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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 M54 Extension - 21mm

ZWO M54 Extension - 21mm

by ZWO
SKU ZWO-M54-M54-21
$18.00 - $18.00
$18.00
$18.00 - $18.00
$18.00
Price Match Policy!
  • Optical Length: 21mm
  • Connection Side 1: M54x0.75 Male Thread
  • Connection Side 2: M54x0.75 Female Thread
  • Clear Aperture: 51mm
  • Material: Black Anodized Aluminum
1 in stock
Qualifies for FREE Shipping in CONUS on orders over $350.
  • Description
  • FAQ
  • Specifications
  • In the Box
  • Warranty

  • Achieve Perfect Back Focus — The ZWO M54 21mm Extension

    The ZWO M54 Extension with 21mm of optical length exists to solve one of the most critical and frustrating problems in astrophotography: achieving the exact back focus distance your field flattener or reducer requires. This precision-machined spacer is the key component for building a standard 55mm back focus train with most ZWO cameras, off-axis guiders, and filter wheels, converting calculation into sharp, round stars across your entire sensor.

    For imagers using full-frame or large APS-C sensors, maintaining a rigid, wide-aperture optical path is essential to prevent vignetting and tilt. The M54 thread standard provides a significantly larger and more stable connection than older T2 (M42) threads. This 21mm extension preserves that robust M54 connection throughout your imaging train, ensuring your camera and corrector remain perfectly orthogonal to the light path.

    Secure M54 Male and Female Threads

    This extension features M54x0.75 male threads on one side and female threads on the other, allowing it to integrate seamlessly into your existing M54 setup. The precisely cut threads ensure a smooth, secure fit that minimizes the risk of tilt, a common source of image degradation with heavier cameras. This robust connection is vital for maintaining collimation and alignment through long imaging sessions.

    The 21mm Back Focus Solution

    The 21mm length is not arbitrary. It is the exact distance needed to complete the 55mm back focus equation for a common ZWO imaging train: 17.5mm (camera sensor to flange) + 16.5mm (M54 OAG) + 21mm (this extension) = 55mm. This single, solid component eliminates the need to stack multiple smaller, less stable adapters, which can introduce flexure and tilt into the system.

    Using a single, correctly sized extension provides a level of mechanical certainty that is difficult to achieve with adjustable spacers or stacked rings. It makes your setup repeatable and reliable, so you can spend less time troubleshooting spacing issues and more time capturing clean data. The result is a sharper field, especially at the corners of your image where spacing errors are most visible.

    A ZWO M54 21mm Extension connected to a ZWO camera and filter wheel.

    Contrast-Preserving Anodized Finish

    Every internal surface of the ZWO M54 Extension is treated with a matte black anodized finish. This critical step ensures that any off-axis stray light entering the adapter is absorbed rather than reflected onto your sensor. By neutralizing these internal reflections, the extension helps preserve the maximum contrast in your images, preventing faint details from being washed out by stray light gradients.

    The Trade-Off: Fixed Length vs. Flexibility

    The primary strength of this extension is also its main limitation: its fixed 21mm length. It is a purpose-built tool for a specific back focus calculation. If your optical train requires a different distance, for example when using a non-ZWO corrector or a different filter wheel, this spacer will not be the correct choice. It prioritizes rigidity and precision over the adjustability of a variable extension tube.

    Building Your Definitive Imaging Train

    Imagine finalizing your setup, knowing your spacing is perfect. With the ZWO M54 21mm Extension, that confidence is built-in. When you see your first full-frame image with pinpoint stars from the center of the Andromeda Galaxy all the way to its faint, dusty edges, you'll understand the value of a rigid, precise optical path. This small component is the foundation of that result, ensuring your expensive corrector performs exactly as its designers intended.

  • How do I get 55mm back focus with this 21mm ZWO spacer?

    This spacer is one part of the standard 55mm formula. It connects to other ZWO components like a 16.5mm OAG or 20mm filter wheel, which then connects to a ZWO camera with a 17.5mm or 17mm flange distance. For example: 17.5mm (camera) + 16.5mm (OAG) + 21mm (this spacer) = 55mm.

    Will this M54 extension connect to my T2 filter wheel?

    No, not directly. This adapter has M54 threads on both sides. To connect to a T2 (M42x0.75) device, you would need an additional M54-to-T2 adapter ring, which would also alter your back focus calculation.

    Is a single M54 spacer better than stacking T2 rings?

    Yes, for two reasons. First, a single, solid spacer is mechanically more rigid and less prone to tilt than several smaller rings screwed together. Second, the larger M54 aperture reduces the chance of vignetting on full-frame and large APS-C sensors.

    What is the thread pitch on the ZWO M54 extension?

    The thread pitch is M54x0.75 on both the male and female threads. This is the standard for most M54 astrophotography accessories.

    Do I need this spacer if I use a ZWO full-frame camera?

    You need this or another combination of spacers if your field flattener or reducer specifies a 55mm back focus distance. This 21mm piece is specifically designed to make achieving that distance simple with other common ZWO M54 accessories.

  • Product Code M54-21
    Optical Length 21mm
    Male Thread M54x0.75
    Female Thread M54x0.75
    Clear Aperture 51mm
    Material Aluminum
    Finish Black Anodized
    • ZWO M54 Extension - 21mm

      ZWO M54 Extension - 21mm

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  • ZWO 2-Year Warranty