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William Optics - RedCat 51 APO 250mm F/4.9
William Optics - RedCat 51 APO 250mm F/4.9
William Optics - RedCat 51 APO 250mm F/4.9
William Optics - RedCat 51 APO 250mm F/4.9
William Optics - RedCat 51 APO 250mm F/4.9
William Optics - RedCat 51 APO 250mm F/4.9

William Optics - RedCat 51 f/4.9 APO Refractor Telescope - L-RC51IX

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    William Optics
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    William Optics RedCat 51 APO f/4.9 Refracting Telescope

    William Optics designs an impeccable astrograph in the Redcat 51 APO!  

    • Features a tilt-adjustment mechanism on the rear to ensure that your camera sensor is squared with the optical axis
    • Features a focusing index on the lens body
    • Features a new mounting ring and dovetail rail

    Quick Overview

    • Ultra-compact 51mm Petzval apochromatic refractor with razor sharp optics and intrinsically flat field
    • Ideal for wide-field astronomical or terrestrial imaging with APS-C or full-frame DSLR, mirrorless, or dedicated astronomy cameras
    • Precision helical focuser, field rotator, and Bahtinov mask for fast and precise set-up
    • Includes slot for 48mm astronomy filters and M48 threads to accommodate a wide range of cameras




    Light-weight with razor sharp optics. It’s ideal for ultra-wide-field imaging of the deep sky and for terrestrial imaging of birds, landscapes, and wildlife.

     Fast, Razor-Sharp Optics and Flat Field with a Large Field of View

    The RedCat 51 / SpaceCat 51 utilizes FPL53 and FPL51 glass in a 4-element Petzval design for excellent color correction and an exceptionally flat field. No field flattener will ever be needed with this scope. The RedCat 51 is constructed to have a big 43mm image circle compatible with APS-C and full-frame DSLRs, mirrorless, and dedicated astronomy cameras. With a camera with a full-frame (36mm x 24mm) sensor, the telescope offers a field of view nearly 10 degrees.

    Usable as a Portable Visual Telescope too

    The RedCat 51  is optimized as an imaging instrument; however, it also serves as a great visual scope. Just add William Optics' erect image diagonal for visual and/or daytime use.


     Precision Focuser and Field Rotator

    The engineered field rotator makes it easy to frame your images. The field rotator is laser etched and fully rotatable by 360 degrees. It is also marked to a precision of one degree.

    Accommodates a 48mm threaded astronomy filter and a T-ring adapter for your camera of choice.


    Ample Back-Focus to Accommodate DSLRs and Many Astro-Imaging Cameras

    The lens assembly can be moved within the telescope body over 33mm by rotating the focusing collar in the front.

     Included Accessories

    •  Super-bright Bahtinov mask that makes it a snap to focus the telescope in an instant 
    •  Dual-spec reversible mounting base for Vixen-style astronomy mounts or an Arca-Swiss mount used in photography tripod heads.
    • A sun-shade/dew-shield which you can invert and slide over the objective cell to minimize the length of the tube for transport and storage. 
    • Beautiful Anodized aluminum alloy Tube

    Redcat 51 Cross-section Aluminum Alloy

    Ideal for Bird, Wildlife, and Nature Photography

    it has a close focusing distance of only 3.7m. Use the 2" nosepiece from the diagonal as an extension tube and the focusing distance drops to 3.0m. 

    • Soft carry case
    • RedCat 51mm f/4.0 OTA - Red OTA with Black Accents
    • M48 Camera Threaded Mount
    • Patented Bahtinov Mask

    • Model Number L-RC51Ix
      Telescope Series William Optics Cat
      Focal Ratio f/4.9
      Optical Design Refractor
      Telescope Aperture 51mm (2")
      Telescope Mount Type No Mount - OTA Only
      Type of Electronics OTA Only - No Electronics
      Number of Refractor Elements 4 (Quad)
      Focal Length (mm) 250 mm
      Image Circle 44mm
      Focuser Style Helical
      Refractor OTA Length - Extended (in.) 12" (305 mm)
      Refractor OTA Length - Retracted (in.) 8.86" (225 mm)
      Telescope OTA Weight (lb.) 3.24 lb. (1.47 kg)

    Other Information

    We work very hard to ensure that we offer the absolute best prices online.  If you find another online store that offers a lower price then us within six months of your purchase date please let us know and we will refund your original payment for the difference.  We want you to feel confident that you are getting the absolute best price for the product you are ordering.  If you find that our own website has a lower price for the same item you have ordered within six months of your purchase date will refund the difference as well.

    To request your partial refund simply e-mail us a link to the same product on our website, or on our competitors website within six months from the date of your order and we will process the credit accordingly.

