. For more options, see our best Barlow lenses, best budget telescope eyepieces and best Celestron telescope eyepieces.

Which star diagonal should you buy?

• Most people with a 1.25-inch focuser: Celestron 1.25-inch Twist-Lock.
• Tight budget: SVBONY SV188P 1.25-inch.
• A 2-inch focuser on a budget: SVBONY SV188P 2-inch, or the SV223 for a ClickLock clamp.
• One 2-inch diagonal for a refractor and an SCT: Celestron 2-inch Twist-Lock.
• SCT or Maksutov, direct thread: Celestron 2-inch SCT XLT.
• Birdwatching and landscapes: Celestron 45 degree erect image diagonal.
• Premium eyepieces: Tele Vue Everbrite for 1.25-inch, Baader MaxBright for 2-inch.
• Newtonian or Dobsonian: you do not need one.

Still choosing a telescope? Take the Telescope Finder, which asks about who it is for, what you want to see and your budget.

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