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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$253.00 - CA$253.00
CA$253.00
CA$253.00 - CA$253.00
CA$253.00

APM Dielectric Mirror Star Diagonal - 2"

APM-2-diag-DX-FL
Only 2 left!

APM 2 inch star diagonal mirror with 99% di-electric coating with fast-lockThe diagonal mirror is produced by APM-Telescopes with highest qualityOp...

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CA$253.00 - CA$253.00
CA$253.00
CA$253.00 - CA$253.00
CA$253.00
Askar 2” High-Precision Dielectric Diagonal
CA$245.00 - CA$245.00
CA$245.00
CA$245.00 - CA$245.00
CA$245.00

Askar 2” High-Precision Dielectric Diagonal

ASK-ASKAR2DIAG

2-inch Barrel Diameter High-Reflectivity Dielectric Mirror Coating Accepts both 2" and 1.25" Eyepieces Compatible with SCT and Refractor/Reflector...

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CA$245.00 - CA$245.00
CA$245.00
CA$245.00 - CA$245.00
CA$245.00
CA$123.00 - CA$123.00
CA$123.00
CA$123.00 - CA$123.00
CA$123.00

Baader 45° Amici Prism Diagonal w/ 24mm Clear Aperture - 1.25"

BAA-2956150

Provides a correctly oriented, erect image for terrestrial viewing Comfortable 45° viewing angle Full 24mm clear aperture prevents vignetting with...

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CA$123.00 - CA$123.00
CA$123.00
CA$123.00 - CA$123.00
CA$123.00
CA$348.00 - CA$348.00
CA$348.00
CA$348.00 - CA$348.00
CA$348.00

Baader 45° Amici Prism Diagonal 45° - 2" SCT Thread

BAA-2956151

Provides a fully corrected, upright, and non-reversed image Comfortable 45° viewing angle for terrestrial observation Attaches directly to 2" SCT ...

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CA$348.00 - CA$348.00
CA$348.00
CA$348.00 - CA$348.00
CA$348.00
CA$976.00 - CA$976.00
CA$976.00
CA$976.00 - CA$976.00
CA$976.00

Baader 90° Astro-Grade Amici Prism w/ BBHS Coating - 2" ClickLock

BAA-2456120
1 in stock

Provides a fully corrected, non-reversed and non-inverted image Comfortable 90° viewing angle 44 mm clear aperture Sealed BBHS® hard-silver coatin...

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CA$976.00 - CA$976.00
CA$976.00
CA$976.00 - CA$976.00
CA$976.00
CA$430.00 - CA$430.00
CA$430.00
CA$430.00 - CA$430.00
CA$430.00

Baader 90° Astro-Grade Amici Prism w/ BBHS Coating - T-2

BAA-2456130

Provides erect, non-reversed images at a 90° viewing angle 31 mm clear aperture Baader BBHS® silver and Phantom Group® anti-reflection coatings 47...

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CA$430.00 - CA$430.00
CA$430.00
CA$430.00 - CA$430.00
CA$430.00
CA$426.00 - CA$426.00
CA$426.00
CA$426.00 - CA$426.00
CA$426.00

Baader 90° Erecting Prism Diagonal - 2"

BAA-2956152

Provides a fully erect, non-reversed image 90° viewing angle for comfortable observing 2" barrel for use with large focusers Includes a 2" to 1.25...

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CA$426.00 - CA$426.00
CA$426.00
CA$426.00 - CA$426.00
CA$426.00
CA$161.00 - CA$161.00
CA$161.00
CA$161.00 - CA$161.00
CA$161.00

Baader 90° Prism Diagonal w/ 32mm Prism - T-2

BAA-2456005
1 in stock

90° comfortable viewing angle Large 32mm prism for full field illumination Baader T-2 thread system for modular connections Excellent for use with...

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CA$161.00 - CA$161.00
CA$161.00
CA$161.00 - CA$161.00
CA$161.00
CA$198.00 - CA$198.00
CA$198.00
CA$198.00 - CA$198.00
CA$198.00

Baader 90° Zenith Prism Diagonal w/ 32mm Prism - 1.25"

BAA-2456005K

90° Zenith Prism Diagonal Oversized 32mm Prism 1.25" Barrel Format Baader T-2 System Threads Includes 1.25" Eyepiece Holder & Nosepiece Des...

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CA$198.00 - CA$198.00
CA$198.00
CA$198.00 - CA$198.00
CA$198.00
CA$706.00 - CA$706.00
CA$706.00
CA$706.00 - CA$706.00
CA$706.00

Baader BBHS Prism Adjustable Star Diagonal - 2"

BAA-2456118
1 in stock

90° prism diagonal for comfortable viewing BaK4 prism with sealed Broad Band Hard Silver (BBHS®) coating Baader 2-inch ClickLock® self-centering e...

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CA$706.00 - CA$706.00
CA$706.00
CA$706.00 - CA$706.00
CA$706.00
CA$332.00 - CA$332.00
CA$332.00
CA$332.00 - CA$332.00
CA$332.00

Baader FlipMirror II Star Diagonal

BAA-2458055

Instantly switch between three different instrument ports Straight-through port for primary camera or spectrograph 90° mirrored port for visual ob...

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CA$332.00 - CA$332.00
CA$332.00
CA$332.00 - CA$332.00
CA$332.00
CA$324.00 - CA$324.00
CA$324.00
CA$324.00 - CA$324.00
CA$324.00

Baader Maxbright Mirror Diagonal - T-2

BAA-2456100
1 in stock

T-2 (M42x0.75) threads on telescope and eyepiece sides for full modularity 99% reflectivity dielectrically coated mirror with 1/10 lambda surface ...

