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Closed October 10–12 for Thanksgiving weekend
Closed October 10–12 for Thanksgiving weekend

Diagonals

A star diagonal turns an unusable viewing angle into a comfortable one, and in doing so becomes one more optical surface between your objective and your eye. How good that surface is determines whether you keep the contrast the telescope worked to produce. David Astro carries diagonals in Canada — dielectric mirrors, prisms and Amici designs in 1.25" and 2" — in stock and shipping from Canada.

The mirror-versus-prism question has a real answer, and it depends on your telescope's focal ratio. Tell us the telescope and we will make a recommendation you can rely on.

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CA$149.00 - CA$149.00
CA$149.00
CA$149.00 - CA$149.00
CA$149.00

David Astro 99% Dielectric Twist-Lock Diagonal - 1.25"

DAV-DIG-TL-1.25
6 in stock

The 1.25" 99% Dielectric Twist-Lock Diagonal is an essential upgrade for astronomers seeking the best in optical performance and convenience. This...

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CA$149.00 - CA$149.00
CA$149.00
CA$149.00 - CA$149.00
CA$149.00
CA$219.00 - CA$219.00
CA$219.00
CA$219.00 - CA$219.00
CA$219.00

David Astro 99% Dielectric Twist-Lock Diagonal - 2"

DAV-DIG-TL-2
Only 5 left!

The 2" 99% Dielectric Twist-Lock Diagonal is an essential upgrade for astronomers seeking the best in optical performance and convenience. This pre...

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CA$219.00 - CA$219.00
CA$219.00
CA$219.00 - CA$219.00
CA$219.00
CA$199.00 - CA$199.00
CA$199.00
CA$199.00 - CA$199.00
CA$199.00

David Astro Penta Prism Diagonal - 1.25"

DAV-PENTA-1.25
Only 4 left!

The David Astro Penta Prism 90-Degree Diagonal is a non inverting non reversing diagonal. It’s particularly suited for use with the David Astro Pre...

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CA$199.00 - CA$199.00
CA$199.00
CA$199.00 - CA$199.00
CA$199.00
Daystar 2" Dielectric Mirror Diagonal
CA$223.00 - CA$223.00
CA$223.00
CA$223.00 - CA$223.00
CA$223.00

Daystar 2" Dielectric Mirror Diagonal

DAY-DS2MD

2.0" Barrel Size Dielectric Mirror Coating For Telescopes with 2" Focusers 90-Degree Viewing Angle Description FAQ Specifications In the Box Wa...

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CA$223.00 - CA$223.00
CA$223.00
CA$223.00 - CA$223.00
CA$223.00
CA$307.99 - CA$307.99
CA$307.99
CA$307.99 - CA$307.99
CA$307.99

Explore Scientific 99% Reflective Diagonal -2"

ES-DD02-00CF

2-inch Diagonal Barrel with Tapered Safety Edge 99% Reflectivity from Dielectric Coatings Includes 1.25-inch Eyepiece Adapter Non-Marring Brass Co...

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CA$307.99 - CA$307.99
CA$307.99
CA$307.99 - CA$307.99
CA$307.99
CA$825.99 - CA$825.99
CA$825.99
CA$825.99 - CA$825.99
CA$825.99

Explore Scientific 99% Reflective Diagonal -3"

ES-DD03-01CF

3-inch diagonal body for large format focusers 99% reflectivity from durable dielectric coatings Includes a 2-inch eyepiece adapter with compressi...

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CA$825.99 - CA$825.99
CA$825.99
CA$825.99 - CA$825.99
CA$825.99
iOptron Dielectric Star Diagonal (1.25")
CA$111.00 - CA$111.00
CA$111.00
CA$111.00 - CA$111.00
CA$111.00

iOptron Dielectric Star Diagonal (1.25")

IOP-6231

1.25" Barrel Diameter Dielectric Mirror Coatings Improves Image Brightness and Contrast Durable, Long-Lasting Reflective Surface Ideal for Refract...

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CA$111.00 - CA$111.00
CA$111.00
CA$111.00 - CA$111.00
CA$111.00
iOptron Dielectric Star Diagonal (2'')
CA$207.00 - CA$207.00
CA$207.00
CA$207.00 - CA$207.00
CA$207.00

iOptron Dielectric Star Diagonal (2'')

IOP-6232

2-inch Barrel for Wide-Field Eyepieces Multi-Layer Dielectric Mirror Coatings High-Reflectivity for Brighter Images Enhanced Coating Durability an...

