Skip to content
Price match 30-day returns* Talk to an astronomer before you buy
William Optics 1.25" 90° Erect Prism for Cat 61 WIFD
William Optics 1.25" 90° Erect Prism for Cat 61 WIFD
William Optics 1.25" 90° Erect Prism for Cat 61 WIFD
William Optics 1.25" 90° Erect Prism for Cat 61 WIFD
William Optics 1.25" 90° Erect Prism for Cat 61 WIFD

William Optics 1.25" 90° Erect Prism for Cat 61 WIFD

by William Optics
SKU WO-D-EP90-125-RC61
Save 5% Save 5%
CA$219.00
CA$219.00 - CA$219.00
CA$219.00
CA$208.00
CA$208.00 - CA$208.00
CA$208.00
Price Match Policy!
  • Provides a fully corrected, erect image for terrestrial viewing
  • Comfortable 90° viewing angle
  • Standard 1.25" barrel accepts most astronomical eyepieces
  • Designed specifically for the William Optics Cat 61 WIFD telescope
1 in stock
Qualifies for FREE Shipping in Canada on orders over $500.
  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • 1.25" 90° Erect Prism for Cat 61 WIFD

    This 90° erecting prism converts the William Optics Cat 61 WIFD from an astronomical instrument into a high-magnification terrestrial spotting scope. It delivers a fully upright and non-reversed image through its standard 1.25" barrel, making the Cat 61's apochromatic optics ideal for birdwatching, landscape observation, or any daytime viewing application.

    The 90° Angle: Comfortable Daytime Viewing with the Cat 61 WIFD

    While straight-through viewing is common in spotting scopes, the 90° angle of this prism provides a more comfortable and ergonomic viewing position, especially when observing targets at a high elevation or for extended periods. This design allows you to use your Cat 61 WIFD on a standard photo tripod without needing to crouch into an awkward position, mirroring the comfort of astronomical observation.

    1.25" Barrel: Universal Fit for Standard Eyepieces

    The prism features a standard 1.25" barrel, ensuring compatibility with the vast majority of astronomical eyepieces. This allows you to leverage your existing eyepiece collection to achieve a wide range of magnifications for terrestrial viewing. You can use a low-power eyepiece for wide scenic views and switch to a high-power one to resolve fine details on distant subjects, all while maintaining a correctly oriented image.

    Erecting Prism vs. Mirror Diagonal: The Right Tool for the View

    It's important to select the right diagonal for your purpose. While this erecting prism is the definitive choice for daytime terrestrial use, a traditional mirror star diagonal is generally preferred for critical astronomical observing.

    • Erecting Prism (This Product): Uses a prism to correct the image both vertically and horizontally. This is essential for terrestrial viewing where an upside-down or mirrored view would be disorienting. The extra glass path can introduce minor chromatic aberration at very high powers on bright astronomical targets.
    • Mirror Star Diagonal: Uses a highly reflective mirror to bend the light path. It corrects the image vertically but leaves it reversed left-to-right. This is perfectly acceptable for astronomy but unsuitable for terrestrial use. Mirror diagonals typically offer slightly higher light transmission and optical purity for faint deep-sky objects.

  • What does the 1.25" 90° Erect Prism do for my Cat 61 WIFD?

    It converts your Cat 61 WIFD telescope, which naturally produces an inverted and reversed image, into a terrestrial spotting scope. The prism corrects the image to be fully upright and correctly oriented left-to-right, making it perfect for daytime activities like birdwatching, nature observation, or surveillance.

    Is this prism better than the standard star diagonal for astronomy with the Cat 61?

    No, for critical astronomical viewing, a high-quality mirror star diagonal is typically recommended. Erecting prisms involve more complex light paths that can introduce very subtle optical aberrations, which might be noticeable at high magnification on planets or double stars. This prism is specifically optimized for daytime terrestrial use where a correct image is mandatory.

    Can I use the 1.25" 90° Erect Prism to watch birds in a nearby park with my Cat 61?

    Absolutely. This is the ideal application for this accessory. Pairing the Cat 61 WIFD's excellent apochromatic optics with this erecting prism will provide sharp, color-correct, and high-magnification views of wildlife, far surpassing what is possible with typical binoculars or small spotting scopes.

    Will I be able to see the Moon correctly oriented with this Cat 61 prism?

    Yes. While a mirror diagonal is often preferred for detailed lunar study, this erecting prism will show you the Moon exactly as it appears to the naked eye or on a map (upright and not mirrored). This can be helpful for identifying specific craters and features from a lunar atlas without having to mentally flip the image.

  • Kind Erecting Prism, 90°
    Barrel / eyepiece size 1.25"
    Telescope-Side Connection 1.25" barrel

    • William Optics 1.25" 90° Erect Prism for Cat 61 WIFD

      × 1

  • William Optics 2-Year Warranty