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Rotation, tilt and focus

The detector should be mounted on LDSS-2 and set to the correct back focal distance and orientation by technical/support staff. The detector position and orientation is determined with the aid of three micrometer heads which measure the distance between reference surfaces attached to LDSS-2 and the detector. A fourth micrometer head measures the rotation about the optical axis. This set of four micrometer readings uniquely defines the correct position of the detector. Thus, once these numbers have been determined, it should be possible to reposition each detector correctly without having to redetermine the correct position using optical tests each time it is interfaced to LDSS-2. However, in practice it is necessary to check that the positioning is correct every time a detector is interfaced to LDSS-2.

The optical system and detector must be aligned so that

• the optimum focus position is within range of the focus adjustment on the camera

• the detector is not rotated with respect to the slit and dispersion directions (i.e. the columns/rows of the CCD are aligned with the slit/dispersion direction)

• the detector is not tilted with respect to the focal surface (i.e. the focus is uniform over the full extent of the detector)

To check the alignments, direct images of the focus mask illuminated by the Tungsten calibration lamp are analysed using routines on the DMS. When satisfied with these adjustments, the best focus must be determined for both spectroscopic observations and for direct imaging. For spectroscopy the focus must be chosen to give the best compromise over the full spectral range required. For photometry the focus should be optimized to give the best images for a particular filter.

The optimum order to do these tests is as follows:

1. Set LDSS-2 to use the focus mask with white light illumination and the filter that you will use to determine the reference focus position
2. Set the camera focus to the default value (LFOCUS 380).
3. Check the rotation of the detector (single exposure with no Hartmann mask selected)
4. Setup the sampling windows for the DMS Hartmann routines.
5. Check that the focus is not out of range (Hartmann test)
6. Check the tilt (Hartmann test)
7. Determine the best overall focus for the reference filter (Hartmann test)
8. Determine the best focus for the other filters that you will be using (Hartmann test)

The following sections deal with the determination of rotation, tilt and focus in more detail.

• Rotation
• Hartmann tests
• Tilt
• Focus