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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.
Baader Expanding Ring - 2" NexStar 4 (C90)/ETX
Baader Expanding Ring - 2" NexStar 4 (C90)/ETX

Baader Expanding Ring - 2" NexStar 4 (C90)/ETX

by Baader Planetarium
SKU BAA-2958500A
$48.00 - $48.00
$48.00
$48.00 - $48.00
$48.00
Price Match Policy!
  • Converts the rear port thread on Celestron C90, NexStar 4, and Meade ETX telescopes
  • Adapts the proprietary thread to a standard 2" Schmidt-Cassegrain (SC) male thread
  • Enables the use of common 2" SCT diagonals, visual backs, and reducers
  • Provides a secure, rigid connection for heavier accessories
  • Precision-machined for a no-flexure fit
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Downloads
  • Warranty

  • Baader Expanding Ring - 2" NexStar 4 (C90)/ETX

    The Baader Expanding Ring for NexStar 4 (C90)/ETX telescopes converts the small, proprietary rear cell thread on these popular Maksutov-Cassegrains to the universal 2" Schmidt-Cassegrain (SC) standard. This single, precision-machined component unlocks compatibility with the vast ecosystem of 2" diagonals, visual backs, and photographic accessories designed for larger SCTs.

    Unlock 2" SC Accessories on Your C90, ETX, or NexStar 4

    The main limitation of compact Maksutovs like the Celestron C90, NexStar 4, and Meade ETX is their non-standard rear port, which restricts you to 1.25" accessories. This expanding ring directly addresses that by threading onto your telescope's rear cell and presenting a standard male 2" SC thread, making your compact scope compatible with a much wider range of equipment.

    • Use 2" Diagonals: Gain the ability to use high-quality 2" diagonals and wide-field 2" eyepieces for breathtaking low-power views.
    • Attach SCT Reducers: Connect standard f/6.3 SCT reducer/correctors to widen your field of view for imaging.
    • Mount SCT-Threaded Cameras: Directly attach imaging cameras and filter wheels that use the 2" SC thread standard for a secure connection.

    From Proprietary Threads to a 2" Rigid Connection

    Unlike simple 1.25" visual backs that often rely on a single thumbscrew and can introduce tilt or flexure, this Baader ring provides a solid, threaded connection. By converting to the 2" SC thread, you create a rigid metal-on-metal foundation for your entire imaging or visual train.

    This is critical when using heavier accessories like a 2" dielectric diagonal paired with a large eyepiece, or an astrophotography camera. The secure threaded interface ensures perfect alignment and eliminates the accessory sag that can compromise image sharpness and collimation, giving you a connection as robust as those found on much larger telescopes.

  • What does the Baader Expanding Ring for NexStar 4/C90/ETX do?

    This adapter converts the proprietary rear thread on compact Celestron NexStar 4, C90, and Meade ETX telescopes to the industry-standard 2" Schmidt-Cassegrain (SC) thread. This allows you to attach a wide variety of accessories that are normally only compatible with larger SCT telescopes.

    Can I use a 2" diagonal on my Celestron C90 with this Baader ring?

    Yes, this is its primary purpose. Once this ring is threaded onto your C90's rear port, you can then attach any standard 2" SCT diagonal or visual back. This lets you use 2" eyepieces for a significantly wider true field of view than is possible with 1.25" accessories.

    Will this Baader Expanding Ring work on a Meade ETX-90?

    Yes, it is designed to be fully compatible with the rear photo port thread on Meade ETX models like the ETX-90 and ETX-125. It provides the same conversion to the standard 2" SC thread, opening up many more accessory options for your ETX.

    Why is a threaded 2" connection better than my stock 1.25" visual back?

    A threaded connection is far more rigid and secure. Stock 1.25" backs often use a simple slip-fit with a thumbscrew, which can allow heavy eyepieces or cameras to sag or tilt. This Baader ring creates a solid, all-metal threaded connection that prevents flexure, ensuring your optical train remains perfectly aligned.

    Is this the same part included in the Baader 2958500 ClickLock set?

    Yes. This expanding ring is the core component of the Baader #2958500 ClickLock visual back set for these telescopes. If you already own that set, you have this adapter. This individual listing is for users who want to build a custom setup or need a replacement part.

    Do I need other adapters to use 2" accessories with this Baader ring?

    This ring provides the necessary male 2" SC thread on your telescope. You will still need an accessory with a corresponding female 2" SC thread to attach to it, such as a 2" SCT visual back, a 2" SCT diagonal, or an SCT-threaded focal reducer.

  • Net weight (kg) 0.05
    Compatible Brands Astro T-2, Baader Planetarium, Celestron
    Inner Diameter / Clear Aperture (mm) 28
    Outer Diameter 56
    Type of Adapter Filter holder, Reducing Adapter
    Optical length (mm) 2.5
    Usage changes NexStar 4 outer thread in the standard 2" SC-thread of all SC-telescopes
    Inner Connection (eyepiece/-camera-sided) M42 x 0.75 (T-2)
    Outer Connection (eyepiece/-camera-sided) 2"/SCT Thread (50.8mm)
    Inner Connection (lens sided) M28.5, 1.4" Thread (35.5 mm)
  • Baader Planetarium Warranty