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CA$324.00 - CA$324.00
CA$324.00
CA$324.00 - CA$324.00
CA$324.00
CA$431.00 - CA$431.00
CA$431.00
CA$431.00 - CA$431.00
CA$431.00

Baader Mirror Diagonal - 2" ClickLock

BAA-2956100

90° mirror diagonal for a comfortable viewing angle Baader 2" ClickLock clamp Oversized dielectric mirror with 1/10 wave surface precision 46.6mm ...

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CA$431.00 - CA$431.00
CA$431.00
CA$431.00 - CA$431.00
CA$431.00
CA$401.00 - CA$401.00
CA$401.00
CA$401.00 - CA$401.00
CA$401.00

Baader Mirror Diagonal w/ BBHS Coating - T-2

BAA-2456103
1 in stock

90° mirror diagonal for astronomical use Broadband Hard Silver (BBHS®) coating, 98-99% reflection from 390nm to 2000nm Female T-2 (M42x0.75) eyepi...

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CA$401.00 - CA$401.00
CA$401.00
CA$401.00 - CA$401.00
CA$401.00
Baader Planetarium 2" BBHS® Mirror – Adjustable Star Diagonal
CA$721.00 - CA$721.00
CA$721.00
CA$721.00 - CA$721.00
CA$721.00

Baader Planetarium 2" BBHS® Mirror – Adjustable Star Diagonal

BAA-2456105
1 in stock

Broad Band Hard Silver (BBHS®) coated mirror, 98-99% reflection from 390nm to 2000nm 47mm clear aperture Secure 2-inch ClickLock self-centering ey...

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CA$721.00 - CA$721.00
CA$721.00
CA$721.00 - CA$721.00
CA$721.00
Baader Planetarium M68 ClickLock® Mirror Diagonal
CA$450.00 - CA$450.00
CA$450.00
CA$450.00 - CA$450.00
CA$450.00

Baader Planetarium M68 ClickLock® Mirror Diagonal

BAA-2956100Z

2-inch ClickLock Eyepiece Clamp M68 x 1mm Male Threaded Telescope Connection 51mm Clear Aperture 109mm Optical Length Dielectrically Coated λ/10 M...

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CA$450.00 - CA$450.00
CA$450.00
CA$450.00 - CA$450.00
CA$450.00
CA$354.00 - CA$354.00
CA$354.00
CA$354.00 - CA$354.00
CA$354.00

Baader Star Diagonal Prism w/ Zeiss Spec Prism & BBHS Coating - T-2

BAA-2456095
1 in stock

Zeiss-Specification Right-Angle Prism with 90° Deflection BBHS® (Broadband Hard Silver) Sealed Reflective Coating T-2 (M42x0.75) Threads on Telesc...

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CA$354.00 - CA$354.00
CA$354.00
CA$354.00 - CA$354.00
CA$354.00
CA$62.00 - CA$62.00
CA$62.00
CA$62.00 - CA$62.00
CA$62.00

Celestron 45-Degree Erect Image Diagonal

CEL-94112-A

Corrects image orientation for terrestrial viewing Produces right-side-up and left-to-right correct images Comfortable 45-degree viewing ang...

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CA$62.00 - CA$62.00
CA$62.00
CA$62.00 - CA$62.00
CA$62.00
CA$222.00 - CA$222.00
CA$222.00
CA$222.00 - CA$222.00
CA$222.00

Celestron Dielectric Star Diagonal with Twist-Lock - 1.25"

CEL-93571

Over 99% reflectivity from 58-layer dielectric mirror coatings 1.25” Twist-Lock holder provides a secure, self-centering grip Diffraction-limited ...

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CA$222.00 - CA$222.00
CA$222.00
CA$222.00 - CA$222.00
CA$222.00
CA$361.00 - CA$361.00
CA$361.00
CA$361.00 - CA$361.00
CA$361.00

Celestron Dielectric Star Diagonal with Twist-Lock - 2"

CEL-93573

Delivers over 99% reflectivity with 58 layers of dielectric coatings Secure, self-centering Twist-Lock mechanism for 2" and 1.25" eyepieces Includ...

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CA$361.00 - CA$361.00
CA$361.00
CA$361.00 - CA$361.00
CA$361.00
CA$257.00 - CA$257.00
CA$257.00
CA$257.00 - CA$257.00
CA$257.00

Celestron Mirror Diagonal with XLT Coatings - 2"

CEL-93527

Interferometer-tested mirror with 1/10 wave or better surface accuracy Features Celestron's proprietary XLT optical coatings for maximum light tra...

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CA$257.00 - CA$257.00
CA$257.00
CA$257.00 - CA$257.00
CA$257.00
CA$62.00 - CA$62.00
CA$62.00
CA$62.00 - CA$62.00
CA$62.00

Celestron Star Diagonal - 1.25"

CEL-94115-A

Provides a comfortable 90° viewing angle Inserts into any 1.25" telescope focuser or visual back Accepts standard 1.25" eyepieces Corrects image o...

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CA$62.00 - CA$62.00
CA$62.00
CA$62.00 - CA$62.00
CA$62.00
CA$59.00 - CA$59.00
CA$59.00
CA$59.00 - CA$59.00
CA$59.00

David Astro 90° Diagonal - 1.25"

DAV-DIAG-1.25
Only 2 left!

The 1.25" Mirror Diagonal is a must-have accessory for astronomers, providing a comfortable viewing angle and improved image quality during telesc...

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CA$59.00 - CA$59.00
CA$59.00
CA$59.00 - CA$59.00
CA$59.00
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

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.