{"product_id":"william-optics-d-ep90-125-rc61","title":"William Optics 1.25\" 90° Erect Prism for Cat 61 WIFD","description":"\u003cul class=\"DAV-feature-list\"\u003e\n\u003cli\u003eProvides a fully corrected, erect image for terrestrial viewing\u003c\/li\u003e\n\u003cli\u003eComfortable 90° viewing angle\u003c\/li\u003e\n\u003cli\u003eStandard 1.25\" barrel accepts most astronomical eyepieces\u003c\/li\u003e\n\u003cli\u003eDesigned specifically for the William Optics Cat 61 WIFD telescope\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c!-- more --\u003e\n\u003cul class=\"tabs\"\u003e\n\u003cli class=\"active\"\u003eDescription\u003c\/li\u003e\n\u003cli\u003eFAQ\u003c\/li\u003e\n\u003cli\u003eSpecifications\u003c\/li\u003e\n\u003cli\u003eIn the Box\u003c\/li\u003e\n\u003cli\u003eDownloads\u003c\/li\u003e\n\u003cli\u003eWarranty\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003cul class=\"tabs-content\"\u003e\n\u003cli class=\"active\"\u003e\n\u003cdiv\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section-center\"\u003e\n\u003ch2\u003e1.25\" 90° Erect Prism for Cat 61 WIFD\u003c\/h2\u003e\n\u003cp\u003eThis 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.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003eThe 90° Angle: Comfortable Daytime Viewing with the Cat 61 WIFD\u003c\/h3\u003e\n\u003cp\u003eWhile 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.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003e1.25\" Barrel: Universal Fit for Standard Eyepieces\u003c\/h3\u003e\n\u003cp\u003eThe 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.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-bg-light\"\u003e\n\u003cdiv class=\"DAV-section\"\u003e\n\u003ch3\u003eErecting Prism vs. Mirror Diagonal: The Right Tool for the View\u003c\/h3\u003e\n\u003cp\u003eIt'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.\u003c\/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cstrong\u003eErecting Prism (This Product):\u003c\/strong\u003e 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.\u003c\/li\u003e\n\u003cli\u003e\n\u003cstrong\u003eMirror Star Diagonal:\u003c\/strong\u003e 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.\u003c\/li\u003e\n\u003c\/ul\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWhat does the 1.25\" 90° Erect Prism do for my Cat 61 WIFD?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eIt 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 \u003cstrong\u003efully upright and correctly oriented left-to-right\u003c\/strong\u003e, making it perfect for daytime activities like birdwatching, nature observation, or surveillance.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eIs this prism better than the standard star diagonal for astronomy with the Cat 61?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eNo, 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.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eCan I use the 1.25\" 90° Erect Prism to watch birds in a nearby park with my Cat 61?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eAbsolutely. 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.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWill I be able to see the Moon correctly oriented with this Cat 61 prism?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eYes. 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.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eHow do I install the 1.25\" 90° Erect Prism on my Cat 61 WIFD?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eInstallation is simple. You insert the 1.25\" nosepiece of the erecting prism directly into the 1.25\" eyepiece holder or visual back of your Cat 61 WIFD's focuser and secure it with the thumbscrews. Then, you can insert any standard 1.25\" eyepiece into the prism's eyepiece holder.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion\"\u003e\n\u003ch3 class=\"DAV-accordion-Q\"\u003eWill this 1.25\" Erect Prism work with other telescopes besides the William Optics Cat 61 WIFD?\u003c\/h3\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"DAV-accordion-A\"\u003e\n\u003cp\u003eWhile designed for the Cat 61 WIFD, this prism should work with most refractor telescopes that use a standard 1.25\" focuser or visual back. However, you must ensure your telescope has enough focuser travel to reach focus, as erecting prisms can sometimes require more \"in-focus\" or \"out-focus\" than standard diagonals.\u003c\/p\u003e\n\u003c\/div\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003ctable class=\"DAV-specifications\"\u003e\n\u003ctbody\u003e\n\u003c\/tbody\u003e\n\u003c\/table\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cul class=\"DAV-grid\"\u003e\n\u003c\/ul\u003e\n\u003c\/li\u003e\n\u003cli\u003e\n\u003cul class=\"DAV-grid\"\u003e\n\u003c\/ul\u003e\n\u003c\/li\u003e\n\u003cli\u003e\u003ca href=\"https:\/\/davidastro.com\/pages\/william-optics-warranty-policy\" rel=\"noopener\" target=\"_blank\"\u003eWilliam Optics 2-Year Warranty\u003c\/a\u003e\u003c\/li\u003e\n\u003c\/ul\u003e","brand":"William Optics","offers":[{"title":"Default Title","offer_id":52853691711773,"sku":"WO-D-EP90-125-RC61","price":219.0,"currency_code":"CAD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0753\/5280\/1565\/files\/WilliamOptics-1-25-90-Erect-Prism-for-Cat-61-WIFD-1_b2794600-6003-4a33-9db5-871534fa1d2d.jpg?v=1788457886","url":"https:\/\/davidastro.com\/fr\/products\/william-optics-d-ep90-125-rc61","provider":"David Astro","version":"1.0","type":"link"}