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.
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
4. Loosen the clamp or thumbscrew that secures the telescope eyepiece.
5. Remove the eyepiece from its mount on the telescope body and set it in a safe place.
6. Insert the extension tube on the telescope accessory mount and tighten the clamp or other locking device.
7. 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.
8. Experiment and use different tube lengths under different light conditions to discover the right focus for your viewing objective.
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