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CA$207.00 - CA$207.00
CA$207.00
CA$207.00 - CA$207.00
CA$207.00
SVBONY SV188 90° Zenith Mirror Diagonal - 1.25"
CA$34.00 - CA$34.00
CA$34.00
CA$34.00 - CA$34.00
CA$34.00

SVBONY SV188 90° Zenith Mirror Diagonal - 1.25"

SV-F9171A

Provides a comfortable 90-degree viewing angle Standard 1.25" barrel fits most refractor and SCT focusers Features a 90% reflectivity mirror for b...

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CA$34.00 - CA$34.00
CA$34.00
CA$34.00 - CA$34.00
CA$34.00
SVBONY SV188P 90° Dielectric Mirror Diagonal - 2"
SVBONY SV188P 90° Dielectric Mirror Diagonal - 2"
CA$88.00 - CA$88.00
CA$88.00
CA$88.00 - CA$88.00
CA$88.00

SVBONY SV188P 90° Dielectric Mirror Diagonal - 2"

SV-F9171D

99% Reflectivity Dielectric Coatings 90-Degree Viewing Angle 11mm Thick Mirror Substrate Durable Aluminum Alloy Body Brass Compression R...

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CA$88.00 - CA$88.00
CA$88.00
CA$88.00 - CA$88.00
CA$88.00
SVBONY SV221 45° Correct Image Prism Diagonal - 1.25"
SVBONY SV221 45° Correct Image Prism Diagonal - 1.25"
CA$25.00 - CA$25.00
CA$25.00
CA$25.00 - CA$25.00
CA$25.00

SVBONY SV221 45° Correct Image Prism Diagonal - 1.25"

SV-W9178B

Produces a fully erect, non-reversed image Comfortable 45° viewing angle for terrestrial observation Standard 1.25" barrel fits most refractor tel...

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CA$25.00 - CA$25.00
CA$25.00
CA$25.00 - CA$25.00
CA$25.00
SVBONY SV221 90° Brass Mirror Diagonal - 1.25"
SVBONY SV221 90° Brass Mirror Diagonal - 1.25"
CA$21.00 - CA$21.00
CA$21.00
CA$21.00 - CA$21.00
CA$21.00

SVBONY SV221 90° Brass Mirror Diagonal - 1.25"

SV-W9178A

90° Deflection Angle for Comfortable Viewing Aluminum-Coated Mirror with >92% Reflectivity Standard 1.25" Barrel and Eyepiece Clamp Accepts 1.2...

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CA$21.00 - CA$21.00
CA$21.00
CA$21.00 - CA$21.00
CA$21.00
SVBONY SV222 45° Correct Image Prism Diagonal FMC - 1.25"
SVBONY SV222 45° Correct Image Prism Diagonal FMC - 1.25"
CA$56.00 - CA$56.00
CA$56.00
CA$56.00 - CA$56.00
CA$56.00

SVBONY SV222 45° Correct Image Prism Diagonal FMC - 1.25"

SV-W9179A

Produces an erect, non-reversed image for terrestrial and astronomical viewing 45° deflection angle provides a comfortable viewing position Phase-...

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CA$56.00 - CA$56.00
CA$56.00
CA$56.00 - CA$56.00
CA$56.00
SVBONY SV222 90° Erect Image Prism Diagonal - 1.25"
SVBONY SV222 90° Erect Image Prism Diagonal - 1.25"
CA$91.00 - CA$91.00
CA$91.00
CA$91.00 - CA$91.00
CA$91.00

SVBONY SV222 90° Erect Image Prism Diagonal - 1.25"

SV-W9179B

Produces a 90° erect, non-reversed image Standard 1.25-inch barrel size BK7 prism with anti-reflection coatings 19mm clear aperture CNC machined a...

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CA$91.00 - CA$91.00
CA$91.00
CA$91.00 - CA$91.00
CA$91.00
SVBONY SV223 90° ClickLock Dielectric Mirror Diagonal - 1.25"
SVBONY SV223 90° ClickLock Dielectric Mirror Diagonal - 1.25"
CA$130.00 - CA$130.00
CA$130.00
CA$130.00 - CA$130.00
CA$130.00

SVBONY SV223 90° ClickLock Dielectric Mirror Diagonal - 1.25"

SV-W9180A

Reflects over 99% of incoming light with a lab-tested dielectric coating Features a 1.25" Clicklock mechanism for fast, secure eyepiece changes Pr...

