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Building a 10" Reflector

Adjusting the Rotating Head

 

The one weakness in this design is the separate rotating head. If it is not adjusted properly the telescope will not stay in collimation as the user rotates the head for a comfortable observing position. As a result, great care must be taken to adjust the upper tube head until the axis of the tube points accurately toward the center of the mirror. The process involves placing a piece of flat corrugated cardboard on the top of the head tube and another on the bottom of the head tube. Each cardboard disk has a small hole in the exact center. A flashlight is placed on the top disk and adjusted until the light shines down through the holes and forms a spot on a piece of paper placed on the mirror. The paper has a cross marked at the exact center. When the light spot is centered on the cross, the axis of the tube is aligned with the center of the mirror. You should have installed the secondary spider and central nut before performing this operation but you must remove the mirror and secondary holder rod or it will block the light beam. However, you should keep the spider and its secondary holder nut in place and let the light beam pass through the nut or whatever is used to hold the shaft of the diagonal rod. If the light does not pass through, the nut is not in the center - it should be in the center - it must be in the center.

 

A piece of white paper is cut to the diameter of the mirror. The paper disk is marked with a cross at the exact center and centered on the mirror and held in place with washers as weights. The cross should be arranged so that the arms are square with the sides of the box. This aides the alignment process.

 

Cardboard disk is cut to diameter of top and a small 1/16" hold punched in the center with the end of the compass or small nail. Disk is carefully centered and held in place with paper tape.

 

A second disk is placed and centered in the track on the top of the tube. A somewhat larger hole, about 1/8", is made with a 6 penny nail. 

 

Hole can be seen from bottom

 

Finished assembly. Head tube is carefully centered on top box. You can see the bottom cardboard disk between the tube and top box.

 

Flashlight is placed on center hole and moved around until light dot appears on mirror below. It is best to darken the room a bit, the light will be faint but clearly visible.

 

Light spot can just be made out at the upper right hand corner of the cross. Moving the flashlight around will cause the light to move its position slightly but one can find the true center very easily. It will suddenly become brighter. Keeping the top hole small is important. The process of adjustment involves first deciding which direction you want to move the light. You move it along one direction of the cross first until it sits on a line and then move it along the other direction until it sits in the center of the cross.

 

To correct the condition seen above move the light spot to the left until it sits on the line. To do this, slightly loosen, but not completely loosen, upper strut nuts A and C. Now completely loosen nuts B and D. Grab top box at A and D and simultaneously pull box down at B and push up at D. This will move spot to the left until it looks like the picture below.

 

To complete adjustment tighten nuts B and D until hand tight. Loosen nuts A and C and now push down at A while pulling up at C until the spot hits the center as shown below.

 

Tighten all nuts and you're done. Remove cardboard disks and replace rotating head tube in its proper position. You can now re-install the spider and secondary mirror. Axis of secondary mirror must be in line with the axis of the head tube. See Installing Secondary.  As a note on collimation, when collimating the telescope have the eyepiece set pointing opposite out from the declination axis. This is done so that when the head is rotated it will not generally move more than about 30 degrees from the position it occupied at collimation. the more you move it, the more out of collimation it can go. Bearing this is mind, making this adjustment is critical. Take you time and be critical. In fact, you can rotate the head and see if the spot moves. It should not move much, if at all.