Alignment is easy using the small opening on one end and thin cross hairs at the other end, light enters the flat mirror via the side port then reflects off the the telescopes optic back through the viewing hole. The collimation eyepiece is more accurate than the simple Collimation Tool and is also useful on refractors with adjustable lens cells.
Optical alignment is essential with telescopes of all designs to enable star images to be round and sharp as well as providing high contrast and detail when viewing the Moon and Planets.
How to use a Cheshire Collimation Eyepiece
Remove the eyepiece or any other accessory from the focuser and rack the focuser all the way in towards the tube.
2.Insert the Collimation Eyepiece into the focuser far enough that the bottom edge of the barrel appears slightly larger than the outer edge of the secondary mirror when looking through the sight hole.
3.Rotate the eyepiece so that the cutout on the side of the barrel is towards an external light source. (If collimating at night, shine a flashlight through the cutout). Never point the cutout side of the eyepiece at or near the sun; serious and instant eye damage can result.
4.Look into the sight hole to determine if the secondary is positioned directly under the focuser tube. The center of the secondary mirror should be positioned directly under the intersection of the cross hairs. If the secondary is not centered in the cross hairs of the Collimation Eyepiece, it will need to be adjusted. This is usually done by rotating the threaded rod that holds the secondary holder. Additionally, the threaded rod can slide up and down through the spider for adjustments in the other axis. To adjust a stalk-type secondary holder, there is usually a center bolt that can be loosened to allow rotation of the secondary holder.
5.Next, you need to adjust the tilt of the secondary mirror so that the entire reflection of the primary mirror is visible in the secondary mirror. Use the three collimation screws located on the top of the secondary mirror holder to center the reflection of the primary mirror underneath the cross hairs of the Collimation Eyepiece.
6.Now that you can see the entire primary mirror reflected in the secondary mirror, you my notice that the reflection of the secondary mirror is not perfectly centered in the primary mirror (see figure 4). To correct this, you must adjust the primary mirror collimation screws located on the bottom of the primary mirror cell. Turn the collimation screws, one at a time, until the centerof the bright ring reflected in the secondary mirror is positioned in the center of the primary mirror. For added ease and accuracy, a small black dot can be placed in the center of the primary mirror and be used to align the primary. Simply adjust the mirror until the dot on the primary is centered in the middle of the bright ring. If the view seen through the focuser tube resembles that of figure 2, then your telescope is properly collimated and ready for observing. However, if the collimation is noticeably off, repeat steps 5 and 6 until aligned.
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