SVBONY SV221 1.25″ 90° Mirror Diagonal

Product Overview

The SVBONY SV221 90° is a 1.25″ mirror star diagonal that turns the light path through a right angle so you can look down into the eyepiece rather than crouching behind the telescope. The aluminium-coated mirror has a reflectivity above 92% and a clear aperture of 29.5 mm, the optical length is about 73 mm, and the diagonal weighs 90.5 g (3.19 oz). The nosepiece carries an M28.5 x 0.6 filter thread, and the eyepiece holder uses a brass pressure ring and screw clamp.

Who It's For

This is a good match if you observe with a refractor, Maksutov-Cassegrain or Schmidt-Cassegrain and want a light, inexpensive 1.25″ diagonal for comfortable viewing near the zenith, or a spare diagonal for a travel or grab-and-go telescope.

Key Features & Design

• 90° right-angle design: comfortable viewing posture, especially for objects high in the sky.
• Aluminium-coated mirror: reflectivity above 92%.
• 29.5 mm clear aperture: full illumination for 1.25″ eyepieces.
• Brass pressure ring: holds the eyepiece without the screw touching its barrel.
• Light construction: ABS housing with aluminium barrels, brass lock screws and a dust cover; 90.5 g in total.

Recommended Uses

• Lunar, planetary and deep-sky observing with 1.25″ eyepieces.
• Comfortable viewing near the zenith on refractors and catadioptrics.
• Casual daytime viewing and birding, keeping in mind the mirror-reversed image described below.

Compatibility and Accessory Notes

• Telescopes: refractors and catadioptrics such as Schmidt-Cassegrain and Maksutov-Cassegrain telescopes with a 1.25″ focuser or visual back.
• Eyepieces: any 1.25″ eyepiece.
• Filters: M28.5 x 0.6 thread on the nosepiece.
• Image orientation: upright but mirror-reversed left to right, as with any single-mirror diagonal. The SV221 45° prism version gives a fully correct image if you need one for land viewing.

What's in the Box

• 1 x SV221 1.25″ 90° mirror diagonal
• 3 x M3 screws
• 1 x dust cap
• 1 x dust plug

Good to Know Before You Order

• Mirror-reversed image: normal for astronomy; for land viewing an erect-image prism diagonal gives a correct view.
• Newtonians and Dobsonians: these telescopes do not use a diagonal, and SVBONY does not recommend the SV221 for them.

Frequently Asked Questions

Is it difficult to set up?

No. It slides into the 1.25″ focuser or visual back, and the eyepiece locks into it with the thumbscrew and brass ring.

Does it reverse the image?

The image is upright but reversed left to right, which is standard for a mirror diagonal.

Can I add filters?

Yes. 1.25″ filters thread into the M28.5 x 0.6 nosepiece.

Will it fit my telescope?

It fits any refractor or catadioptric with a 1.25″ focuser or visual back.

Bottom Line

In short: a light 1.25″ 90° mirror diagonal with a 29.5 mm aperture, over 92% reflectivity, a brass compression ring and filter threads for refractors and catadioptric telescopes.