Knobs for SkyWatcher Newtonian Primary with Factory Collimation Screws

Like many Chinese Dobs, the SkyWatcher has a push-pull screw arrangement for the primary mirror cell. See the photo below. Three Phillips screws pass through the OTA and thread into the mirror cell and provide a pulling force; three Allen set screws thread into the OTA and press against the back of the mirror cell and thus provide a pushing force. The pull screws are sometimes called the collimation screws, and the push screws are sometimes called the lock screws. The pull screws normally have a rubber grommet between the inside back of the OTA and the mirror cell to theoretically hold the cell in place when the push screws are loosened during the collimation process. This usually doesn’t work very well.

 

 

The proximity of the push and pull screws next to each other requires our knobs to be staggered. See photo below. To install knobs, remove a single Phillips screw and replace it with a black knob with spacer. The spacer provides support so the black knob and screw can pull the mirror cell toward the back of the scope. If there’s a rubber grommet between the OTA and mirror cell, be sure it remains in place. Tighten the knob to the same tension as the factory screw you just removed and collimation should be relatively unaffected. Repeat the exchange with the other two Phillips screws, one at a time. Next, remove one Allen set screw and replace it with a red knob. The red knob and screw can provide the pushing force without any spacers. Again, if you match tension of the knob with the (now removed) factory screw, collimation should be relatively unaffected. Exchange the other two push screws, one at a time.

 

 

Now collimate using whatever method works best. We recommend using a laser collimator and adjust the secondary mirror followed by the primary. You will need to manipulate the push and adjacent pull screw until collimation is achieved and the push screws are snug. There is no need to tighten the push screws beyond just snug enough to hold the mirror cell in place.

 

 One caution: try not to set the telescope down on the red knobs, using them as feet. This can stress the collimation system.