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June 24th: Store is closed for Quebec National Holiday. Orders placed that day will ship on June 25th.
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!
  • Adds exactly 21mm of spacing to an M42 optical train
  • M42x0.75 female threads on one side, male threads on the other
  • Machined aluminum with a black anodized finish
  • Solves critical back focus requirements for cameras and correctors
4 in stock
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • ZWO 21mm M42 to M42 Extension

    The ZWO 21mm M42 to M42 Extension adds a fixed 21mm of length to any optical train using standard M42x0.75 T-threads. This spacer is a fundamental component for achieving the precise back focus required by field flatteners, coma correctors, and ZWO's own line of cooled cameras which standardize on a 55mm back focus distance.

    Achieving 55mm Back Focus with a 21mm Spacer

    Correct spacing is not optional in astrophotography; it's the difference between sharp stars edge-to-edge and distorted, comatic images. This 21mm extension is a key part of the standard ZWO 55mm back focus solution. When combined with a 16.5mm spacer and the 17.5mm sensor-to-flange distance of many ZWO cameras, this 21mm piece completes the required 55mm path, ensuring your corrector performs exactly as designed.

    • Precision Machining: Machined from black anodized aluminum to maintain optical alignment and eliminate internal reflections that can reduce contrast.
    • Standard M42 T-Threads: Features male M42x0.75 threads on one side and female M42x0.75 threads on the other, making it a universal fit for most astronomy cameras, filter wheels, and off-axis guiders.
    • Fixed, Non-Flexing Length: Unlike variable extenders, this fixed 21mm tube provides a rigid, repeatable connection that prevents tilt in the imaging train.

  • What is the ZWO 21mm M42 Extension used for?

    The ZWO 21mm M42 Extension is used to add a precise 21mm of distance in an astrophotography imaging train. Its primary purpose is to help achieve the specific back focus distance required by optical correctors like field flatteners or coma correctors, most of which require a 55mm distance from the corrector's flange to the camera sensor.

    How do I use the ZWO 21mm extension to reach 55mm of back focus with my ZWO camera?

    Many ZWO cameras have a 17.5mm back focus distance from the front flange to the sensor. To achieve the standard 55mm, you need to add 37.5mm of spacers. This is typically done by combining this 21mm extension with a 16.5mm extension (55 - 17.5 = 37.5; 21 + 16.5 = 37.5). The exact combination depends on your specific camera and other accessories like filter wheels or off-axis guiders.

    Are the threads on the ZWO 21mm M42 Extension the same as T2 threads?

    Yes. The terms M42, T-thread, and T2-thread all refer to the same metric thread standard: M42x0.75. This 21mm extension is compatible with any accessory that uses this common standard.

    Can I stack multiple ZWO 21mm extensions?

    While you can physically thread multiple extensions together, it is generally not recommended. Stacking many spacers can introduce potential tilt and flex into your imaging train. It is always better to use a single extension of the correct length if one is available.

    Will this 21mm extension fit between my ZWO EFW filter wheel and ASI1600MM Pro camera?

    Yes. The ZWO EFW and ASI1600MM Pro both use M42 T-threads. This 21mm extension can be placed between them or elsewhere in the M42 train to achieve the necessary spacing for your specific corrector and telescope configuration.

    What is the ZWO 21mm M42 extension made of?

    The extension is machined from aluminum and features a black anodized finish. This construction ensures it is lightweight, rigid, and has a non-reflective internal surface to maintain image contrast.

  • SKU ZWO-T2-EXT-21
    MPN ZWO T2-EXT-21
    Connector Side A M42 (T or T2) Female
    Connector Side B M42 (T or T2) Male
    Length 21 mm
    Warranty 2 Years

    • 21mm M42 to M42 Extension

      × 1

  • ZWO 2-Year Limited Warranty