Pick a bright star and center it in the field of the telescope. Study the image of the star while racking it in and out of focus using an eyepiece that yields 30 to 60 power for every inch of aperture. If an unsymmetrical focus pattern is present, then collimation is necessary. (If the telescope is properly collimated, the out of focus star image will appear as a concentric ring pattern similar to that shown in Figure
To collimate, the telescope should be on either a motor driven (i.e., tracking) equatorial mount that is approximately polar aligned or pointed at a stationary star without the motor drive running. Polaris, the North Star, is the perfect collimation star for northern hemisphere observers since it appears motionless against the background sky long enough to perform the collimation procedure. Polaris is the last star in the handle of the Little Dipper (Ursa Minor) and its distance above the northern horizon is always equal to your latitude angle.
Prior to collimating, locate the three (3) mounting screws on the objective lens housing on the front of the tube. (These screws attach the objective lens housing to the main tube and should not be removed). It may be necessary to remove the lens shade from the front of the tube to allow easy access to the collimation screws. Next to each mounting screw is a shorter Allen screw (collimation screw) that pushes against the optical tube to pivot the objective lens housing (see Figure
two (2) turns!
With Polaris or another bright star centered in the field of view, focus with your highest power eyepiece (i.e., one with the shortest focal length). This includes eyepieces in the 4mm to 6mm range. The star should be well centered in the field of view of the eyepiece. It may be helpful for two people working together, while one views and instructs the other which screws are correctly turned and by how much. Start by loosening one Phillips head (mounting) screw about 1 turn and advancing the Allen screw to see if the motion is correct. If not, undo what you did and try another set of screws.
After making the first adjustment, it is necessary to |
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the field of view. It can then be judged for symmetry by going just inside and outside of exact focus |
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and noting the star’s pattern. Improvement should be seen if the proper adjustments are made. Since |
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Figure | ||
three (3) sets of screws are present, it may be necessary to move at least two (2) sets of screws to | ||
achieve the necessary lens movement. DO NOT over tighten the outer mounting screws! | A collimated telescope | |
should appear as a | ||
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Once in collimation, your telescope should not need additional collimation unless the telescope has | symmetrical ring pattern | |
similar to the diffraction | ||
been bumped or jarred severely. | disk seen here. |
Refractor Collimating Eyepiece- 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 site 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 pin hole. If the objective lens appears oval, then it may be necessary to collimate the telescope as described above.
Collimation of a Schmidt-Cassegrain
Figure
The three collimation screws are located on the front of the secondary mirror housing.
The optical performance of your telescope is directly related to its collimation, the alignment of its optical system. Your telescope was collimated at the factory after it was completely assembled. However, if the telescope is dropped or jarred severely during transport, it may have to be collimated. The only optical element that may need to be adjusted, or is possible, is the tilt of the secondary mirror.
To check the collimation of your telescope you will need a light source. A bright star near the zenith is ideal since there is a minimal amount of atmospheric distortion. Make sure that tracking (with an optional motor drive) is on so that you won’t have to manually track the star. Or, if you do not want to power up your telescope, you can use Polaris. Its position relative to the celestial pole means that it moves very little thus eliminating the need to manually track it.
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