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Achieve Perfect Focus with this 3.5" Extension Tube
The Tele Vue Extension Tube - 3.5" is a precision-machined 2-inch barrel extender designed to solve common back-focus issues in complex imaging and visual setups. It provides a rigid, non-flexing 89mm extension, allowing you to reach focus when your focuser's drawtube travel is insufficient for your accessory train. This component is essential for observers and imagers who use accessories like Barlows, focal reducers, or off-axis guiders that alter the focal plane.
Fixed 3.5" (89mm) Optical Extension
This extension tube provides a fixed 3.5 inches (89mm) of outward travel for your focuser. This specific length is often required when using certain accessories, like a Tele Vue Paracorr with a DSLR or specific eyepiece combinations on refractors that have limited focuser travel. Using a fixed tube provides a more rigid and stable connection than stacking multiple smaller extenders, which can introduce flexure and tilt into the optical path.
The primary benefit is achieving focus where it was previously impossible. For imagers, this means bringing your camera sensor to the precise focal plane required for sharp stars across the frame. For visual observers, it allows the use of long focal length eyepieces or Barlow lenses that would otherwise remain blurry.
Standard 2" Barrel Interfaces for Universal Fit
The Tele Vue X3C-0009 is engineered for broad compatibility across the Tele Vue ecosystem and with standard 2-inch accessories. The male end is a standard 2-inch barrel that inserts securely into your telescope's focuser or diagonal, held in place by your existing compression rings or setscrews. The female end accepts any standard 2-inch accessory, from eyepieces to camera nosepieces.
This universal fit eliminates guesswork. You can confidently integrate it into your existing collection of 2-inch gear without worrying about proprietary threads or adapters. The machined shoulders ensure that accessories seat squarely, which is critical for maintaining optical alignment and preventing image degradation.
Internal Baffling for Maximum Image Contrast
An extension tube is more than just a spacer; it is an active part of your optical train. The interior of the Tele Vue 3.5" Extension Tube is machined with anti-reflection threads and finished in a deep matte black. This treatment acts as a light baffle, trapping off-axis stray light that could otherwise reflect off the inner walls and reach your eyepiece or sensor.
The direct visual consequence is higher contrast. By preventing internal reflections, the background sky in your images remains darker, and subtle details in faint nebulae or galaxies are not washed out. This commitment to stray light control is a core principle of Tele Vue design and ensures that adding this extender does not compromise the performance of your primary optics.
A Critical Note on Telescope Balance
While providing a crucial function, adding an 89mm extension and a heavy accessory to the back of your telescope significantly shifts the center of gravity. This is the product's main trade-off. Before use, you must ensure your telescope is properly re-balanced on its mount to prevent tracking errors or, in extreme cases, unwanted slewing.
For observers with smaller mounts, this added leverage may require additional counterweights or shifting the optical tube assembly forward in its rings. Properly balancing the system is not just a recommendation; it is essential for maintaining the stability and tracking accuracy of your entire setup, particularly for long-exposure astrophotography.
Is This the Right Extension Tube For You?
This 3.5" extension tube is the right choice when you have a significant focus shortfall, typically encountered when adapting imaging systems or using specific Barlow/eyepiece combinations. It is for the observer who needs a robust, single-piece solution to bridge a large gap in focuser travel without introducing flex.
If you only need a small amount of extension (less than 2 inches), Tele Vue offers shorter tubes that may be a better fit and have less impact on balance. However, when your camera or eyepiece simply won't come to focus and you have inches to go, the X3C-0009 is the specific, high-quality tool designed to solve that exact problem.
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When do I need a 3.5" extension tube?
You need an extension tube when your focuser runs out of outward travel before your eyepiece or camera reaches focus. This often happens with refractors when observing straight-through (no diagonal) or when adding accessories like a focal reducer or off-axis guider to an imaging train.
Will this extension tube cause vignetting?
With its 2-inch diameter, this tube is designed to pass the full light cone for sensors up to full-frame (35mm format) and any 2-inch eyepiece without causing vignetting. It will not be the limiting factor in your optical train.
Is it better to use one long tube or stack several short ones?
One long, rigid tube is always better. Stacking multiple shorter extension tubes creates more connection points, which can lead to flexure and tilt in the optical path, degrading image quality. The 3.5" tube provides a single, solid connection.
How does this affect my telescope's balance?
Adding this tube pushes your eyepiece or camera further back, shifting the center of gravity. You will need to re-balance your telescope by sliding the optical tube forward in its rings or by adding more counterweights to ensure proper tracking.
Does the Tele Vue 3.5" extension have filter threads?
No, this extension tube does not typically have threads for 2-inch filters. Filters should be placed on the nosepiece of your camera or on the barrel of your eyepiece before inserting them into the extension tube.
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| Product Number |
X3C-0009 |
| Extension Length |
3.5 inches (89mm) |
| Barrel Diameter |
2 inches |
| Telescope Side Connection |
2" Male Barrel |
| Accessory Side Connection |
2" Female Barrel with compression lock |
| Material |
Black Anodized Aluminum |
| Internal Design |
Anti-reflection threaded and blackened |
| Weight |
Approx. 6.4 oz (181 g) |
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Tele Vue 3.5" Extension Tube - 2"
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Tele Vue 5-Year Limited Warranty
