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IMPORTANT: Our store is located in Canada. All orders that include products manufactured outside of Canada or USA will be subject to tariffs and duties, regardless of the order value. US customers are responsible for all applicable duties and tariffs, and those will be billed by the carrier, except for the Lacerta upgrade kit, for which we collect them at checkout.
ZWO M42 Extension - 21mm
ZWO M42 Extension - 21mm

ZWO M42 Extension - 21mm

by ZWO
SKU ZWO-T2-EXT-21
$18.00 - $18.00
$18.00
$18.00 - $18.00
$18.00
Price Match Policy!
  • Optical Length: 21mm
  • Connection Side 1: M42x0.75 Female Thread
  • Connection Side 2: M42x0.75 Male Thread
  • Outer Diameter: 2" (50.8mm)
  • Material: Aircraft-Grade 6061 Aluminum
  • Finish: Matte Black Anodization
4 in stock
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • The 55mm Back Focus Solution: ZWO M42 T2 Extension - 21mm

    This is the ZWO M42 Extension - 21mm, a precision-machined T2 spacer designed for one critical purpose: achieving the industry-standard 55mm back focus required by most field flatteners and coma correctors. For imagers using a ZWO ASI cooled camera, this 21mm extension is often the final, essential piece that connects your camera and filter wheel to a fully corrected optical system, ensuring sharp, aberration-free stars across your entire frame.

    Achieving Critical Back Focus with M42 Threads

    The universal M42x0.75 thread, commonly known as the T2-thread, is the standard for amateur astronomy. This 21mm extension uses male and female T2 threads to provide a secure, flexure-free connection between your camera and other accessories. Its rigid design prevents the optical tilt that can ruin an image, a common point of failure in less precise spacing solutions.

    When combined with a ZWO ASI cooled camera (17.5mm back focus) without filter wheel or filter drawer, this 21mm spacer completes the optical path. The calculation is simple: 17.5mm (camera) + 21mm + 16.5mm totals 55mm. This configuration allows your corrector to perform exactly as designed.

    Engineered for Precision Optical Length

    In astrophotography, spacing is not approximate. The 21mm optical length of this ZWO M42 extension is machined to tight tolerances, ensuring you hit your back focus target without guesswork. This precision is what separates a good imaging setup from a great one, transforming elongated stars in the corners of your image into sharp, pinpoint sources of light.

    This fixed length is a deliberate design choice. Unlike variable extensions, a solid tube offers zero chance of slipping or changing length during an imaging run. This reliability means you can assemble your imaging train with confidence, knowing this component will hold the required spacing perfectly throughout the night.

    Lightweight and Reflection-Controlled Construction

    ZWO manufactures this T2 extension from aircraft-grade 6061 aluminum for a combination of low weight and high strength. This prevents the spacer from adding unnecessary torque or balance issues to your focuser tube, which is especially important for smaller telescope mounts.

    The interior of the extension is treated with a matte black anodization. This finish is not cosmetic; it is an optical surface designed to absorb stray light and prevent internal reflections from bouncing onto your sensor. The result is higher contrast and a darker background sky in your final images, allowing faint details in nebulae and galaxies to stand out.

  • Which ZWO spacer do I need for 55mm back focus?

    If you are using a ZWO cooled camera (17.5mm back focus) without an EFW or filter drawer, this 21mm spacer is the one you need. When added to the camera's included 16.5mm adapter, the total distance becomes exactly 56mm.

    Will this M42 extension work with non-ZWO cameras or filter wheels?

    Yes. The M42x0.75 (T2) thread is an industry standard. This 21mm extension will connect to any accessory that uses standard male or female T2 threads, regardless of the manufacturer.

    Does this ZWO T2 extension cause vignetting?

    With its 42mm clear aperture, this extension is designed to be free of vignetting on sensors up to APS-C size. For full-frame sensors, you may need to use larger M48-threaded extensions to avoid darkening in the corners of your image.

    Can I stack multiple ZWO M42 extensions?

    Yes, you can thread multiple extensions together to create a custom length. However, for maximum rigidity and to minimize potential tilt, it is always best to use a single extension of the correct length whenever possible.

    Is this the same as an M48 extension tube?

    No. This is an M42 (42mm diameter) extension. M48 extensions have a larger 48mm diameter and a different thread pitch (M48x0.75). The two are not compatible without a dedicated M42-to-M48 adapter.

  • Product Code T2-EXT-21
    Optical Length 21 mm
    Thread Type M42x0.75 (T2-Thread)
    Connection Side 1 M42x0.75 Male
    Connection Side 2 M42x0.75 Female
    Clear Aperture ~38 mm
    Outer Diameter 2" (50.8mm)
    Material 6061 Aluminum
    Finish Matte Black Anodized

    • ZWO M42 Extension Tube - 21mm

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  • ZWO 2-Year Warranty