5 Best Telescope Star Diagonals (October 2026)
The best telescope star diagonals include the Celestron 1.25" Star Diagonal for Refractor for a basic 1.25-inch setup, the Alstar Telescope Diagonal 90 Degree 1.25Inch Telescope for threaded or barrel mounting, and the Astromania Telescope Diagonal Prism 1.25" for an upright, non-reversed view.
On this page (6 sections)
The best telescope star diagonal for most 1.25-inch refractors is the Celestron 1.25" Star Diagonal for Refractor, a 90° mirror diagonal that accepts standard 1.25-inch eyepieces. If you need an upright, non-reversed view, consider the Astromania prism; if you want a twist-lock eyepiece holder, consider Celestron’s dielectric model.
A diagonal turns the light path so you can look into the eyepiece from a more comfortable angle, especially when viewing objects high in the sky. Before choosing one, confirm the focuser or visual back size and whether your telescope design can use a diagonal.
The parts of a telescope and their functions guide explains where the focuser and diagonal fit in the optical path. For reflectors, read about telescope focusers for reflectors and check the telescope maker’s compatibility guidance.
Our picks at a glance
-
1
Best for a basic 1.25-inch setupCelestron 1.25" Star Diagonal for Refractor
9.5/10 our score
See it on Amazon -
2
Best for threaded or barrel mountingAlstar Telescope Diagonal 90 Degree 1.25Inch Telescope
9.4/10 our score
See it on Amazon -
3
Best for an upright, non-reversed viewAstromania Telescope Diagonal Prism 1.25"
9.3/10 our score
See it on Amazon
- 4
Check itCelestron 1.25" Twist-Lock Dielectric Star Diagonal
Best for a twist-lock eyepiece holder - 5
Check itSVBONY SV188 Dielectric Star Diagonal 1.25
Best for a metal-body mirror diagonal
How we chose telescope star diagonals
The best telescope star diagonals need to fit your telescope’s opening and suit the image orientation you want. We compared the stated barrel size, telescope compatibility, viewing angle, optical design, and handling features.
All five picks are 1.25-inch options. Their compatibility is not universal: several are meant for refractors or catadioptric telescopes, and the Astromania and SVBONY models are not recommended for reflectors.
A 90° viewing angle is common across this group, but the optical designs differ. Mirror diagonals generally leave the image reversed left to right, while the Astromania Amici prism is designed to provide an upright, non-reversed view.
Our score weighs specs and features against what you need from a telescope star diagonal.
Compare the best telescope star diagonals
The main differences are telescope fit, optical design, and how the eyepiece is held. Use the table to narrow the choices, then confirm your telescope’s connection size and type.
| Product | Best for | Barrel size | Viewing angle | Telescope fit | Our score |
|---|---|---|---|---|---|
| Celestron 1.25" Star Diagonal for Refractor | a basic 1.25-inch setup | 1.25 in | 90° | Refractors and catadioptric telescopes | 9.5 |
| Alstar Telescope Diagonal 90 Degree 1.25Inch Telescope | threaded or barrel mounting | 1.25 in | 90° | 1.25-inch refracting telescopes | 9.4 |
| Astromania Telescope Diagonal Prism 1.25" | an upright, non-reversed view | 1.25 in | 90° | 1.25-inch focusers, excluding reflectors | 9.3 |
| Celestron 1.25" Twist-Lock Dielectric Star Diagonal | a twist-lock eyepiece holder | 1.25 in | 90° | Refractors and Cassegrain-type telescopes | 9.1 |
| SVBONY SV188 Dielectric Star Diagonal 1.25 | a metal-body mirror diagonal | 1.25 in | 90° | Refractors and catadioptric telescopes | 8.8 |
The best star diagonals for telescopes
The Celestron standard diagonal leads for common 1.25-inch refractor and catadioptric setups, while the other models suit specific mounting, image-correction, or holder preferences. Match the diagonal to your telescope before comparing coatings or extras.
Celestron 1.25" Star Diagonal for Refractor: Best for a basic 1.25-inch setup
- 90° viewing angle
- Fits 1.25-inch focusers or visual backs
- Accepts 1.25-inch eyepieces, Barlow lenses, and accessories
A straightforward choice for a compatible 1.25-inch focuser or visual back, with a 90° angle for looking at objects overhead.
Pros
- Fits 1.25-inch focusers or visual backs
- Accepts standard 1.25-inch accessories
- 90° angle can make overhead viewing more comfortable
Cons
- Image is left-to-right reversed
- Not a fit for 2-inch focusers
The Celestron 1.25" Star Diagonal for Refractor is a simple match for a telescope with a 1.25-inch focuser or visual back. Its 90° viewing angle directs the eyepiece to a more comfortable position when you look at objects overhead.
