How to Use an Extension Tube

An extension tube for a telescope or other optic acts between the objective lens and the eyepiece of the instrument by spacing and funneling light. Extension tubes affect focus capability by moving the lens within the eyepiece further from the mirror within the telescope.

Things You'll Need

• Telescope and eyepiece
• Accessory adapter (optional)
• Extension tube for your optics model

Instructions

Get the Right Tube

1. Select the object you want to view, such as a galaxy, a specific star or a feature of a certain planet. This will give you an idea of the field of view you need to achieve.

2. Decide on the optical effect you wish to attain via focus, such as enhancing detail on the moon's surface or bringing a distant star into sight.

3. Get some advice from your telescope manufacturer's instruction manual, astronomical literature or through astronomer or optics shop contacts on how much focal extension to shoot for. This will tell you which length of extension tube to select.

Achieve the Right Tube Effect

1. Loosen the clamp or thumbscrew that secures the telescope eyepiece.

2. Remove the eyepiece from its mount on the telescope body and set it in a safe place.

3. Insert the extension tube on the telescope accessory mount and tighten the clamp or other locking device.

4. Attach the telescope eyepiece to the free end of the extension tube and secure it with its locking device. Since attachment of eyepieces to extension tubes varies widely by model, it's very important to carefully follow your manufacturer's instructions during this process to avoid damaging the telescope.

5. Experiment and use different tube lengths under different light conditions to discover the right focus for your viewing objective.

Tips & Warnings

• Extension tubes typically come in a range of diameters and lengths. They usually attach at the T-mount site in front of the eyepiece.

• Trial and error are unavoidable when using extension tubes. Experience will help you determine which length of extension you will need to use in order to achieve the desired focus.

• Extension tubes are also used with 35mm cameras to enhance the focus for still or macro photography. You can most likely get one specifically designed for your camera though the manufacturer.

• Metal or plastic extension tubes have no lens components, so they represent an economical investment with a demonstrative effect on astronomical or other viewing.

• An extension tube is not a magnifier and should not be used in place of an eyepiece.

• An extension tube provides no "white light" solar protection. You will still need to use a specially made solar filter in order to safely view solar eclipses.