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secondary spacing is off by about 2mm. Remember you want better than 1mm spacing
accuracy for the CDK12.5.
Adjusting the Secondary Spacing on the CDK20
Since the CDK20 secondary mirror is spring loaded, small adjustments in spacing can be
made by tightening or loosening the collimating screws by equal amounts. For large
spacing adjustments, one may need to adjust the center knob shown in figure 6. To move
the secondary toward the primary, loosen the center knob and tighten the collimating
screws by equal amounts until everything feels snug. To move the secondary away from
the primary, loosen the tree collimating screws by equal amounts and tighten the center
knob.
Determining the Direction to Move the Secondary (CDK20)
To determine which way to move the secondary, gently slide the Ronchi ocular out of the
spacing tube. If as you do this, the Ronchi reaches null, the focal plane is too far back
and you need to move the secondary mirror away from the primary. If as you slide the
ronchi out of the spacing tube, the number of lines grows, the focal plane is too far
forward and you need to move the secondary toward the primary.
For every one unit the secondary spacing is changed, the focal plane moves five units.
Or, if the focal plane is within 5mm (about two ronchi lines), the secondary spacing is
within its 1mm tolerance. Figure 8 shows a Ronchi simulation of the CDK20 with the
Ronchi screen set at the focal plane and also with the focal plane 10mm away from the
Ronchi screen. The image on the left is what you will see if the primary to secondary
spacing is perfect. The image on the right is what you will see if the primary to
secondary spacing is about 2mm off.