It accepts standard 1.25-inch eyepieces, Barlow lenses, and other accessories with a 1.25-inch insert barrel. The mirror design presents an upright image that is reversed from left to right, a common orientation for astronomical observing.
Celestron describes it for refractors and catadioptric telescopes. It is the best starting point here if you want a conventional diagonal and don't need image correction for daytime use.
Check the focuser opening before you buy. If your telescope has a different connection, a compatible adapter may be needed.
Alstar Telescope Diagonal 90 Degree 1.25Inch Telescope: Best for threaded or barrel mounting
- 1.25-inch connector
- 90° viewing angle
- Threaded and barrel mounting options
This diagonal is designed for 1.25-inch refractors and offers threaded or barrel mounting. Its image is vertically erect with lateral inversion.
Pros
- Two mounting options
- Multi-coated optics
- 1.25-inch filter threads
Cons
- Designed for refractors
- Image remains horizontally reversed
The Alstar Telescope Diagonal 90 Degree 1.25Inch Telescope suits a 1.25-inch refractor and offers threaded or barrel mounting. Its 90° angle directs the eyepiece upward for a more comfortable position when observing overhead.
The product data describes multi-coated optics and internal treatment intended to reduce reflections. The diagonal has 1.25-inch filter threads and a thumbscrew to hold an eyepiece in place.
Its image is vertically erect with lateral inversion, so left and right remain reversed. Choose it if the mounting options or filter threads suit your setup, and confirm that your telescope is a refractor with the right connection.
Astromania Telescope Diagonal Prism 1.25": Best for an upright, non-reversed view
- 1.25-inch barrel
- 90° erecting image prism
- Image is upright and non-reversed
An Amici prism corrects image orientation for upright, non-reversed views. Astromania says it fits 1.25-inch focusers, but it is not recommended for reflectors.
Pros
- Corrects image orientation
- Accepts standard 1.25-inch eyepieces and filters
- Can be used for terrestrial viewing
Cons
- Not recommended for reflector telescopes
- Prism design differs from a standard mirror diagonal
The Astromania Telescope Diagonal Prism 1.25" is the choice for upright, non-reversed views. Its Amici prism is designed to correct image orientation, which can be useful for daytime terrestrial viewing as well as astronomy.
It accepts standard 1.25-inch eyepieces and includes a 1.25-inch filter thread. The product data describes multi-layer dielectric coatings and internal glare-reducing threads.
Astromania says the diagonal fits 1.25-inch focusers but is not recommended for reflector telescopes. The prism design makes it distinct from a standard mirror diagonal, so choose it when corrected image orientation matters and your telescope is compatible.
Celestron 1.25" Twist-Lock Dielectric Star Diagonal: Best for a twist-lock eyepiece holder
- Twist-lock centers the eyepiece
- 58 dielectric coating layers
- 90° viewing angle
The twist-lock holder secures and centers standard 1.25-inch eyepieces. Celestron says its dielectric coatings provide an over 99% reflective surface.
Pros
- Twist-lock keeps the eyepiece centered
- Accepts 1.25-inch accessories
- Dielectric coatings
Cons
- Heavier than the basic Celestron option
- For compatible refractor and Cassegrain-type setups
The Celestron 1.25" Twist-Lock Dielectric Star Diagonal suits you if you want a holder that centers the eyepiece. Turning the lock secures a standard 1.25-inch eyepiece without using a traditional thumbscrew.
Celestron says the diagonal has 58 dielectric coating layers and a reflective surface above 99%. It also accepts 1.25-inch Barlow lenses, threaded filters, and other accessories.
Its 90° angle is intended for refractors and Cassegrain-type telescopes. Choose this model for its twist-lock design and stated coating performance, while checking that your telescope accepts a 1.25-inch diagonal.
SVBONY SV188 Dielectric Star Diagonal 1.25: Best for a metal-body mirror diagonal
- 90% reflectivity flat mirror
- Full-metal body
- Compatible with refractors and catadioptric telescopes
A 1.25-inch mirror diagonal with a full-metal body and 90% reflectivity. SVBONY says it works with refractors and catadioptric telescopes, but not reflectors.
Pros
- Full-metal body
- Accepts 1.25-inch eyepieces and filters
- Includes dust covers
Cons
- Not suitable for reflector telescopes
- Additional adapters are required for SCT telescopes
The SVBONY SV188 Dielectric Star Diagonal 1.25 is a full-metal mirror diagonal for compatible refractors and catadioptric telescopes. Its 90° viewing angle can make it easier to observe objects overhead.
