How to Collimate a Refractor Telescope

Written by joel douglas
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How to Collimate a Refractor Telescope
Collimate your refractor telescope to improve its optical performance. (refractor telescope image by Jim Mills from

Refractor telescopes use a series of precisely spaced glass lenses housed in an optical tube to focus the light from distant objects. A perfectly aligned refractor telescope will let you observe fine detail on the moon and planets. If the lenses come out of alignment, however, a refractor telescope will only provide blurry views of the night sky. You can check and realign the glass lenses in your refractor through a process known as collimation.

Skill level:

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Things you need

  • Magnifying eyepiece
  • Phillips-head screwdriver
  • Allen wrench

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  1. 1

    Set up the refractor telescope at night. Aim the telescope at the North Star, Polaris, which lies at a distance equal to your latitude in the northern sky. Although you can use other stars to test collimation, Celestron recommends using Polaris because it remains stationary all night. Other stars change position throughout the night, requiring you to reorient the scope during the collimation process.

  2. 2

    Insert a magnifying eyepiece into the telescope's focuser. Look through the eyepiece and verify that Polaris is centred in the field of view. Adjust the focus knob until the star appears sharp in the eyepiece.

  3. 3

    Turn the focus knob counterclockwise to defocus the telescope. Examine the image through the eyepiece. You should see a series of evenly spaced, concentric rings. Now turn the focus knob clockwise to defocus the telescope in the other direction. Examine the image. You should again see a series of evenly spaced, concentric rings identical in appearance to the first defocused image. If the rings do not appear concentric, you need to collimate the telescope.

  1. 1

    Remove the dew shield and cap from the front of the telescope. Look through the front of the telescope. You should see three Phillips-head screws and three Allen screws evenly spaced around the outer glass lens.

  2. 2

    Loosen the Allen screws slightly, and then tighten the Phillips-head screws.

  3. 3

    Look through the eyepiece and test the collimation again using the defocusing method. If the defocused images of Polaris do not resemble evenly spaced, concentric rings, continue adjusting the Phillips-head and Allen screws.

  4. 4

    Loosen the Phillips-head screws and tighten the Allen screws instead if you notice that the defocused images become less concentric after loosening the Allen screws and tightening the Phillips-head screws.

  5. 5

    Continue loosening and tightening the Allen and Phillips-head screws until the defocused images of Polaris resemble evenly spaced, concentric rings.

Tips and warnings

  • If your refractor telescope lacks the Allen and Phillips-head collimation screws, you will need to contact the manufacturer for professional collimation.

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