    Our 100% Price Guarantee has some limitations:

    • You must purchase the item from our website before requesting your Price Match Guarantee
    • Promotions such as rebates and buy one, get one free offers are not eligible
    • The item must be in stock on the competitors website
    • The competitor must be an online store, they may not have a retail location
    • The website can not be a discounter or auction website (ie; eBay, overstock, etc..)
    • The competitor must be an Authorized Retailer of the product in question
    • The Price Match Guarantee includes the item price and the shipping charges, it excludes sales tax

    Order Confirmation As soon as you place your order you will receive an order confirmation e-mail.  This means that we have received your order in our system and pre-authorized your credit card for the purchase.  As soon as we receive your order we automatically reach out to our suppliers to confirm that it is in stock and available for immediate shipment.  If your item is on backorder or unavailable we will void the pre-authorization and reach out to you via e-mail.  If your item(s) are available for immediate shipment (within 5 business days) we will process the charges and submit the order for shipment.

    Order Shipment If your order is received, the inventory is in stock and your credit card information matches the information, In most cases, we can ship within 5 business days. We will send you tracking information within 24 hours of your order leaving the warehouse to the e-mail address your provide when checking out.  If you do not receive tracking information from us within six business days of your order please email us at

    Miscellaneous Shipping Information

    • We cannot ship to PO boxes.
    • Non-standard shipping surcharges will apply to isolated service areas, such as remote areas, areas not readily accessible by a delivery vehicle, islands, Hawaii & Alaska etc., as defined by the shipper. These circumstances occur less than 1% of the time.

    Delivery Times

    You can estimate the deliver time to your home or business by viewing the "Ships Within" information listed with every product. This refers to the time from when you place your order, to the time it leaves the warehouse. The total delivery time for your order is calculated from the time your order is placed, until you receive it. The diagram below shows how the total delivery time is broken down into pre-ship processing time and shipping time. 

    Processing time is the time it takes from when you submit your order to when the product leaves the warehouse. The "Ships Within" time frame listed with each of our products on the website applies to this time frame. For example, if your order a product that reads "Ships Within: 2 Weeks", this means that the product typically leaves the factory within two weeks. If there will be any additional time required to process your order, we will notify you immediately to confirm the delivery period. This time is needed for products that are shipped to you directly from our manufacturers. The manufacturers need this time to fulfill your order. In some cases, the items are custom built.  Shipping time is measured when your order leaves the warehouse to when it arrives at your address. Actual shipping time can vary between 2-5 business days. Contact our helpful customer service experts at (888-264-2244)  for more information.


    Did your shipment arrive damaged or defective?

    Please report damaged or defective merchandise to us immediately by email phone. Photos of the items and the packaging would be greatly appreciated. This will speed up our resolution process. We strongly recommend at the time of delivery, if your notice any cartons that look dented, crushed or just not right, open them immediately and inspect the contents. If the contents appear to be damaged, please notate it on the delivery ticket, refuse that item only and contact us as soon as possible for instructions.

    Brands/Products that do NOT qualify for our 30 Day Return Policy:


    APM products do not qualify for our 30 day return policy.

    Custom Orders

    Additionally, items that are special ordered, customized, or engraved - cannot be returned or refunded under our 30 day return policy. 

    If your engraved item needs to be repaired within 30 days of purchase, please reach out to us and we will have the product repaired or replaced, re-engrave if necessary, and return it to you in new condition.

    Customer Reviews

    Based on 2 reviews
    Loc Nguyen
    William Optics Red Cat 51

    Look beautiful , as expected , thank you very much, try to take picture with it
    but got no luck , cloudy all week, over all the scope is very nice.

    Awesome Little Scope!

    I bought this little APO a couple of months ago to take widefield shots that won't fit in single frame when I use a 80mm Apochromat. Among the objects I have captured are the North America Nebula, the (entire) Veil Nebula, parts of the Antares Region, and large swaths of the sky in Orion. In addition, objects like the Pleiades look very good with a bit of space around them. As the season progresses, I look forward to working more in Orion, and photographing the Ring and Cone Nebulae. I bought the WO diagonal for this refractor, but I haven't done any visual observing, but it is a great tool for astrophotography. If you're new to AP, I would STRONGLY consider a small APO like this. It is easy to find objects in the sky, since the field of view is wide and forgiving, and the focus is sharp enough to pick up double stars (like Polaris). The only thing I found is that with an AVX or larger, you will need to acquire a smaller (say 2kg or 5lbs) counterweight. The 5kg (11 lbs) counterweights that come with the AVX and larger mounts have to be placed so high on the bar that collisions are a threat. The color and clarity of this scope are excellent, and there are a lot of nice touches that you find in Taiwanese and Japanese products - like the included Bahtinov mask. If you have a larger mount, consider attaching a longer dovetail; the WO dovetail is a bit short.


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