How do I collimate my refractor telescope?
October 8, 2008
Collimation is the proper alignment of the optical elements–lenses and mirrors in your telescope. Collimation is important for getting the best out of your scope. Poor collimation will result in optical aberrations and distorted images.
The optical axis of the objective (main) lens must be aligned with the optical axis of the eyepiece. This is done when your scope is made at the factory and usually needs no further adjustment. However, you can collimate your objective if it loses alignment by being dropped or jarred.
Your refractor includes a collimating eyepiece that can help you to roughly check the alignment of your telescope in the daytime. The collimating eyepiece has a pinhole sight that helps you determine if the optics are properly aligned with the tube. With the focuser racked in all the way and the diagonal removed, place the collimating eyepiece inside the focuser tube. If the telescope is properly collimated, you should be able to see the entire edge of the objective lens when looking through the pinhole. If the objective lens appears oval, you need to collimate your scope.
Prior to collimating, locate the three Philips-head mounting screws on the white objective lens housing or cell on the front of the tube. (These screws should not be removed.) Remove the lens shade from the front of the tube. Next to each mounting screw is a shorter Allen-type collimation screw that pushes against the back half of the white objective cell to pivot the front half holding the lens. In order to make an adjustment, the mounting screw is loosened while the Allen screw is turned in or out. Do not remove or back out the mounting screws more than one to two turns, and do not over tighten the outer mounting screws! Only one of the three sets is adjusted at a time. Normally, motions on the order of 1/8 turn of the collimation (Allen) screw will make a difference, with only about 1/2 to 3/4 turn being the maximum required.
To star collimate in the Northern Hemisphere, point at a stationary star like the North Star (Polaris). It can be found in the north sky, at a distance above the horizon equal to your latitude. It’s also the end star in the handle of the Little Dipper. Polaris is not the brightest star in the sky and may even appear dim, depending upon your sky conditions. For the Southern Hemisphere, point at Sigma Octantis.
If there is a lot of twinkling or unsteadiness in the atmosphere, this is what astronomers call a night of poor seeing. It is best to wait until a night of very good seeing (stable atmosphere) before star collimating your scope.
With Polaris or another bright star centered in the field of view, focus with a high-power eyepiece in the 4 mm to 6 mm range. The star should be well centered in the field of view of the eyepiece. Look at the diffraction pattern both inside and outside of focus. Proceed with collimation if an asymmetrical focus pattern is present. If you can, have an assistant to turn the screws according to your instructions. Start by loosening the Phillips-head mounting screws about 1 turn and advance the Allen screw to see if the adjustment is correct. If not, undo what you did, and try another set of screws.
After making adjustments to one set, you need to re-center the star in the field of view. Check again for symmetry by moving just inside and outside of exact focus and noting the star’s diffraction pattern. Improvement should be seen if the proper adjustments are made. Since three sets of screws are present, it may be necessary to move at least two sets of screws to achieve the necessary lens movement.
Once in collimation, your telescope should not need additional collimation unless the telescope gets severely jarred or bumped again.