Diagonals

Star diagonals provide a more comfortable viewing angle when using refractors, Maksutov-Cassegrain, or Schmidt-Cassegrain telescopes. Upgrading the diagonal from 1.25-inch format is often beneficial as this will allow the use of 2-inch eyepieces. Owners of Schmidt-Cassegrain telescopes larger than eight inches in aperture should seriously consider upgrading to a 2-inch diagonal.

  • Celestron 1.25-inch Errect Image 45 Degree Diagonal
  • Celestron 1.25-inch Errect Image 45 Degree Diagonal

  • Celestron's 1.25 inch erect image diagonal allows you to look into the telescope at a 45 degree angle which ir more comfortable for terrestrial observations. read more...
  • Our price: $35.00

  • Celestron 1.25-inch Star Diagonal
  • Celestron 1.25-inch Star Diagonal

  • 94115-A Celestron Star Diagonal - 1-1/4" is standard equipment on many Celestron telescopes. read more...
  • Our price: $35.00

  • Celestron 2-inch SCT Diagonal XLT
  • Celestron 2-inch SCT Diagonal XLT

  • Upgrading to the Celestron 2-inch XLT Schmidt-Cassegrain Diagonal allows you to take advantage of the wider field of view provided by 2-inch eyepieces. read more...
  • Our price: $134.95

  • Meade Series 5000 2" Diagonal Mirror
  • Meade Series 5000 2" Diagonal Mirror

  • Meade Series 5000 2-inch Enhanced Mirror Diagonal with SCT and 1.25-inch eyepiece adapters. read more...
  • Our price: $139.00

  • Orion 1.25-inch 45-degree Correct Image Diagonal
  • Orion 1.25-inch 45-degree Correct Image Diagonal

  • For terrestrial or astronomical use, this diagonal produces a right-side-up, non-reversed image with correct daytime orientation read more...
  • Our price: $38.00

  • Orion 1.25-inch Correct-Image Star Telescope Diagonal
  • Orion 1.25-inch Correct-Image Star Telescope Diagonal

  • For terrestrial or astronomical use, this 1-1/4 inch 90-degree diagonal produces an upright non-reversed image. read more...
  • Our price: $39.00

  • Orion Dielectric Mirror Star Diagonal 1.25-inch
  • Orion Dielectric Mirror Star Diagonal 1.25-inch

  • 90-degree diagonal features sophisticated, lab-tested dielectric coatings that boast an astounding 99 percent reflectivity read more...
  • Our price: $89.00

  • Orion Dielectric Mirror Star Diagonal 2-inch
  • Orion Dielectric Mirror Star Diagonal 2-inch

  • 90-degree 2" mirror diagonal features sophisticated, lab-tested dielectric coatings that provide an astounding 99% reflectivity read more...
  • Our price: $139.00

  • Stellarvue 1.25-inch Erecting Prism
  • Stellarvue 1.25-inch Erecting Prism

  • This 45 degree erecting prism is useful for terrestrial observing, providing an erect image and a comfortable viewing angle. read more...
  • Our price: $69.00

  • Stellarvue Enhanced 2-inch Dielectric Diagonal
  • Stellarvue Enhanced 2-inch Dielectric Diagonal

  • The Stellarvue Deluxe 2 inch star diagonal features a 1/10 wave mirror and a 99% reflective dielectric coating that enhances contrast and performance. read more...
  • Our price: $189.00

  • TeleVue 45 Degree Erecting Prism 1.25"
  • TeleVue 45 Degree Erecting Prism 1.25"

  • While star diagonals correct a refractor's inverted image, they don't correct the left-right orientation. For terrestrial viewing, choose an erecting prism to achieve a natural view. read more...
  • Our price: $59.00

  • TeleVue 60 Degree Everbrite 1.25"
  • TeleVue 60 Degree Everbrite 1.25"

  • The TeleVue 60 Degree Everbrite diagonal is designed for comfortable terrestrial use and presents an image that is correct in the up-down perspective but left-right reversed. read more...
  • Our price: $239.00

