V112907
6
Adjusting the Secondary Spacing on the CDK12.5
The CDK12.5 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 screw driver 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 and that moves the focus 3mm.
4.
Take the four collimation knobs and rotate them in equal amounts until the
assembly is tight again.
5.
Check the Ronchi screen and repeat 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 screw driver 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 and that moves the
focus 3mm.
4.
Take the four collimation knobs and rotate them in equal amounts until the
assembly is tight again.
5.
Check the Ronchi screen and repeat process as necessary until you get a null.
Determining the Direction to Move the Secondary (CDK12.5)
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 on the CDK12.5, 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. And it equals an error in primary to secondary
spacing of about +/- .7mm. Figure 8 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