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CA$130.00 - CA$130.00
CA$130.00
CA$130.00 - CA$130.00
CA$130.00
SVBONY SV223 90° ClickLock Dielectric Mirror Diagonal - 2"
SVBONY SV223 90° ClickLock Dielectric Mirror Diagonal - 2"
CA$151.00 - CA$151.00
CA$151.00
CA$151.00 - CA$151.00
CA$151.00

SVBONY SV223 90° ClickLock Dielectric Mirror Diagonal - 2"

SV-W9180B

Features a 2-inch barrel with a >99% reflectivity dielectric mirror Convenient Clicklock clamping mechanism for secure eyepiece attachment Defl...

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CA$151.00 - CA$151.00
CA$151.00
CA$151.00 - CA$151.00
CA$151.00
Tele Vue 2" SCT RichField Kit 1
CA$655.00 - CA$655.00
CA$655.00
CA$655.00 - CA$655.00
CA$655.00

Tele Vue 2" SCT RichField Kit

TV-SCP-8202

SCT Adapter (ACC-0003): threads onto a standard SCT rear cell 2" 90° Everbrite Diagonal (DDP-8004), 99% dielectric mirror 2"-1.25" HiHat adapter i...

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CA$655.00 - CA$655.00
CA$655.00
CA$655.00 - CA$655.00
CA$655.00
Tele Vue 60º Everbrite Diagonal - 1.25" 2
Tele Vue 60º Everbrite Diagonal - 1.25" 1
CA$517.00 - CA$517.00
CA$517.00
CA$517.00 - CA$517.00
CA$517.00

Tele Vue 60º Everbrite Diagonal - 1.25"

TV-DPC-6012

Reflectivity: 99% Everbrite Dielectric Coatings Viewing Angle: 60° Barrel Size: 1.25" Construction: Machined Aluminum Housing Eyepiece Holder: Non...

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CA$517.00 - CA$517.00
CA$517.00
CA$517.00 - CA$517.00
CA$517.00
Tele Vue 90º Enhanced Aluminum Diagonal - 1.25" 1
CA$302.00 - CA$302.00
CA$302.00
CA$302.00 - CA$302.00
CA$302.00

Tele Vue 90º Enhanced Aluminum Diagonal - 1.25"

TV-DSC-0125

Diagonal Type: 90° Mirror (Star) Barrel Size: 1.25" Reflectivity: 96% Eyepiece Holder: Brass Clamp Ring Filter Threads: Yes (1.25") Weight: 5 oz (...

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CA$302.00 - CA$302.00
CA$302.00
CA$302.00 - CA$302.00
CA$302.00
Tele Vue 90º Everbrite Diagonal - 1.25" 1
CA$455.00 - CA$455.00
CA$455.00
CA$455.00 - CA$455.00
CA$455.00

Tele Vue 90º Everbrite Diagonal - 1.25"

TV-DPC-1250

Reflectivity: 99% Dielectric Coating Substrate Material: Pyrex Barrel Size: 1.25" Optical Angle: 90° Eyepiece Holder: Non-marring Brass Clamp Ring...

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CA$455.00 - CA$455.00
CA$455.00
CA$455.00 - CA$455.00
CA$455.00
Tele Vue 90º Everbrite Diagonal 1
CA$630.00 - CA$630.00
CA$630.00
CA$630.00 - CA$630.00
CA$630.00

Tele Vue 90º Everbrite Diagonal - 2"

TV-DDP-8004
1 in stock

Reflectivity: 99% Dielectric Coatings Mirror Substrate: Pyrex Body Construction: CNC-Machined Aluminum Eyepiece Holder: Non-Marring Brass Clamp Ri...

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CA$630.00 - CA$630.00
CA$630.00
CA$630.00 - CA$630.00
CA$630.00
William Optics 1.25" 90° Erect Prism for Cat 61 WIFD
CA$219.00 - CA$219.00
CA$219.00
CA$219.00 - CA$219.00
CA$219.00

William Optics 1.25" 90° Erect Prism for Cat 61 WIFD

WO-D-EP90-125-RC61
1 in stock

Provides a fully corrected, erect image for terrestrial viewing Comfortable 90° viewing angle Standard 1.25" barrel accepts most astronomical eyep...

