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Focussing CCDs on ISIS to avoid astigmatism from the grating
Focussing the Blue Arm
When focussing a detector on the blue arm:
- Set the blue collimator focus to 5600
- Reduce the Hartmann shifts to about 10 to 13 microns (~0.7 to 1 pixels) by using the capstans on the CCD
- The final focus can be done with the collimator
Focussing the Red Arm
When focussing a detector on the Red arm:
- Remove any dichroic or filter
- Set the red collimator focus to 9100
- Reduce the Hartmann shifts to about 10 to 13 microns (~0.7 to 1 pixels) by using the capstans on the CCD
- The final focus can be done with the collimator
- When the dichroic is introduced into the beam the collimator focus in the red arm must be increased by around 2300
- When a colour filters is introduced into the beam the collimator focus must be increased by ~ 1000