SVBONY states 90% reflectivity and says the diagonal accepts standard 1.25-inch eyepieces, filters, and other accessories. Dust covers help protect it when it is stored.
The product data says it is not suitable for reflector telescopes. It also notes that an additional adapter is needed for SCT telescopes, so check your connection before choosing it.
How do you choose a telescope star diagonal?
Choose a star diagonal by matching its barrel to the telescope’s focuser or visual back, then check the telescope type and image orientation. Optical coatings and holder design are useful comparisons after compatibility is clear.
Which barrel size fits your telescope?
The options here have 1.25-inch barrels or connections. A 1.25-inch diagonal is designed for a 1.25-inch focuser or visual back; a larger focuser may require a suitable adapter or a different diagonal.
Check the telescope manual for the connection size. The SVBONY SV48P 102mm telescope review can help if you’re comparing a refractor setup and its accessories.
Which telescope types can use a diagonal?
Refractors commonly use a diagonal at the focuser, and some catadioptric telescopes use one at the visual back. The specific connection and included accessories vary, so check the telescope instructions.
Newtonian reflectors use a different eyepiece arrangement. A diagonal made for refractors may not fit their optical path; our Dobsonian vs Newtonian telescope comparison explains the common reflector designs.
Does a mirror or prism give the view you want?
A mirror diagonal usually provides an upright view that is reversed left to right. That is generally practical for astronomical targets, where you can turn a star chart to match the eyepiece view; see what a star chart is.
An erecting prism corrects orientation for a conventional terrestrial view. The Astromania option is the only pick here described as providing upright, non-reversed views.
How much do coatings and holders matter?
Coatings describe how the diagonal’s optical surfaces handle reflected light, while the holder affects how you secure an eyepiece. Celestron states over 99% reflectivity for its Twist-Lock model, and SVBONY states 90% for the SV188.
A twist-lock may appeal if you prefer its clamping method, while a thumbscrew is a familiar alternative. These details matter after fit, telescope type, and image orientation.
Can a telescope star diagonal help with different targets?
A star diagonal changes the eyepiece position, not the telescope’s ability to resolve a target. Choose the telescope and eyepiece for the target, then use a compatible diagonal for viewing comfort and the image orientation you prefer.
For satellite observing, start with the telescope and field of view; our guide to the best telescopes for spotting satellites in orbit covers that decision. For deep-sky observing, the best telescope for galaxies guide helps you compare aperture and other telescope needs.
Which telescope star diagonal should you choose?
Choose the Celestron standard diagonal for a compatible 1.25-inch refractor or catadioptric setup and a conventional mirror view. Pick Astromania if you need an upright, non-reversed image, and choose the Celestron Twist-Lock if its centered holder is your priority.
Our top 3 picks at a glance
-
1
Best for a basic 1.25-inch setup
Celestron 1.25" Star Diagonal for Refractor
9.5/10 our score
See it on Amazon -
2
Best for threaded or barrel mounting
Alstar Telescope Diagonal 90 Degree 1.25Inch Telescope
9.4/10 our score
See it on Amazon -
3
Best for an upright, non-reversed view
Astromania Telescope Diagonal Prism 1.25"
9.3/10 our score
See it on Amazon
Frequently asked questions
What size star diagonal do I need?
Match the diagonal barrel to your telescope’s focuser or visual back and your eyepieces. These picks use 1.25-inch fittings, so they suit setups with a compatible 1.25-inch opening.
A 1.25-inch diagonal won't fit a 2-inch focuser without a suitable adapter.
Check your telescope manual or measure the opening before choosing.
Can I use a star diagonal with a reflector telescope?
Usually, a standard star diagonal is not suited to a Newtonian reflector because its eyepiece points out from the side of the tube. The Astromania and SVBONY options here are explicitly not recommended for reflectors.
See our guide to what a reflecting telescope is and check your telescope’s design before buying.
Does a star diagonal reverse the image?
A standard mirror star diagonal gives an upright image that is reversed left to right. That orientation is usually fine for astronomical viewing, where there is no fixed up or down.
An erecting prism such as the Astromania corrects the image to upright and non-reversed, which can help for daytime terrestrial viewing.
Do dielectric coatings matter on a star diagonal?
Coatings affect how much light the diagonal reflects. The Celestron Twist-Lock model has 58 dielectric layers and a maker-stated reflectivity above 99%, while the SVBONY SV188 states 90% reflectivity.
Those figures describe reflectivity.
Telescope fit, optical alignment, and the image orientation you want also matter.
