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For every one unit the secondary spacing is changed on the CDK12.5/17, the focal plane
moves 7.5 units. You want to keep the error at the focal plane to 5mm or better, which
equals just under two lines on the Ronchi. This corresponds to an error in primary-to-
secondary spacing of about +/- .7mm. Figure 10 shows a Ronchi simulation for 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 off by about 2mm. Remember, you want better than
1mm spacing accuracy for the CDK12.5/17.
Adjusting the Secondary Spacing (CDK12.5 / CDK17)
The CDK12.5 / 17 secondary mirror is not spring loaded, so you must always keep
tension in the collimating screws and the central bolt. To move the secondary
toward
the
primary mirror:
1.
Loosen the four collimation knobs. Loosen them in equal amounts to maintain
your collimation.
2.
Hold the secondary housing with one hand. This is to keep the housing from
rotating. (
The four screws sit in shallow detents and we would like them to go
back in the detents when the procedure is complete)
.
3.
Take a flat-head screwdriver and place the tip in the central bolt of the secondary
assembly. Rotate the central bolt in a
clockwise
direction.
For reference,
rotating ¼ turn moves the secondary .4mm, which moves the focal plane
3mm.
4.
Rotate the four collimation knobs in equal amounts until the assembly is tight
again.
5.
Check the Ronchi screen and repeat this process as necessary until you get a null.
To move the secondary
away
from the primary:
1.
Loosen the four collimation knobs. Loosen them in equal amounts to maintain
your collimation.
2.
Hold the secondary housing with one hand. This is to keep the housing from
rotating. (
The four screws sit in shallow detents and we would like them to go
back in the detents when the procedure is complete)
.
3.
Take a flat-head screwdriver and place the tip in the central bolt of the secondary
assembly. Rotate the central bolt in a
counter-clockwise
direction.
For
reference, rotating ¼ turn moves the secondary .4mm, which moves the focal
plane 3mm.
4.
Rotate the four collimation knobs in equal amounts until the assembly is tight
again.
5.
Check the Ronchi screen and repeat this process as necessary until you get a null.
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 the Ronchi reaches null as you do this, the focal plane is too far back and
