Skip to content

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.

Filters

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...

View full details
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...

View full details
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...

View full details
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 ...

View full details
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 Full 2-inch clear aperture prevents vignetting Baade...

View full details
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
1 in stock

90° comfortable viewing angle Amici prism design provides correctly oriented, erect images Features a full 1.25" clear aperture to prevent vignett...

View full details
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...

View full details
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 Maxbright-series prism...

View full details
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
1 in stock

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

View full details
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

Broad Band Hard Silver (BBHS®) Coated Prism 2-inch ClickLock® Self-Centering Eyepiece Clamp Fully User-Collimatable Prism Mount CNC-Machined Alumi...

View full details
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
1 in stock

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

View full details
CA$332.00 - CA$332.00
CA$332.00
CA$332.00 - CA$332.00
CA$332.00
CA$76.00 - CA$76.00
CA$76.00
CA$76.00 - CA$76.00
CA$76.00

Baader M55/M68 Adapter for 2" Baader Mirrors & Prisms

BAA-2956110

Converts Baader 2" diagonals to a rigid M68 threaded connection Attaches to the M55 female thread on the diagonal body Provides a standard M68 x 1...

View full details
CA$76.00 - CA$76.00
CA$76.00
CA$76.00 - CA$76.00
CA$76.00
CA$139.00 - CA$139.00
CA$139.00
CA$139.00 - CA$139.00
CA$139.00

Baader Maxbright Amici Prism 1.25"w/ Integral 2X Glasspath Corrector

BAA-2956155

Provides a fully corrected, erect image for terrestrial viewing Designed for the Baader Maxbright Binoviewer Standard 1.25" barrel format Integrat...

View full details
CA$139.00 - CA$139.00
CA$139.00
CA$139.00 - CA$139.00
CA$139.00
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

Features Baader's T-2 System threads for full modularity High-reflectivity Maxbright mirror for bright, high-contrast views All-metal housing for ...

View full details
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

2-inch Barrel Format Baader ClickLock Eyepiece Clamp High-Reflectivity Mirror Design Precision Machined Body Self-Centering Clamp Mechanism Des...

View full details
CA$431.00 - CA$431.00
CA$431.00
CA$431.00 - CA$431.00
CA$431.00
CA$698.00 - CA$698.00
CA$698.00
CA$698.00 - CA$698.00
CA$698.00

Baader Mirror Diagonal w/ BBHS Coating - 2" ClickLock

BAA-2456115
Only 1 left!

2" BBHS star diagonal with 2" ClickLock terminal and black case. Provides greatest reflectance over most spectral range from 390 to 2000nm New inc...

View full details
CA$698.00 - CA$698.00
CA$698.00
CA$698.00 - CA$698.00
CA$698.00
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

BBHS® BroadBand Hard Silver Coating Protective Dielectric Overcoat Baader T-2 System Component Modular Threaded Connections Optimized for High Ref...

View full details
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

BBHS® Broad Band Hard Silver coating for maximum spectral range and color fidelity User-collimatable mirror in a CNC-machined aluminum housing The...

View full details
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 (Zeiss) Threaded Telescope Connection All-Metal Mirror Diagonal Housing Replaces Standard Slip-Fit Diagonals E...

View full details
CA$450.00 - CA$450.00
CA$450.00
CA$450.00 - CA$450.00
CA$450.00
CA$81.00 - CA$81.00
CA$81.00
CA$81.00 - CA$81.00
CA$81.00

Baader Q-Turret Quad Eyepiece Revolver - 1.25"

BAA-2957010
1 in stock

Holds four 1.25" eyepieces for rapid switching Lightweight design is compatible with most focusers Metal threads for secure connections Accepts pl...

View full details
CA$81.00 - CA$81.00
CA$81.00
CA$81.00 - CA$81.00
CA$81.00
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 Baader BBHS® (Broadband Hard Silver) Coatings T-2 (M42x0.75) Threads on Input and Output Faces Solid, All-Me...

View full details
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...

View full details
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 ...

View full details
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...

View full details
CA$361.00 - CA$361.00
CA$361.00
CA$361.00 - CA$361.00
CA$361.00

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.