  • TeleVue 90 Degree Enhanced Aluminim Diagonal 1.25"
  • TeleVue 90 Degree Enhanced Aluminim Diagonal 1.25"

  • TeleVue's basic 1-1/4 inch star diagonals feature a solid body construction for strength and rigidity of alignment and enhanced aluminum mirror coating with 96% reflectivity. read more...
  • Our price: $130.00

  • TeleVue 90 Degree Enhanced Aluminum Diagonal 2"
  • TeleVue 90 Degree Enhanced Aluminum Diagonal 2"

  • The 2 inch Enhanced Aluminum mirror coating provides a 96% reflective surface across all but the extreme ends of the visual spectrum. read more...
  • Our price: $219.00

  • TeleVue 90 Degree Everbrite Diagonal 1.25"
  • TeleVue 90 Degree Everbrite Diagonal 1.25"

  • TeleVue Everbrite diagonals feature advanced broadband di-electric coatings and solid body construction providing both greater durability and 99% reflectivity. read more...
  • Our price: $185.00

  • TeleVue 90 Degree Everbrite Diagonal 2"
  • TeleVue 90 Degree Everbrite Diagonal 2"

  • TeleVue 2-inch Everbrite star diagonal with 99% reflective di-electric coating features solid body construction for strength and rigidity of alignment. read more...
  • Our price: $285.00

  • TeleVue Long SCT Adapter
  • TeleVue Long SCT Adapter

  • This adapter is used to attach a 2" TeleVue diagonal to a Schmidt-Cassegrain telescope. read more...
  • Our price: $48.00

Buy Telescope Star Diagonals

When you aim a refractor, SCT or Maksutov_Cassegrain telescope up at the night sky, the light cone exiting the telescope points downwards. This can make for an awkward viewing experience, as the higher the telescope points, the closer to the ground you have to be to see through it.

Star diagonals, also known as telescope diagonals, fix these problems, providing a more comfortable viewing angle. The only telescope type that doesn’t need a star diagonal is the Newtonian reflector, which has a focuser that points out sideways from the top of the telescope

Telescope star diagonals are simple accessories, deflecting the light from a telescope by 90 degrees. All the viewer needs to do is slide the telescope diagonal into the focuser. Star diagonals work in one of two ways:

  • Prism: Prism star diagonals use a triangular-shaped prism, with the base of the prism positioned at 45 degrees. When the incoming light hits the base of the prism, it’s deflected by 90 degrees, so it goes into the eyepiece. The light path in a prism diagonal is slightly longer than that of a mirror diagonal, and prisms tend to be more prone to chromatic aberration. However, prisms don’t degrade over time and transmit as much light as a mirror, producing high-contrast images for a longer period than most regular mirror diagonals.
  • Mirror: Mirror diagonals work similarly to prism star diagonals, with a mirror oriented at exactly 45 degrees to the incoming beam of light so the light is reflected directly into the eyepiece. The efficacy of these mirror diagonals depends on the high reflectance of the mirror, which is why dielectric diagonals, using dielectric mirrors, are so popular — they reflect up to 99% of incident light, producing a brighter image. Dielectric mirrors also don’t degrade over time, unlike regular mirrors.

Diagonals are available in multiple sizes, but upgrading the diagonal from 1.25-inch format to a 2-inch star diagonal is often beneficial as it would allow the use of 2-inch eyepieces. Owners of Schmidt-Cassegrain telescopes larger than eight inches in aperture should seriously consider upgrading to a 2-inch diagonaldielectric mirror diagonal.

Skies Unlimited offers quality star diagonals from a variety of brands, including Tele Vue and Stellarvue star diagonals. We also provide the expertise you need to make the best decision for your viewing needs.

Established in 2004 by amateur astronomers striving to make astronomy more accessible to the masses, Skies Unlimited strives to provide our customers with the equipment they need along with the knowledge and advice to support them in their new hobby. No matter what you need, from advice on recommendations to product specifications, we’re here to help.

Learn More Today

Lookinf for a new star diagonal? Skies Unlimited can help. Call us today or visit our store located just outside of Pottstown, Pa., and we’ll get you the information you need to continue improving your observational experience.

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