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CA$219.00 - CA$219.00
CA$219.00
CA$219.00 - CA$219.00
CA$219.00
William Optics 2" 90° Dura Bright Dielectric Diagonal for SCT threaded Scope
CA$286.00 - CA$286.00
CA$286.00
CA$286.00 - CA$286.00
CA$286.00

William Optics 2" 90° Dura Bright Dielectric Diagonal for SCT threaded Scope

WO-D-DIG2D-C-DB-SCT
1 in stock

Enables use of 2-inch eyepieces for wide-field views 90° viewing angle for comfortable observing at the zenith Dura-Bright dielectric coatings for...

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CA$286.00 - CA$286.00
CA$286.00
CA$286.00 - CA$286.00
CA$286.00
CA$152.00 - CA$152.00
CA$152.00
CA$152.00 - CA$152.00
CA$152.00

William Optics 45° Erecting Prism - 1.25"

WO-D-EP125
6 in stock

Provides a fully corrected image (upright and left-right correct) Comfortable 45° viewing angle for terrestrial observation Fits standard 1.25" fo...

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CA$152.00 - CA$152.00
CA$152.00
CA$152.00 - CA$152.00
CA$152.00

Mirror Diagonals, Prisms and Optical Path

Every diagonal adds roughly 90 to 130 mm of optical path, which affects whether your telescope reaches focus — a detail that matters more than most buyers expect.

  • Dielectric mirror diagonals: Multi-layer coatings exceeding 99% reflectivity across the visible spectrum, with no chromatic effect at any focal ratio. The default choice for most telescopes.
  • Prism diagonals: Total internal reflection with excellent light throughput and slightly better scatter control, and they can outperform mirrors at f/10 and slower. In fast refractors they introduce chromatic aberration.
  • Amici roof prisms: Produce a correctly oriented image for terrestrial use, at the cost of a diffraction spike on bright stars that makes them poor for astronomy.
  • 1.25 inch versus 2 inch: A 2 inch diagonal is required to use 2 inch eyepieces and passes a wider field; a 1.25 inch is lighter and adds less back-focus.
  • Compression ring clamping: Even radial grip on the eyepiece barrel rather than a single set screw, which prevents marking and keeps heavy eyepieces square.
  • Back-focus consequences: Newtonians rarely accept a diagonal at all; refractors and Cassegrains generally have enough travel. Check before assuming.

Why Choose David Astro for Diagonals

A diagonal is not a neutral component, and the right one depends on the telescope in front of it.

  • Focal ratio matched: We recommend a prism only where it will genuinely outperform a dielectric mirror, and never in a fast refractor where it would introduce colour.
  • Back-focus checked: We confirm your telescope has the inward travel to reach focus with the diagonal and your eyepiece or camera.
  • Weight considered: A heavy 2 inch diagonal plus a heavy eyepiece can unbalance a small refractor, and we will mention it before you discover it in the dark.
  • Authorized dealer: Genuine diagonals with verified coatings and manufacturer warranty, in Canadian stock.
  • Ships from Canada: No duties or brokerage fees on Canadian orders.

Diagonals in Canada — Frequently Asked Questions

Dielectric mirror or prism?
Dielectric for almost all telescopes, and especially anything faster than f/8. A good prism can edge ahead on planetary contrast at f/10 and slower, where its chromatic contribution is negligible.
Do I need a 2 inch diagonal?
Only if you want to use 2 inch eyepieces for wide low-power fields. If your widest eyepiece is 1.25 inch, a 2 inch diagonal adds weight and back-focus with no benefit.
Why does my telescope not reach focus with a diagonal?
A diagonal adds around 100 mm to the optical path. Newtonians generally lack the inward travel to absorb it. Refractors and Cassegrains are designed with a diagonal in mind and usually have no issue.
Is an Amici prism suitable for astronomy?
Not really. It corrects image orientation for terrestrial viewing but produces a diffraction spike across bright stars that is distracting on the Moon and planets.

Choosing a diagonal?

Tell us your telescope and focal ratio and we will recommend mirror or prism. Authorized dealer, Canadian stock.